THE EFFECTS OF MONOCALCIUM PHOSPHATE ON CROPS GROWN
IN SOME BUGANDA SOILS
A thesis presented by
PETER HOTSON LE MARE
for the Degree of Doctor of Philosophy in the Faculty of Science of the University of London
May 1973
Rothamsted Experimental Station Harpenden Herts 2
ABSTRACT
In field experiments at Namulonge, Uganda, in 1961-63 125 kg/ha of triple superphosphate diminished yields of cotton and beans, but larger dressings increased them. This work at Rothamsted investigated the cause of these unusual responses. Ryegrass and cotton were grown in pot experiments in a glasshouse and in controlled environment cabinets. Monocalcium phosphate (MCP) added to Namulonge soil increased the amounts of phosphorus and manganese in ryegrass; this effect was not caused by triple-point solution, formed by hydrolysis of MCP, dissolving soil manganese as. dicalcium phosphate also increased manganese uptake.
Manganese concentrations in cotton grown in nutrient solutions without soil, were increased by added phosphorus and decreased by calcium; a small Ca:P ratio in the nutrient medium caused manganese toxicity, but extra calcium prevented it. With large Ca:P ratio, excess manganese was precipitated in older leaves as nodules containing Mn (perhaps Mn02); younger leaves had only slight toxicity symptoms. The Ca:P ratio in the nutrients appeared to be involved in controlling the effect of manganese on auxin oxidation.
The ratio of Ca:P in applied nutrient solutions controlled the concentration of manganese in'cotton grown in Namulonge soil without added manganese. Large Ca:P ratios decreased, small ratios increased manganese concentrations; with the mole ratio Ca:P = 1:2, (the ratio in triple superphosphate) the smallest concentrations of calcium and phosphorus in the nutrient solution increased plant manganese, but larger concentrations at the same ratio decreased it. It appears, therefore, that moderate amounts of triple superphosphate depress yield 3
because the Ca:P ratio is too small to prevent harmful amounts of manganese accumulating in plants, not because phosphorus is 'fixed' in the soil.
Quartz breccia ridges in Buganda contain much manganese so soils below them have accumulated manganese; crops grown in the soils may take up too much manganese when fertilisers with small Ca:P ratios are applied. 4
ACKNOWLEDGEMENTS
I acknowledge help of various kinds that I received whilst I
investigated the fertility of the Namulonge soils.
When I returned to England in 1969 G. W. Cooke encouraged me to
investigate the causes of the field responses at Namulonge, and provided
facilities in the Chemistry Department at Rothamsted. He supervised the
project and helped by criticising the manuscript.
Dr. A. H. Cornfield was Director of Studies at Imperial College,
London.
E. Jones took soil samples at Namulonge and sent them to Rothamsted.
Discussions with him were very helpful, especially during my visit to
Namulonge in May 1972. Z. M. S. Kanamwangi provided information on the
distribution of'lunyu' soil in Buganda. At Rothamsted J. M. Hill assayed
leaf samples for enzyme activities; he, I. F. Bird and A. J. Keys helped
by criticising the manuscript of Chapter 10. J. D. D. Mitchell and
R. Selby helped with the pot experiments. Other help is gratefully
acknowledged: H. A. Smith, M. Roberts, Brenda Messer, V. Cosimini,
R. A. G. Rawson (chemical analyses); J. H. A. Dunwoody and A. Todd
(statistical analysis); F. D. Cowland and Lynda Woods (photographs and
maps); and Mrs. Maureen Broom (typing the thesis).
I am also grateful to other colleagues at Rothamsted and Namulonge
who helped by discussion, and in other ways, to complete the work.
The project was financed by the Overseas Development Administration
of H. M. Foreign and Commonwealth Office. 5
CONTENTS
Chapter Page
Title 1
Abstract 2
Acknowledgements 4
Contents 5
1 Introduction 12
Review of earlier work on response to 12 phosphate in Uganda
The unusual response to phosphate at 13 Namulonge
Other examples of the effect observed 16 at Namulonge
Uganda 16
Kenya 16
India 18
North America 19
Fertilisers that may cause the unusual 20 response
Hydrolysis of monocalcium phosphate in 21 soil
A first hypothesis for the unusual 24 response at Namulonge
2 The Namulonge Environment 27
Geology of Namulonge Research Station 27
Soils 29
Characterisation of Namulonge soils 29
Sendusu series 3o
Nalumuli series 3o
Nakyesasa series 33
Climate 33 6
Chapter Page
Soils and Crops Used in the Experiments 36
Soils 36
Soil 1 36 Soil 2 36 Soil 3 36
Crops 37
4 Effects of Small Amounts of Monocalcium 38 Phosphate on Ryegrass Grown in Three Namulonge Soils
Experimental 38
Results 39
Dry matter 41 Plant phosphorus 41 Plant manganese 41
Discussion 49
5 Laboratory Experiments on Separating the Derivatives of Monocalcium Phosphate Hydrolysis
The method 51
Testing the method in Namulonge soil 52
Composition of residues after 52 hydrolysis Movement of phosphate into soil 55 Phosphorus recovered from the 58 soil Time needed for hydrolysis of 58 monocalcium phosphate
6 The Effect of Monocalcium Phosphate, and Its 65 Hydrolysis Derivatives on Manganese in Ryegrass Grown in Namulonge Soils
Experimental 67
Soils 67 Phosphate treatments 67 Procedure 68 7
Chapter Page
Results 72
Plant density 72 Plant manganese 72 Dry matter 75 Plant phosphorus 81
Discussion 88
7 Possible Effects of Manganese in Cotton 91
Hypothetical causes, of the unusual 91 response
The effects of excess manganese in 92 the plant
The interaction of manganese and iron 93
Auxin oxidation by manganese 94
Cotton experiments at Rothamsted 94
8 Effects of Triple-Point Solution on Cotton 96 Grown in Two Namulonge Soils
Experimental 96 Soils 96 Nutrient treatments 97 Phosphate 97 Iron 97 Basal nutrients 97 Procedure 99 Design 99
Results 100
Observations during growth 100
Leaf production 100 8
Chapter Page
Dry matter and plant composition 102
Results for shed leaves 102
Results for plants at harvest 104
Dry matter 104 Plant manganese 106 Plant phosphorus 106 Plant calcium 109
Discussion 109
9 Interactions of Phosphorus, Calcium and 114 Manganese Applied to Cotton Grown in Nutrient Solutions
The effects of applying two factors 115 together at various ratios
Experimental 116
Treatments 116 Basal nutrients 116 Procedure 117
Results 118
Observations during growth 118
Calcium deficiency 118
Manganese toxicity 119
Dry matter and chemical composition 124
Dry matter 124 Plant manganese 128 Plant phosphorus 137 Plant calcium 137
Discussion 140 9
Chapter Page
10 The Effects of Calcium, Phosphorus and 142 Manganese on Some Enzyme Systems Controlling Auxins in Cotton
Experimental 145
Activities of peroxidase and 145 IAA-oxidase
Procedure 145
Inhibition of IAA oxidation 146
Procedure 146
Results 147
Activity of peroxidase 147
Inhibition of IAA oxidation 147
Discussion 153
11 The Effects of the Ratio of Calcium to 157 Phosphorus in Nutrient Solutions on Cotton Grown in Namulonge Soil
Experimental 157
Treatments 158
Results 158
Presentation of results 158 Dry matter 159 Plant manganese 159 Plant phosphorus 165 Plant calcium 165
Nutrient ratios in plants 168
Calcium : manganese ratio 168 Phosphorus : manganese ratio 171
Soil pH 171
Discussion 174 10
Chapter Page
12 General Discussion : Implications of the 177 Work for Tropical Agriculture
Review of experiments at Rothamsted 177
Behaviour of monocalcium phosphate 177 (MCP) in soil
Manganese nutrition of cotton 178 Effects of calcium and phosphorus, 180 and their interactions with manganese, in cotton grown in Sendusu soil
Results of experiments at Rothamsted 183 related to field results at Namulonge
Phosphate fixation and sigmoid 183 response curves Phosphorus concentration in plants 185 The effect of calcium in triple 186 superphosphate Soil physical conditions 187 Effects of other fertilisers 188
Differences in the effects of manganese 189 on various crops and their implications for fertilising at Namulonge
The source of manganese in Namulonge soil 192
References 195
Appendices
Ia Observations on the Phosphate Potential 203 of Some Tropical Soils
Ib Experiments on the Effects of Phosphate 203 Applied to a Buganda Soil
II The Geology of South Mengo District, 204 Buganda, with Special Reference to Sources of Manganese
III Analysis of Crop Samples 208
IV Yields of Dry Matter, and Concentrations 209 of Phosphorus and Manganese in Crops 11
Chapter Page
Appendices (Continued)
V Yields of Dry Matter, and Concentrations 212 of Phosphorus and Manganese in Crops (Chapter 6)
VI Yields of Dry Matter, and Concentrations 215 of Calcium, Phosphorus and Manganese in the Cotton (Chapter 8)
VITA Yields of Dry Matter, and Concentrations 219 of Calcium, Phosphorus and Manganese in Cotton (Chapter 9)
VIIB Assays of Peroxidase Activity and 221 Inhibition of IAA Oxidation in Cotton Leaves (Chapter 10)
VIII Yields of Dry Matter and Concentrations 222 of Calcium, Phosphorus and Manganese in Cotton (Chapter 11)
IX Concentrations of Phosphorus in Water 226 Extracts of Soil 35 and 80 Days After Applying Monocalcium Phosphate and Triple-Point Solution to the Soil Surface (Chapter 5)
X Composition of Nutrient Solutions 227 Applied to Cotton Grown in Sendusu Soil (Chapter 11) 12
CHAPTER 1
INTRODUCTION
Review of earlier work on response to phosphate in Uganda
Traditional peasant agriculture in Uganda is sustained by shifting cultivation that maintains the soil fertile enough for subsistence and a modest cash income from crops sold off the farm; but more intensive farming to increase crops for sale depletes the soil of plant nutrients.
Namulonge Research Station was opened in 1950 to find ways of increasing rain-grown cotton in Africa. From the start the farm was used to develop systems of intensive farming suited to the local Buganda environment (Hutchinson & Prentice, 1959) and these entailed bigger demands on the soil's nutrient reserves. Chemical studies of the soil and crops began in 1952 to find how to improve and maintain fertility by using manures and fertilisers correctly. Concurrently with the work similar problems at Ukiriguru, Tanzania, were studied and comparing results from the two places helped to understand both.
Namulonge (0° 31' N, 32° 37' E) is in Buganda province, about 40 km north of Lake Victoria. The warm, humid climate, with average rainfall of 1250 mm per year without a prolonged dry season, permits profuse natural vegetation or, under cultivation, two crops per year.
For many years the Department of Agriculture recommended a rotation of three years grass and three years cropping (Martin, 1944) but few farmers adopted it. Martin considered this system provided crops with adequate nutrients and he regarded maintaining soil structure and 13
preventing erosion as most important. Griffith (1949a)recognised that
intensive cropping and cattle production would deplete soils of nutrients.
He found that only nitrogen fertiliser increased yields but foresaw that
the severely weathered ferrallitic soils were unlikely to supply enough
phosphorus for big yields as farming intensified.
Buganda is about 1300 km from the East African coast so imported
fertilisers were costly, but a deposit of rock phosphate exists at
Tororo in eastern Uganda. In Griffith's early experiments ground rock
phosphate was compared with a sodaphosphate being prepared from it with
sodium carbonate in Kenya. Responses were small (Manning & ap Griffith,
1949) but few experiments tested a soluble phosphate so the need for
fertiliser phosphate remained in doubt.
The failure of fertilisers to increase yield appreciably in Buganda,
and the large yield variation between seasons, led to studies of the
relation of cotton yield to rainfall and other aspects of the climate
(Manning, 1956; Hutchinson, Manning & Farbrother, 1958). Meanwhile work
on the mineral nutrition of the crop at Namulonge concentrated on
phosphate.
The unusual response to phosphate at Namulonge
In some field experiments with cotton at Namulonge triple super-
phosphate increased yields but many responses were small; sometimes
phosphate advanced maturity. Before the Research Station opened the
land grew indigenous vegetation that developed during two years after
the peasant farmers left, so for a few years crops were sustained by
accumulated reserves of fertility. As this was depleted the need for
fertiliser phosphate was expected to increase but responses remained small. 14
Study of water use by the crop, in relation to rainfall, showed
that from 1952 to 1960 water deficiency often limited cotton yield;
recent comparison of data for rainfall at Namulonge in 1952 to 1960 with
longer records for neighbouring stations, and with Namulonge records for
1961 to 1972, shows that the earlier period was relatively dry. This
hindered long-term studies of nutrient needs but by 1959 there was enough
evidence to justify detailed work on the behaviour of fertiliser phosphate
in Namulonge soils, and this led to the investigations described here.
Concurrently with the Namulonge work factors controlling fertility
in a long-term manurial experiment at Ukiriguru, Tanzania, were
investigated. Crops at Ukiriguru were known to respond well to
phosphate so this experiment was used to compare data for Schofield's
(1955) phosphate potential with older estimates of available soil
phosphate; all were related to crop yield but the phosphate potential
gave the best relationship so in 1958 it was used in a survey of cotton
fields at Namulonge (LeMare, 1960).
The potential was well related to concentration of phosphorus in
leaves; and suggested that phosphate fertiliser should be effective on
soils with little available phosphate. The relationships of phosphate
potential with soil pH and organic matter indicated that as values of
these increased potential values decreased, so that more phosphate was
available to crops. The relationship with organic matter indicated that
the better yields on newly cleared land were caused partly by better
phosphate nutrition.
The relationship of phosphate potential with pH was consistent with
the possibility that Namulonge soils supplied too little phosphorus to 15
crops because fertiliser phosphate precipitated with iron and aluminium, as strengite and variscite, and that larger dressings than had been used were necessary to increase yield: until then 320 kg/ha of triple super- phosphate was the largest dressing. An apparently similar condition at
Kitale, Kenya, was observed by E. D. Bumpus and R. G. Poultney who reported (Private communication) that some soils needed at least 600 kg/ha of triple superphosphate to ensure good yields of wheat.
The hypothesis that the relationship between applied phosphate and yield was sigmoidal, having positive curvature with small dressings, was tested in pot and field experiments at Namulonge (LeMare, 1968a, b;
Appendix I). In the first pot experiment the response curve for sorghum, grown in an arable soil of pH 5.3 having little organic matter, was sigmoid.
In an uncultivated soil of pH 5.9 and with more organic matter, the curvature was wholly negative. In the second pot experiment the inter- action of lime and phosphate was examined in the soil that gave the sigmoid curve. Lime did not affect the shape of the curve, but tended to lessen yield.
The pot experiments were followed by two field experiments on soils that had been cropped for 10 and 5 years. Dressings of triple super- phosphate were in the geometric series 0, 62.5, 125 ... 2000 kg/ha to determine precisely the response with small dressings, and to measure the amount of fertiliser needed for maximum yield. The experiments showed that small dressings (62.5 and 125 kg/ha) decreased yield but the harmful effect was reversed by more fertiliser so with 250 kg/ha of triple superphosphate yield was similar to that without any; more fertiliser increased yield of cotton and beans but triple superphosphate had little 16
effect on maize. The typical form of response is shown in Figure 1.1.
The work described in this thesis examined why small amounts of triple superphosphate decreased yield in the Namulonge soils, although large amounts increased it.
Other examples of the effect observed at Namulonge
Before starting experiments to explain the unusual response, similar effects, reported by other workers, were sought. Few were found and authors who gave data showing the effect did not comment on it; the only author (Terman, 1960) who commented did not publish his results.
Uganda. After the pot experiments, and at about the time of field experiments at Namulonge, Stephens (1966) examined the effects of triple superphosphate on maize and cotton at Kawanda, about 16 km south-west of
Namulonge. He applied 500 kg/ha of triple superphosphate in each of three years and concluded from his results that although fertiliser phosphorus was taken up by the crops, it tended to decrease cotton yield; maize was little affected.
In seven experiments to test new maize and sorghum varieites under good and poor soil conditions, Jowett (1971) measured their responses to nitrogen and phosphate fertilisers in northern and eastern Uganda. Both nutrients increased yield of maize and their interaction was small.
However, with sorghum, although each nutrient alone increased yield their interaction was negative so that phosphate diminished yield if nitrogen was given.
Kenya. During 1948 to 1950 experiments to measure responses to fertilisers in the Kenya highlands showed that most crops responsed well to phosphate,
17
4000 Plant dry matter
3000
/ha 2000 kg ld, ie Y
1000 PIS
Seed cotton la
0
0 125 250 500 1000 2000
Triple superphosphate, kg/ha
Figure 1.1
Relationship between mean yields of four crops of cotton and applied triple superphosphate at Namulonge, 1961-63 18
but in two of seven lucerne experiments the response curve was sigmoid and yield with 42.5 kg/ha of double superphosphate was less than without fertiliser; in one experiment sodaphosphate (supplying the same amount of phosphorus) also decreased yield, but it was the better fertiliser in this and most of the other experiments (Holme & Sherwood, 1954).
In later experiments Seitzer et al (1970) measured the effects of nitrogen and phosphorus fertilisers on wheat in Kenya. Single or triple superphosphate supplied 0, 17.5 and 35 kg P/ha, but the authors did not state which fertiliser was used in each experiment. Phosphate increased yield in 22 of 26 experiments, but in some it did so only without nitrogen fertiliser. In five of the twenty-two experiments the results resembled Jowett's (1971): without nitrogen, phosphate increased yield linearly and the NP interaction was negative; with 10 kg N/ha phosphate did not affect yield but with 20 kg N yield decreased linearly with increasing phosphate. The positive response to phosphate without nitrogen showed that soil phosphate was not adequate for maximum yield but although nitrogen alone also increased yield, with it superphosphate was harmful. The phosphate dressings were within the range used in the
Namulonge experiments but the largest was not enough to show whether very large dressings of phosphate fertiliser would reverse the trend.
India. In pot experiments, Goswami et al (1969) measured yield and total phosphorus in jute grown in soils treated with small amounts of super- phosphate. The fertiliser was labelled with 32P and plant phosphorus derived from fertiliser was estimated; total plant phosphorus was determined and the difference estimated the amount of soil phosphorus taken up. In all experiments superphosphate increased the amount of 19
fertiliser phosphorus in the plants, but with it less soil phosphorus was taken up so that in two experiments on alluvial soils (pH 6.0 and
6.3) total plant phosphorus was greater without than with 9 kg P/ha as superphosphate.
North America. Terman (1960) reported field experiments on cotton, maize, small grains and forage crops in south-eastern United States. He stated that 'a slight to marked turn-down occurred in the response curves at relatively low rates of triple superphosphate' in 10 of 32 cotton experiments, so he excluded them from his average responses. He suggested the 'turn-down' indicated seedling injury or unbalanced nutrition.
Terman used 0, 50, 100, 150 and 200 kg/ha of triple superphosphate, similar to the smaller dressings at Namulonge.
Hamilton and Lathwell (1964) reported a pot experiment on the effects of monocalcium phosphate (MCP)and diammonium phosphate (DAP) on lucerne grown in extremely phosphate deficient Lima silt loam (pH 7.4) from New
York State. Phosphate dressings in geometric series supplying 0, 24,
48 ... 384 kg P/ha were mixed with the soil. With each salt the relationship between total plant phosphorus and yield was sigmoid but whereas all dressings of DAP increased yield, the smallest dressings of
MCP diminished it slightly.
Fried and Dean (1952), in their paper proposing the 'A' value as an estimate of available soil nutrient, presented data which indicated that in one of their soils, Davidson clay loam, 'A' diminished when a small dressing of monocalcium phosphate was added, but it increased with more
MCP. The authors, and later Scott Russell et al (1954), discussed the anomaly of the overall linear trend of 'A' associated with increments of 20
MCP. They suggested that some of the applied P reacted with soil
constituents so that it was not available to plants and caused the
apparent increase of available soil P as more MCP was given. Although
both groups of authors recognised this anomaly both ignored the initial 31 decrease of 'A'. 'A' values cannot be measured without adding some P 32 31 as carrier for P: Fried and Dean's smallest dressings was 2.5 kg P/ha 31 but their data show 'A' was least with 5 kg P/ha. Over the range 2.5,
5, 10 and 20 kg 31P/ha as band-placed monocalcium phosphate the curvature
was positive with probability 0.025; for MCP mixed with the soil the size of positive curvature was similar but 'A' values were much bigger,
as was the standard error.
In Fried and Dean's experiments 5 kg 31P/ha did not decreased the
dry matter yields, so the smaller 'A' must reflect a smaller concentration
of phosphorus in the oat tops that were analysed.
Most authors gave inadequate or no statistical information to test the significance of their effects. However, formal tests of statistical
'significance' at conventional values of probability may be misleading, but a number of similar examples of small differences may demonstrate a real effect. The experiments cited support the results from Namulonge, and indicate that a harmful effect of small dressings of phosphate fertiliser occurs widely.
Fertilisers that may cause the unusual response
In my pot experiments at Namulonge (LeMare, 1968a) ammonium and
potassium dihydrogen orthophosphates did not decrease yields of sorghum
plants, but in a later pot experiment a small dressing of dicalcium 21
phosphate diminished yield of cotton plants slightly (Kabaara & Jones,
1966). In the field experiments at Namulonge, and in all experiments by other workers in which phosphate fertiliser decreased yield, single or triple superphosphate, or monocalcium phosphate, was used; in one experiment sodaphosphate also slightly diminished yield (Holme & Sherwood,
1954). The results indicated that the effect was biggest with monocalcium phosphate and may be characteristic of it; however, the apparent greater frequency of the effect with this salt may be fortuitous because single and triple superphosphate were the commonest fertilisers in experiments testing the effect of phosphate.
The present work sought the factor that decreased yield; a hypothesis based on the known chemical behaviour of monocalcium phosphate in soil was the basis for the initial investigations.
Hydrolysis of monocalcium phosphate in soil
The chemical behaviour of monocalcium phosphate (MCP) in soil was elucidated by the Tennessee Valley Authority (TVA) as part of a comprehensive study of reactions of phosphate in soils (Huffman, 1962).
When MCP is placed in soil it absorbs water vapour and a nearly saturated solution develops so TVA workers investigated its behaviour in very concentrated solutions. MCP did not dissolve congruently; instead it hydrolysed and dicalcium phosphate precipitated. Two systems occurred: within one hour dicalcium phosphate dihydrate (DCPD) was associated in meta stable equilibrium with MCP and a 'triple-point solution', containing calcium and phosphorus, at pH 1.48; within seven days a stable system developed in which MCP was in equilibrium with dicalcium phosphate anhydrate (DCPA) and a triple-point solution at pH 1.01 (Lindsay &
Stephenson, 1959a). The solubility isotherms are shown in Figure 1.2, from the paper by Lindsay and Stephenson. 22
— STABLE ISOTHERMS -- METASTABLE ISOTHERMS O - SOLUBILITY POINTS ON THE ISOTHERMS CALCULATED FROM 5 LITERATURE. A- EXPERIMENTAL POINTS FOR DISSOLUTION OF MCP IN WATER TPS ••-•Z 17 do• +-3do. -J J4 mrpst4.2,24bc = , N.:•••3,15 min \ 0 • V .-t-1,2 mfr. 0 , v ., 0,• - 0 . % 03 0 +1,'■ .4` A ob./ CS. • • 0. 4;' Ae. • CP/ 4%°- / • 4C. • • <2 • \-‘ • 0C•' .t%Cal s". / 0 '
O 0.5 1.0 1.5 Ca IN SOLUTION, MOL/L.
Figure 1.2
Solubility isotherms in the concentrated region of the CaO-P205-H20 system at 25°C, and points showing the effect of time on the dissolution of monocalcium phosphate (MCP) in water.
(Copied from Lindsay & Stephenson, 1959a)
TPS = triple-point solution MTPS = metastable triple-point solution 23
The hydrolysis was represented (Brown & Lehr, 1959) by
1Ca(H PO )H 0 + water —> f(CaHP04 ) + (2-f)Ca0 + f)P 0 2 4 2 (12 2 5 where f is the fraction of P remaining'as dicalcium phosphate after hydrolysis.
If R is the mole ratio of Ca0 : P205,
R - I- f or 1-f 2
f = 1-R 2-R
The TVA workers measured R experimentally. In the metastable system, the triple-point solution in equilibrium with DCPD and MCP contained -1 -1 1.44 mol 1 Ca0 and 1.99 mol 1 P205,
1.44 0.276 00 -. 0.724 ; 1.276 - 0.216 R = 1.99
The stable triple-point solution in equilibrium with DCPA and MCP -1 Ca0 and 2.25 mol 1-1 P 0 contained 1.34 mol 1 2 5
00 R = 0.595 ; f = 0.288
Thus, at equilibrium DCPD and DCPA contained 21.6 and 28.8% respectively of the phosphorus originally present in MCP. The corresponding solutions therefore contained 78.4 and 71.2% of the original phosphorus.
In soils, equilibrium was not maintained because the triple-point solution (TPS) moved away by capillarity from the site where MCP was 24
placed; hydrolysis proceeded and only DCP remained at the site of
placement. Lehr, Brown and Brown (1959) investigated the behaviour of
MCP in sand and soil. In sand the residue of DCP contained 21.3% of
phosphorus applied as MCP. In soils the residues contained 18 to 33% of the added P, depending upon soil factors, including moisture content.-
As the acid solution moved from where MCP was placed, it dissolved calcium,
iron, aluminium, manganese and other soil constituents; iron and aluminium precipitated as phosphates close to the placement site, but manganese moved further and remained in solution.
A first hypothesis for the unusual response at Namulonge
Chemical work at Namulonge (LeMare, 1968c; Appendix I) showed that strengite, FePO .2H 0, did not control solubility of phosphate, but in 4 2 soil at pH 5, the concentrations of aluminium and phosphate two years after applying up to 1000 kg/ha of triple superphosphate were consistent with the solubility product for variscite, A1P0 .2H 0 so this mineral 4 2 may have controlled the solubility of phosphate in untreated soil, and possibly some of the phosphate applied as triple superphosphate was converted to it. Although Bache's (1963) work indicated that variscite is not likely to form at pH > 3.1 it may form as a product of aluminium, dissolved by the triple-point solution, and phosphate contained in the triple-point solution.
Although the nature of the phosphate in Namulonge soils was not established, its solubility was very small, near that of variscite, so much of the added phosphate would not be available to crops. Because the solubility measurements were made two years after fertiliser was applied, they did not show how soon the fertiliser phosphate was unavailable 25
but if it occurred within a few weeks the crop's major source of fertiliser phosphorus would be the DCP residue, which TVA workers showed contained 20-30% of the applied phosphorus. DCP may react with soil constituents also, so the proportion of fertiliser phosphorus useful to the crop would be very small, although it would increase in relation to the amount given.
In the field experiments, all dressings of triple superphosphate increased dry matter of plants sampled six weeks after applying fertiliser but small dressings decreased harvest yields of the first and later crops. The hypothesis adopted when the present work began was that DCP was the major source of phosphate for the first as well as later crops; manganese, dissolved by the acid triple-point solution, was harmful but its effect was not observed whilst the phosphorus in DCP was available to the crop. Where small dressings were given, DCP provided adequate phosphorus for early growth but not enough to sustain the crop; the harmful effect of manganese became dominant so yields decreased.
Many Buganda soils contain much manganese (Chenery, 1960) but amounts extracted by neutral normal ammonium acetate were usually small
(Radwanski, 1960) and evidence of excess manganese in crops was rare.
However, these criteria of harmful amounts of manganese in soils may have been misleading. The field experiments began in 1961; rainfall was heavy in 1961 and 1962 so the soil was very wet and possibly poorly aerated; if manganese was dissolved by triple-point solution leaving ++ granules of fertiliser, it was likely to remain in solution as Mn ions, and be readily available to crops. 26
Workers elsewhere showed that when monocalcium phosphate was applied to soils, the concentrations of manganese in crops increased. Fruhstorfer
(1956) reported that deficiency of manganese in potatoes was alleviated by superphosphate alone, and that manganese sulphate was more effective when mixed with superphosphate. Larsen (1956) reported a pot experiment in which solutions of monocalcium'phosphate were applied to barley growing in a soil with little available manganese; large concentrations of phosphorus eliminated symptoms of manganese deficiency. In citrus, monoammonium and monocalcium phosphates increased concentrations of manganese in leaves of sour orange (Bingham, Martin & Chastain, 1958; Bingham & Garber, 1960).
Field experiments (Larsen, 1964) on manganese deficient soils showed that concentrations of manganese in leaves of oats and sugar beet increased as dressings of triple superphosphate increased in the geometric series,
0, 500, 1000 ... 8000 kg/ha. Larsen suggested that phosphate affected translocation of manganese in the plants or, more likely, that plants took up manganese that was dissolved by triple-point solution in the soil.
The first pot experiment at Rothamsted, described in Chapter 4, investigated the effects of monocalcium phosphate (MCP) on dry matter yield and concentration of manganese in ryegrass growing in Namulonge soils; in the second experiment (Chapter 6) the effects of MCP were compared with the effects of its two hydrolysis derivatives, dicalcium phosphate and triple-point solution, applied separately to Namulonge soils. The results of these experiments with ryegrass were the basis for experiments with cotton. 27
CHAPTER 2
THE NAMULONGE ENVIRONMENT
Geology of Namulonge Research Station. The geology of the area round
Namulonge was described by Hepworth (1956). , Within the 900 hectares
of the Research Station (including the Livestock Husbandry Farm at
Nakyesasa) most of the geology of the surrounding area is represented.
Figure 2.1, based on an unpublished map by Hepworth, shows the main
features of the geology at Namulonge.
The estate is a strip of land lying north west - south east about
5.7 km long by 2.1 km at its widest. The northern boundary is the
Nasirye river which lies along a line of en eschelon faulting associated
with a major tear-fault. For about 3.3 km the southern boundary follows
-another to /--fault along a breccia ridge about 1200 m above sea level,
and an associated valley was formed where the fault line meets the upper
reaches of the Lwajali river, along which the boundary continues for about
1.3 km. Within the estate there is a minor ridge running along a tear-
fault under the eastern residential area; another minor tear-fault
caused the Katabusolo valley at the head of which is the dam. These
minor fault lines are nearly parallel with the southern ridge and with
the overall direction of the Nasirye river. The agricultural
significance of the breccia, which contains much manganese, was not
realised until 1972.
The rock underlying most of the estate is grey mobilised granite.
Outcrops of the granite contain garnet, epidote, hornblende and biotite. . • •
. • • •
NALUMULI
NAKYESASA • • . •
8•*514 • CAI, • • • • • . • • • • 1‘18 /414,,7 .(jA/Gu
444 oo
•
• • Fig 2.1 The Geology of Namulonge gam ems Tear-faults • 1 Granite outcrops • Quartzite outcrops • Schists GAYAZA Mixed zone of granite and schists Mobilized granite
Rivers:and valleys
Scale 1:10,000 approx 29
The Nalumuli area is a zone of mixed granite and schist where mica flakes occur on the surface and weathered granite 6 m below it.
Nakyesasa is underlain by schists; mica flakes are common and the schists outcrop in a cutting on the pediment.
A band of quartzite, sheared by the tear-faults, runs north - south through the estate and quartz gravel occurs in places.
I examined the breccia ridges at Namulonge in May 1972. Much of the quartz material was friable; it had a dark brown coating on the surface and in the interstices. Tests with orthophosphOric acid and periodate, and with hydrogen peroxide, showed the brown material was manganese dioxide.
Soils
The first account of the soils of Uganda (Griffith, 1949b) was followed by a more detailed reconnaissance survey of the country
(Chenery, 1960) from which a general account was prepared (Harrop, 1962).
The soils of Buganda were surveyed and mapped at 1:500,000 by Radwanski
(1960).
Characterisation of Namulonge soils. The soils of Namulonge are members of Griffith's 'Buganda' series; Harrop classed them as 'ferralitic sandy clay to sandy clay loam soils with a dominant red colour developed on basement complex'. Radwanski (1959) examined the Namulonge soils and classed them as intergrades of his 'Buganda' and 'Mirambi' series: red or brown clay or sandy loams with a quartzose horizon at 18 cm or lower.
In places iron concretions overlie laterite, probably associated with the Tanganyika surface of late-Tertiary age. 30
For many years this classification seemed adequate to describe the
Namulonge soils but Figure 2.2 shows a soils classification that is based on geological considerations; it recognises the influence of the quartz breccia that occurs from Sendusu to Kirimantungu. Three series occur.
Sendusu series. Red brown clay loam surface soil overlying red subsoil. Friable, iron-stained quartz gravel, derived from the quartz breccia, is unevenly distributed in the soil. The underlying rock is mobilised granite, but in the lower part of the slope rounded quartz stones occur about 1.5 m deep, probably indicating the base level of the
Tanganyika surface (Pallister, 1959).
Plates 2.1a and b show two profiles of this series photographed in March 1956 before the land was cleared and ploughed. The sites of the profiles are shown in Figure 2.2. Angular quartz, derived from the breccia ridge, is well illustrated in Plate 2.1b. These profiles were the upper two of five extending down slope from the field where the unusual response to triple superphosphate was first observed.
Nalumuli series. The surface soil is dark reddish brown clay loam; subsoil is red, becoming yellowish red with increasing depth. Granite or mica schist occurs about 1 m deep on the crest of the ridge, but is very deep on the lower slopes; in one pit the rock was below 8.25 m.
Angular quartz occurs near the surface close to the line of quartzite that runs south-west to north-east through the estate; probably little of the angular quartz in Nalumuli is derived from quartz breccia because this area is separated from the breccia ridge by the Katabusolo valley.
Rounded quartz pebbles and stones, probably associated with the Tanganyika surface, also occur, as in the Sendusu series. •. NALUMULI
3.2 G.,..DG.E.LAL AsC4 DAKM:/44,4NA NAKYESASA BRECCIA RI j .•
UNG1J •
Fig 2.2 The Soils of Namulonge • Sites of soils used at Rothamsted Pits — see plate 2.1 a &b A Sites of Field Experiments (Le Mare 1968b) Sendusu series Nalumuli series Nakyesasa series
Scale 1:30,000 approx 30 em
a b
Plate 2 .1 Soil profiles at Sen du su, Namulonge, sho\ving quartz fragments from the breccia (b) 33
Nakyesasa series. These soils are also red brown clay loams with red subsoil. They are on schist and contain mica flakes. They were not studied because they occur on the Livestock Husbandry Farm, which was not farmed as part of Namulonge Research Station.
Climate
The mean annual total rainfall during 1950 to 1970 was 1254 mm.
Namulonge has a bimodal rainfall distribution with rainfall maxima in
April and November. Figures 2.3 and 2.4 show the rainfall for 1961 and
1962 when the field experiments were done; total rainfall in these two years was 1562 and 1510 mm respectively. The Figures also show the 1:1 confidence limits of rainfall calculated from 20 year's data (T. K. Thorp,
Private communication). The interval between the two limits indicates the expected range of rainfall in 50% of all years. Although the mean totals for the two seasons of each year are similar, rainfall is more reliable during the first; cotton is grown in the second season.
Other weather data vary little during the year: the mean monthly maximum temperature is about 29°C in January, decreasing steadily to about 26°C in July; the mean monthly minimum is 16 ± 1°C. The mean number of sunshine hours varies from 5.5 daily in August to 7.1 in 1 January, and the range of radiation is from 375 Cal Cm per day in
July to 436 Cal Cm-1 per day in January. Figure 2.3 Rainfall, mm in 20 days 100 200 0
Thin lines:1:1confidence limitscalculatedfrom Thick line:rainfall in1961(total1562mm) Rainfall atNamulongeinoverlappingperiods oftwentydays,plottedatten-dayintervals Jan FebMarAprMayJune JulyAugSeptOctNovDec 3 IIIIIIIIIIIIIIIIIIII,Ilititi,,I,,0
6 records for1950-1969 9
12
15 Ten-day period
18
21
24
27
30
33
36 100 200
Rainfall, mm in 20 days Figure 2.4 Thin lines:1:1confidence limitscalculatedfrom Thick line:rainfallin1962(total1510 mm) Rainfall atNamulongeinoverlappingperiods oftwentydays,plottedatten-dayintervals records for1950-1969 4 12 1
11 15 1
Ten-day period ►
fi 18
ti
21
il 24 l
i t 27 i
telt 30
33 1,
1 36 0 100 200 300 36
CHAPTER 3
SOILS AND CROPS USED IN THE EXPERIMENTS
Soils
The pot and field experiments at Namulonge that demonstrated the unusual effect of triple superphosphate ended about seven years before the present work began. Meanwhile most of Namulonge farm, including the sites of the experiments, had received lime and phosphate fertiliser so there was little choice of soils with suitable history, when samples were taken for the present work. Samples of soil, 0-20 cm deep, were taken from three sites, shown on Figure 2.2, in November 1969.
Soil 1 was from Sendusu. The land had not been cultivated for at least
23 years; when sampled it carried mixed vegetation of elephant grass
(Pennisetum purpureum), short grass and shrubs.
Soil 2 came from field 9, Nalumuli, Namulonge Research Station. This had been cropped twice each year for 20 years, usually with maize and cotton. Manure, fertiliser and lime were used from 1964 and these improved productivity (Passmore, 1969).
Soil 3 was from the north end of field 10, which adjoins field 9 at
Nalumuli and had the same cropping history but the area sampled had not received manure, fertiliser or lime since the farmers left in 1946.
Soil reaction (pH in 0.01 M CaC12) and phosphorus extracted by
0.5 M NaHCO solution were: 3 37
Soil pH NaHCO -P 3 ppm
1 5.75 6 2 6.00 13 3 5.8o 4
The pH range was smaller than in the Namulonge experiments. Soil 1
came from the same area as soil 926 that was used, in the pot experiments
at Namulonge and its pH was similar. Soil 3 was selected to represent
soil 925 of the Namulonge pot experiments, and the soil of experiment C2
in field 318 in which small dressings of triple superphosphate were harmful
(LeMare, 1968a, b). Field 10, from which soil 3 was taken, is about
2.4 km south-east of field 318: it was the nearest site with a suitable
history. However, its pH (5.80) was greater than that of the soils used
in experiments at Namulonge.
Soil 2 was chosen because it was similar to soil 3 but had received
lime, manure and fertiliser which improved its productivity, and to
provide a standard for the two untreated soils.
Crops
Ryegrass was grown in the first two pot experiments at Rothamsted
to examine the effects of monocalcium phosphate. (MCP) in Namulonge soils.
•The grass was cut approximately monthly during the experiments to assess
effects of MCP and its derivatives, on plant growth and composition. When
some of the effects were known for ryegrass their importance for cotton
was examined in pot experiments in controlled environment cabinets. 38
CHAPTER 4
EFFECTS OF SMALL AMOUNTS OF MONOCALCIUM PHOSPHATE ON
RYEGRASS GROWN IN THREE NAMULONGE SOILS
The first experiment measured the effects of small amounts of monocalcium phosphate (MCP) on leaf dry matter, and phosphorus and manganese concentrations in ryegrass grown in three Namulonge soils.
The experiment was designed to see whether MCP was harmful to ryegrass, and how phosphorus and manganese concentrations in the crop were related to increasing dressings of MCP in the three soils.
Experimental
The experiment was in conical brown glass pots, 10 cm high and
10 cm diameter at the top. MCP was applied at 0, 4.1, 8.2, 12.3 and
16.4 mg in 200 g of soil per pot. Each pot received 25 ml of nutrient solution supplying 50 mg N as NH4NO3, 50 mg K as and 10 mg Mg as K2SO4 MgSO4.
The nutrient solution was added to the soil from a pipette; after
4 hours the soil was stirred in a Kenwood mixer for 1 minute and left overnight in a covered pot. Next day 150 g of the soil were mixed for
11 minutes with 400 g of sharp white quartz chips.
The hole in the base of each pot was covered with nylon gauze and quartz chips were put in the bottom of the pot to make its weight to
720 g; half the soil-quartz mixture was put in and pressed, followed by
50 g of soil. MCP, in units of 4.114g, was placed in small depressions 39
in the soil and covered with soil. The remaining soil-quartz mixture was added and pressed; 0.25 g of perennial ryegrass (Lolium perenne var. S23) was sprinkled on, covered with 0.5 g fine quartz, and sprayed with water. Pots were covered until the seed germinated. During growth the pots stood in saucers containing demineralised water.
The positions of soil in the pot, and of MCP within it, are in
Figure 4.1. The MCP crystals were placed in pure soil to simulate granules of triple superphosphate in soil in the field.
The amounts of MCP were chosen in relation to the amounts of triple superphosphate that decreased yields in the field experiments, in which minimum yield was with 125 kg/ha of triple superphosphate. Assuming a 6 15 cm layer of soil weighs 2 x 10 kg/ha, the concentration of P in the soil in the field was about 12 lig/g, irregularly distributed in granules containing about 5 mg P each. In the pots the concentrations were 0, 5,
10, 15, 20 pg/g in units of 1 mg. Thus the ratio of applied P to soil
P in the pots covered the range in soil in the field experiments, but the amount of P in each unit of MCP was smaller than in a granule of triple superphosphate.
The 15 experimental treatments (3 soils x 5 phosphate dressings) were randomised in 5 blocks.
Experimental and analytical methods that were not specifically adapted to this or later experiments are reported in the Appendices.
Results
The ryegrass was cut 34, 59 and 90 days after sowing, dried at o 80C and weighed. Concentrations of phosphorusi, manganese a.F141.-ft+am+witem
Figure 4.1
The positions of soil and monocalcium phosphate (MCP) in the pots
Soil + quartz
ca MCP:
Soil
Soil + quartz
Quartz • • • • • Gauze
MCP 41
in the leaf dry matter were determined by the methods in Appendix III.
Dry matter. Yields are shown in Figure 4.2. Without MCP yields were best, about 1 g/pot, in soil 2 but MCP did not increase yield significantly in this soil at any cut. MCP linearly increased yield of cuts 1 and 2 in soil 1 and of all cuts in soil 3; the effect of MCP on total dry matter was slightly bigger in soil 3.
Plant phosphorus. The phosphorus concentrations in the leaves is shown in Figure 4.3. Results for soils 1 and 3 were similar: concentration increased linearly with all dressings of MCP in the first and second, but not in the third cut; concentration, and the slope of the relationship decreased with time. In soil 2 also, concentrations of phosphorus increased with MCP but were greater in the'second than in the first cut.
The effect of MCP on the amount of phosphorus taken up by the ryegrass is shown in Figure 4.4. In soils 1 and 3 it was linearly related to the amount of MCP applied; 50 and 46% of the applied P was recovered respectively. Recovery of phosphorus from soil 2 was not well related to the amount applied.
Plant manganese. Manganese concentrations in the leaf dry matter are in Figure 4.5. At the first cut concentrations of manganese varied little, either between phosphate treatments or between soils. The mean value was 148 ppm in the dry matter.
At the second cut manganese concentrations were much greater. In plant dry matter from soil 2 the mean was 290 ppm Mn but there was no trend associated with the amount of MCP applied. In soils 1 and 3
42
Soil 2 Soil 3
e
0 2.0
4)
1.5
1.0
0 A 0.5 ❑ I I I
0 I t 1 t t 1 1 t 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 P applied, mg/pot
Figure 4.2
Effect of phosphorus, applied as MCP, on yield of dry matter
❑- Cut 1; 0 - Cut 2; L - Cut 3; * - Sum of 3 cuts - SE of difference between two points, I for all soils
43
Figure 4.3
Effect of phosphorus applied as•MCP on phosphorus concentration in ryegrass
0- Cut 1; C) - Cut 2; A- Cut 3 I-. SE of difference between two points
0 1 2 3 4 0 1 • 2 3 4 0 1 2 3
P applied, mg/pot 44 ••■
Figure 4.4
The effect of phosphorus applied as MCP on amount of phosphorus taken up by ryegrass
❑- Cut 1; C)- Cut 2; Li - Cut. 3 1 - SE of difference between two points
Soil 1 Soil 2 7
6
0 5 horus, 3 Phosp
2
0 0 1 2 1 2 3 4 0 1
P applied, mg/pot 45 500
Soil 1 Soil 2 Soil 3
400 m pp r,
tte 300 ma dry
in
se ne a Mang
100
0; t 1 t t • • t 0 1 2 3 4 0 . 1 2 3 4 0 1 • 3 4
P applied, mg/pot
Figure 4.5
The effect of phosphorus- applied as MCP on manganese concentration in ryegrass. Experiment 1
- Cut.1; 0- Cut 2; Q - Cut 3; SE of difference between two points - 46
manganese concentrations increased linearly at 8 and 11 ppm Mn/mg P applied as MCP; without MCP the concentrations were 422 and 327 ppm Mn for soils 1 and 3 respectively.
At the third cut manganese concentrations of dry matter grown in soils 1 and 3 without MCP were smaller than at the second cut, 167 and
184 ppm, but they increased with MCP: in soil 1 the rate of increase was 20 ppm Mn/mg of P applied, more than twice what it was at the second cut; in soil 3 the rate was similar to that of the second cut. In soil 2 the effect of MCP on manganese concentration was not well related to dressings of MCP.
Figure 4.6 shows the effect of MCP on the total amount of manganese in three crops of ryegrass. From soil 2 the plants took up 0.65 mg Mn/pot, irrespective of the amount of MCP given, but from soils 1 and 3 the amount of manganese taken by the ryegrass was closely related to applied MCP; without MCP the crops from soils 1 and 3 contained 0.55 and 0.45 mg Mn/pot respectively; with MCP the amounts increased by 0.075 and 0.05 mg Mn/mg of applied P so that with the largest dressing (16 mg MCP = 4 mg P/pot) the crops contained 0.85 and 0.65 mg Mn/pot, showing that these soils supplied large amounts of manganese (the nutrient solutions contained none), and that with MCP the crops took more from the soil.
The ratio of P:Mn in the ryegrass is plotted against applied P in
Figure 4.7. Its size, and relation to applied MCP, were similar in soils
1 and 3: in the first cut the ratio increased rapidly with applied MCP, because P concentration increased while Mn remained constant; it also increased linearly at the second cut but the rate was smaller. At the third cut the ratio diminished with all dressings in soil 1 but in soil 3
47
Figure 4.6
Effect of MCP. on total manganese removed by three cuts of ryegrass I - . SE of difference
or
1.0 Soil 1
0
E. a) 0.5 tn a) z
1 2 3 0 1 2 3
P applied, mg/pot 48
Soil 1 Soil 2 Soil 3
30
Mn 20 P: io t ra le Mo
10
1 0 1 2 3 0 1 2. 3 4 0 1 2 P applied, mg/pot
Figure 4.7
The relationship between P:Mn ratio -in ryegrass, and phosphorus applied as monocalcium phosphate
❑- 1st cut; 0 - 2nd cut; A - 3rd cut 49
it decreased linearly to a minimum with 3 mg P/pot, then increased slightly with 4 mg P. In crops grown in soil 2 the ratio was greater than in those from other soils; it was biggest with 2 mg P/pot.
Discussion
This experiment was designed to see how monocalcium phosphate added
to the three soils affected ryegrass growth and composition, especially whether small dressings of MCP lessened yield, as in the field
experiments at Namulonge, and if they affected the manganese concentration
in the crop.
Yields were best in soil 2. In this soil MCP affected ryegrass yield very little, although it increased phosphorus concentration in the
crop; it had little effect on manganese concentration. The relationships
between added MCP and dry matter, and concentrations of phosphorus and
manganese, were irregular, especially at the third cut; the cause is not known but it may have been related to changes in soil constituents
effected by the lime, manure and fertiliser applied in the field.
In soils 1 and 3 MCP increased the concentrations of phosphorus and
manganese in the crops but at successive cuts the effect on phosphorus
diminished while that on manganese increased. These results were
compatible with the field experiments at Namulonge where all dressings of
triple superphosphate improved early growth but small dressings of triple
superphosphate were harmful after about three months. The results
supported the hypothesis that phosphorus in the triple-point solution,
formed by hydrolysis of MCP, precipitated with iron and aluminium and
that the crop's main source of phosphorus was the dicalcium phosphate 50
residue; as this was depleted, manganese became harmful. In soil 3 the
minimum in the relationship between the P:Mn ratio and MCP indicated that the phosphorus-manganese balance in the crop was affected by the
amount of MCP applied, when dressings were small. To test the hypothesis
in more detail MCP and its hydrolysis derivatives were tested separately
with ryegrass in soils 1 and 3. The method for separating the
derivatives, and laboratory tests of it, are described in Chapter 5, and
the effects of the derivatives on ryegrass in Chapter 6. . 51
CHAPTER 5
LABORATORY EXPERIMENTS ON SEPARATING THE DERIVATIVES OF
MONOCALCIUM PHOSPHATE HYDROLYSIS
Investigations by Tennessee Valley Authority (TVA) workers elucidated the hydrolysis of monocalcium phosphate (MCP) (Brown & Lehr, 1959;
Lindsay & Stephenson, 1959a), identified many compounds formed from MCP and its hydrolysis derivatives by reaction with soil constituents
(Lindsay & Stephenson, 1959b, c, d; Moreno, Lindsay & Osborn, 1960), and tested some of these compounds on crops (Lindsay & Taylor, 1960).
The TVA workers studied these problems using large salt:soil ratios
(Lindsay & Stephenson, 1959a), large pellets of MCP containing 15 mg P
(Bouldin & Sample, 1958, 1959), and synthetic triple-point solution
(Lindsay & Stephenson, 1959b) and dicalcium phosphate (Terman, Bouldin &
Lehr, 1958).
This chapter describes tests of a method of applying separately to soils the two hydrolysis derivatives of an amount of MCP equivalent to a granule of triple superphosphate. The method was developed with 60 mg
MCP (14.8 mg P) so that results from it could be compared with those from
TVA methods. Later it was used in pot experiments with amounts of MCP equivalent to granules of triple superphosphate.
The Method
A piece of filter paper was fixed with 'Araldite' epoxy resin to the ground end of a glass tube 50 mm long x 6 mm diameter. MCP was put 52
into the tube which was then placed on moist soil, covered and left to stand in the laboratory. Water vapour from the soil passed through the filter paper to the MCP, which hydrolysed. The triple-point solution, formed by hydrolysis, was absorbed by the soil through the paper, leaving the dicalcium phosphate residue in the tube.
Testing the method in Namulonge soil
The method was tested in 45 g soil, wetted to 80% of field capacity, in PVC tubes 34 mm internal diameter, lined with polythene film closed at the bottom. 60 mg of acetone-washed MCP (crystal size < 0.5 mm) was placed in tared glass tubes with No. 50 filter paper on one end.
Each soil received three MCP treatments: none, MCP placed directly into a 3 mm depression in the soil, and MCP in glass tubes placed in a similar depression. The PVC cylinders were closed with 'Parafilm' thermoplastic sheet and left to stand in the laboratory. The arrangement of soil and MCP in the cylinder is shown in Figure 5.1. Treatments were replicated four-fold.
After 25 days all glass tubes were removed and their contents analysed; after 35 and 80 days water soluble phosphorus in the soil was
determined.
Composition of residues after hydrolysis. The glass tubes were removed
and adhering soil brushed off; they were dried at 80°C and weighed. The
residues were dissolved in 5 ml of 2N HC1, diluted to 100 ml with water,
and the phosphorus and calcium concentrations of the solutions determined;
means of the four replicates are in Table 5.1. 53
Moist plug
'Parafilm'
MCP .. .
Filter paper • . • , :. . •„ . . .
':• • ; • • • • . „ -• . % • • • • _.• „ .., •.. .• Soil • • • • ••- ...•.• • - •• • • s .• - , •, • • •• • a r • , • • • • „ • • • • 4. Polythene sleeve . • ' - . • .• • • • • • . • . .• 7. . „ . • . ' ' • • •. . • .• • - . . • ' . . •• . . . PVC cylinder . .. -. • . .
Figure 5.1
Method of applying triple-point solution to soil and remvoing the dicalcium phosphate residue 54
TABLE 5.1
Weight and composition of residues
P in residue Residue P P, % of Mole ratio as % of Soil mg mg residue P:Ca applied P
1 15.4 2.95 19.1 1.00 19.9 2 15.4 2.88 18.8 1.02 19.5 3 15.0 2.81 18.7 1.01 19.0
Mean 15.3 2.88 18.9 1.01 19.5 55
The P:Ca mole ratio was 1.00 to 1.02 in the three soils, sufficiently close to unity to show the residue composition was that of dicalcium phosphate (DCP).
Anhydrous DCP contains 22.8% and the dihydrate 18.0% of phosphorus; the residue contained 18.9%, corresponding to a mixture containing
18.7% as anhydrate and 81.3% dihydrate DCP.
The residues from the three soils contained 19.0 to 19.9% of the phosphorus put into the tube as MCP. These results agree with those from TVA experiments (Lehr, Brown & Brown, 1959) which showed that 18 to
33% of applied phosphorus remained at the pellet site; the actual percentage was affected by several factors, including soil moisture and soil type.
Movement of phosphate into soil. After 35 and 80 days the cylinders of soil of one replicate were separated into 5 mm sections. The sections were weighed (each was about 5.5 g) and shaken for one hour with water at 1:15 soil:water ratio; 20 ml aliquots of the suspension were centrifuged at 10,000 rpm for 15 minutes and the phosphorus concentration of the extract was measured. The detailed results are in Appendix Table 1,c; the mean results for treatments, soils and time are in Figures 5.2a, b and c.
Figure 5.2a shows that the concentration of phosphorus in the water extract of surface soil was 7.5 ppm more if MCP was placed directly in the soil than if placed in a tube, because the dicalcium phosphate residue remained in the surface layer. Below the surface, movement of phosphate was similar for both placement methods. In the Section depth, E 25 20 10 15 5 0 0 a) Theeffectofmethod ofplacement
• O - MCPplaceddirectly insoil - MCPplacedintube with Figure 5.2 filter paperbase 5 I
The movementoftriple-pointsolutionthroughsoil 10 t
15 0 1
l b) Differencesbetween soils
A - O - P inwaterexract,ppm Soil Soil 1; 5 1
3 0 -Soil2; 10 1
15 0 t
4
O O The effectoftime - 35days - 80 days 5 1
10 1
15 vi rn t
. 57
5-10 mm layer phosphorus concentration of the extract was about 8 ppm; it decreased linearly to 0.4 ppm in the 15-20 mm layer and in the
20-25 mm layer the concentration was that for untreated soil, 0.1 ppm P.
Figure 5.2b shows that movement in soils 1 and 3 was similar, but at the surface the concentration of extract from soil 3 was less than soil 1, perhaps because more phosphorus precipitated with soil constituents. Soil 2 had more water extractable phosphorus in the sub- surface layers than the other soils, but less in the surface.
Figure 5.2c shows there was less water extractable phosphorus throughout the soil column after 80 days than at 35 days, indicating the applied phosphate became less soluble with time.
At 35 days the experiment was comparable with that of Bouldin and
Sample (1959), who placed a pellet of MCP containing 15 mg P in Hartsells fine sandy loam; after 35 days Bouldin and Sample extracted layers of soil, 3/16 inch thick, with water at 1:15 ratio. Their extract of surface soil contained 10 ppm P; in sub-surface layers the concentrations decreased linearly to a very small value at 20 mm. Comparable results for the mean of three soils from my experiment showed that with MCP applied to the soil the concentration of phosphorus in the water extract decreased linearly from 15.3 ppm at the surface to 0.5 ppm in the 15 to
20 mm layer; at 20 to 25 mm the concentration was 0.1 ppm. When MCP was applied in a tube and phosphate moved through filter paper, the concentration was 7.4 ppm in the surface layer but below this the concentrations were almost the same as those for MCP applied directly to the soil. Thus, for
MCP applied to the soil the results agreed well with the similar method used by Bouldin and Sample. The similarity of sub-surface results for the 58
two application methods showed that the tube with filter paper satisfactorily applied the triple-point solution to soil without the
DCP residue.
Phosphorus recovered from the soil. For the soils that received phosphate in solution through the filter paper, the amount of P received by the soil was the difference between that in the MCP and that left in the residue; for MCP placed in the soil 14.8 mg P was applied. The percentages of P recovered from the soil in the water extracts are in Table 5.2.
More phosphate was recovered when it was applied directly to the soil (13.9%) than when it passed through a filter paper (12.6%). At
80 days there was little difference between soils and the mean recovery was 11.5%, but at 35 days less was recovered from soils 2 and 3 (13.8% 11.!‘ and 13.4%) than from soil 1 ( .6%). Soluble phosphate was precipitated more rapidly in the two arable soils, but over a long period precipitation was similar in all the soils.
Time needed for hydrolysis of monocalcium phosphate. In the experiment just described, 25 days was allowed to ensure complete hydrolysis of
MCP and movement of triple-point solution from the placement position, so that the results were comparable with those of the TVA experiments described. However, other investigations by TVA workers showed that hydrolysis was complete within seven days (Lindsay & Stephenson, 1959a).
In pot experiments to investigate the effect of the hydrolysis derivatives on crop growth, it is desirable not to leave the soil in the pots awaiting sowing longer than necessary, so three experiments were done to see how soon hydrolysis was complete. 59
TABLE 5.2
Per cent of added P recovered in water extracts after
35 and 80 days
P via filter paper P direct Mean
35 8o 35 80 35 80 Soil days days days days days days Mean
1 16.8 10.4 18.5 12.4 17.6 11.4 14.5 2 12.7 11.7 15.0 11.7 13.8 11.7 12.8 3 13.7 10.6 13.2 12.4 13.4 11.5 12.4
Mean 14.4 10.9 15.6 12.2 14.9 11.5 13.2 Mean 12.6 13.9 13.2 60
In the first of these experiments the moisture content was 16%, as in the experiment already described; 60 mg of MCP was allowed to hydrolyse for 1, 2, 4 and 8 days. In the second experiment soil moisture content was also 16% but a moist cotton wool plug was put into the top ' of the glass tube, to provide a more humid atmosphere over the MCP, because the movement of water vapour through the filter paper seemed slow in the first experiment; tubes stood for 1, 2 and 3 days. In the third experiment soil moisture was 18% and tubes had a plug of moist cotton wool; they stood for 2, 4, 6 and 8 days.
The results for the three experiments are shown in Figure 5.3a, where the weight of phosphate compound in the glass tube is plotted against the number of days from the start of each experiment. The broken and dotted horizontal lines represent the expected weights of dicalcium
phosphate anhydrate (DCPA) and dicalcium phosphate dihydrate (DCPD) respectively if hydrolysis of MCP is complete (Lindsay & Stephenson, 1959a).
The solid lines show the progress of hydrolysis in the three experiments.
Variation between replicates was large initially but was small after
4 days.
A moist plug in the glass tube hastened hydrolysis, which was
complete by the eighth day without a plug. Hydrolysis was quicker in
soil containing 18% than 16% of water, and appeared complete on the
fourth day. However, Figure 5.3b shows that the P:Ca mole ratio of the
residue in soil at 18% was 1.12 on the fourth day, showing that hydrolysis
was not complete. On the sixth and eighth day the P:Ca ratio was 1.02,
showing that the residue composition was very near that of dicalcium
phosphate, and that hydrolysis of MCP was sufficiently complete for
testing the derivatives on crops. 60 O - 16% moisture without moist plug ❑- 16% with moist plug •- 18% with moist plug 50
g be, tu s 40 las g in d
n 30 u o Theoretical omp weight for P c
f 20 DCPA DCPD o t h ig We 10
0 1 0 1 2 3 4 5 6 7 8 Days after placement
Figure 5.3a
The effect of time on weight of residue Figure 5.3b residue ofMCPhydrolysis The effectoftimeonP:Camoleratiothe
Mole ratio P:Ca 1 0
Days afterplacement 2
62 . 4 63
The residue weight reached the theoretical value for dicalcium phosphate on the fourth day but the mole ratio, P:Ca, was too big for
DCP; by the sixth day, although the mole ratio was near 1, the residue weight was less than the theoretical value. These results suggest that some DCP passed through the filter paper with the triple-point solution as hydrolysis proceeded.
In the pot experiment described in the next chapter ryegrass was grown with both hydrolysis derivatives so the extent of hydrolysis could not be checked. But in an experiment described in Chapter 8 cotton was grown using the triple-point solution only, so the phosphorus in the residues was measured. 10.4 mg MCP was allowed to hydrolyse for
6 days in tubes with a moist plug over soil at 16.7% moisture. Table 5.3 shows the mean per cent of applied P that remained in the residues.
The mean percentage of the applied P that remained in the residue was 20.1. The results, obtained with 10.4 mg MCP, confirmed those of the earlier experiments in which 60 mg MCP was used, and showed that the method was satisfactory for separating the hydrolysis derivatives of very small amounts of MCP. 64
TABLE 5.3
Phosphorus remaining in residues after hydrolysis.
Cotton Experiment 1 (See Chapter 8)
MCP applied P Mean weight of % of applied P per pot per pot P in residue remaining in mg mg mg residue
10.4 2.56 0.519 ± 0.0064 20.3
20.8 5.12 1.030 ± 0.0136 20.1
31.2 7.68 1.529 ± 0.0132 19.9
Mean 20.1 65
CHAPTER 6
THE EFFECT OF MONOCALCIUM PHOSPHATE, AND ITS HYDROLYSIS
DERIVATIVES ON MANGANESE IN RYEGRASS GROWN IN
NAMULONGE SOILS
The pot experiment described in Chapter 4 showed that monocalcium
phosphate (MCP) increased manganese concentration of ryegrass grown in
the two Namulonge soils that had not been limed or manured in the field.
This result supported the hypothesis that triple-point solution (TPS),
formed by hydrolysis of MCP, dissolved soil manganese and so made it
available to plants; the residue of dicalcium phosphate (DCPR) was
assumed not to affect manganese. This chapter describes a pot experiment
to test the hypothesis by comparing the effects of these two derivatives,
and of MCP, on manganese concentration in ryegrass.
The hypothesis is represented in Figure 6.1, where manganese in the
crop is shown as a linear function of phosphorus applied in these three forms: m and m are the manganese concentrations without applied 0 1 phosphorus and with p mg of phosphorus as MCP, so the slope of the line
is
- m m1 0 p
The hypothesis assumes that only TPS increases manganese concentration,
so the same concentration should occur if the TPS is separated from the
DCP residue and applied to the soil alone. TPS contains approximately
80% of the phosphorus in MCP from which it is derived, so the slope of 66
Figure 6.1
Hypothetical effect of phosphorus applied as MCP and its hydrolysis derivatives on the concentration of manganese in the plant
TPS
m MCP 1
m DCP
I
0.8 p P
P applied 67
the line relating manganese concentration to phosphorus in TPS will be
(m1 - m0) 1.25 (m1 - mo)
0.8 p p
Thus, for TPS and MCP the ratio of the slopes of the lines relating
manganese concentration to phosphorus applied should be 1.25:1; and
for p mg of phosphorus applied as TPS, the concentration should be
1.25(m1 - m0) m0. Dicalcium phosphate should not affect manganese,
so with all dressings concentration should be m0.
Experimental
Soils. Sendu'su and Nalumuli unmanured soils were used. The samples
were taken in August 1970 from the sites of soils 1 and 3 of the first ryegrass experiment.
Phosphate treatments. The amounts of MCP used for the treatments were similar to those in granules of triple superphosphate. In a sample of
Fison's triple superphosphate 43.2% and 53.7% of the granules were in
the 2-3 mm and 3-4 mm fractions respectively; these contained 3.2 and
6.6 mg P per granule.
The laboratory experiments described in the previous chapter showed that phosphorus originating in MCP was divided between DCP residue and
TPS in a ratio close to 1:4. In the pot experiment described here the
amounts of phosphorus applied per pot were in this ratio (3 and 12 mg) so that the residue obtained with the larger dressing of TPS provided the DCP treatment at the smaller dressing; the residue from MCP used to 68
supply the smaller dressing (3 mg P) of TPS provided a treatment supplying 0.75 mg P per pot. The largest dressing of DCP was not obtained by hydrolysis; instead a sample of dicalcium phosphate dihydrate (DCPD) prepared by Albright and Wilson Ltd (Grade 0, Batch
129; Sample No. W9147; 29 November 1956) was used. Dressings supplying
0.75 and 3 mg P were applied to the soil in one position, simulating one small granule of triple superphosphate. Two dressings of 6 mg P, equivalent to two large granules of triple superphosphate, provided the treatments supplying 12 mg P. The treatments are listed in Table 6.1.
The eight treatments on each soil were replicated five times and arranged in five 4 x 4 balanced lattices.
The appropriate amounts of MCP and DCPD were weighed into glass tubes with glass fibre filter paper fixed to one end. The tubes were kept in a desiccator over silica gel until the experiment was set up.
Procedure. 50 ml of nutrient solution containing 50 mg N as NH4NO3,
50 mg K as and 10 mg Mg as MgSO was added. to 300 g soil and K2SO4 4 left overnight. Next day the weights of amber glass pots were standardised at 650 g by adding white quartz chips; 80 g of moist soil was mixed with 300 g of quartz chips and 165 g of this mixture put into the pot, followed by 220 g of the moist soil; the remaining soil-quartz mixture was put into a numbered polythene bag for use later. The soil surface in the pot was levelled and pressed. Glass tubes, containing monocalcium phosphate (for treatments to provide MCP, TPS and DCPR) and dicalcium phosphate dihydrate, were stoppered with moist cotton wool and were pressed about 2 mm into the soil. The pot and tubes were covered with 'Parafilm' thermoplastic adhesive sheet; pots that did not receive P 69
TABLE 6.1
Phosphate treatments
Weight of MCP Treatment P applied No. of placement hydrolysed mg/pot positions mg
No added phosphate O 0
Monocalcium phosphate 3 1 12.2 (MCP) 12 2 48.8
1 15.2 Triple-point solution 3 12 2 60.8
Dicalcium phosphate 0.75 1 15.2 residue 3 1 60.8
Dicalcium phosphate 12 2 dihydrate 70
were also covered with 'Parafilm'. The pots were left to stand for six days while MCP hydrolysed and TPS moved into the soil. Figure 6.2 shows a pot with two tubes for placing 12 mg P; for dressings of 3 mg P one tube was put in the centre of the pot.
After six days the 'Parafilm' was removed and 0.2 g of dry soil
(< 0.5 mm) was put on top of the residue in the glass tube. The tube was pressed about 2 mm into the soil surface of the appropriate pot and the contents pushed out with a glass rod, which was brushed clean; the tube's contents were then covered with a little of the surrounding soil.
In pots to test MCP and DCPD the contents of the glass tubes were transferred to the soil in which they had stood for the past six days; for pots to test DCPR, residues were from tubes in pots testing TPS.
After placing the phosphate, about 150 g of the soil-quartz mixture, previously set aside in a numbered polythene bag, was placed on the soil surface and pressed; 0.25 g of ryegrass seed was sown and covered with the remaining soil-quartz mixture. The surface was sprayed with water and the pots were covered with saucers.
The pots were in systematic order on mobile benches of size
75 x 155 cm, one for each soil. They were left in the potting house for four days until the seed germinated; then they were uncovered, transferred to the glasshouse and randomised.
The experiment was maintained for 42 weeks and eight cuts of grass were taken during this time. 71
Figure 6.2
Method of applying triple-point solution to soil
Moist cotton wool 'Parafilm' thermoplastic sheet Glass tube
MCP
••
Fibreglass Moist filter soil
Moist soil - quartz mixture
quartz gauze 72
Results
Plant density. As germination was uneven the pots were scored for plant density three days after randomising them; a score of 5 represented greatest density. The results are in Table 6.2. Plant density was poorer in Nalumuli soil than in Sendusu soil. In both soils it was best in untreated pots and there was a trend along the benches, especially in
Nalumuli soil. An effect like this had not been observed before, although pot experiments with ryegrass were usually not randomised until after germination. The reason seemed to be that sunshine passing through a high, narrow window warmed the pots differentially as the sun rose. Plant
density improved when the pots were transferred to the glasshouse but
variations seemed to affect yields of the first cut in both soils and of
the second cut in Nalumuli soil.
Plant manganese. Because the experiment was designed to measure the
effect of the phosphate treatments on concentrations of manganese in the
crop, these are discussed first.
In cuts 1 to 6 the effects of different sources of phosphorus on
manganese concentration were very small, being little bigger than their
standard errors; in cuts 7 and 8 the effects were bigger but replicates
were bulked so standard errors are not available. Results averaged over
all sources for each cut are in Figure 6.3 which shows that, except for
cuts 6 and 7, manganese concentration increased at successive cuts from
both soils. In the first two cuts, increasing dressings of phosphate
tended to diminish concentration of manganese but there was little effect
in the third and fourth cuts. In cuts 5 to 8 manganese concentrations of 73
TABLE 6.2
Scores for plant density three days after germination
Scores
Treatment P applied Sendusu Nalumuli Mean mg soil soil
No added phosphate 0 4.4 3.4 3.9
3.2 Monocalcium phosphate 3 3.4 3. 0 12 2.6 2.2 2.4
Triple-point solution 3 3.2 2.8 3.0 12 2.8 2.0 2.4
Dicalcium phosphate 0.75 3.0 2.4 2.7 residue 3 3.0 1.6 2.3 Dicalcium phosphate 12 3.0 1.6 2.3 dihydrate 400 SENDUSU SOIL NALUMULI SOIL
300
S
4 tter,
ma 200 dry in
nese a 03 • ng Ma at, 2
• 1 100
3 12 0 3 12
P applied, mg/pot
Figure 6.3
The effect of applied phosphorus on manganese concentration in eight cuts of ryegrass grown in two Namulonge soils 75
crops grown in the two soils differed greatly: in Sendusu soil phosphate increased manganese concentration; but in Nalumuli soil phosphate caused little effect, except at the eighth cut when it decreased concentration.
Because the effect of applied phosphate changed between the fourth and fifth cuts, the results for the different P sources are presented for cuts 1 to 4, and for cuts 5 to 8, separately in Figure 6.4. This shows that the effects of the three P sources on manganese concentration differed little except in Sendusu soil where TPS at 3 mg P per pot did not increase manganese concentration; however the coefficients for the linear regression of concentration on the amount of applied phosphorus, shown in Table 6.3 were similar for all sources.
The effect of the sources of phosphorus on the amount of manganese taken up by the crops is shown in Figures 6.5a and b. These also show that MCP and its derivatives affected plant manganese similarly.
Dry matter. Total dry matter yields for cuts 1 to 4, 5 to 8 and 1 to 8 are shown in Figure 6.6. In both soils yields were greater, and responses to P were less, in the first four cuts than in the second four cuts,. Responses to MCP and TPS were similar but the response to DCP differed from these. In both soils, at the first cut both dressings of the DCP residue and the prepared DCP dihydrate greatly decreased yield.
This was not repeated in cuts 2 to 4, but in cuts 5 to 8 the 'smallest dressing of DCP residue in Nalumuli soil decreased yield slightly; however, in the total produce of cuts 1 to 4 and 5 to 8 the decrements were less than their standard errors. In cuts 1 to 4, the dressings of
DCP increased yield very little but in cuts 5 to 8 3 mg P as DCP residue 76
400 SENDUSU SOIL NALUMULLSOIL m pp r, te t ma dry in nese a Cuts 1-4 Mang
0.75 3 12 0.75 3 12
P applied, mg/pot
Figure 6.4
The effect of applied phosphorus on manganese concentration of ryegrass
0 - MCP; TPS; 0 - DCPR; - DCPD 77,
3
2 t o /p mg rop, c in
nese 1 a I ng Ma
I t t 0 t t 0.75 3 12 0.75 3 12
P applied, mg/pot
Figure 6.5a
The effect of applied phosphorus on total manganese in ryegrass grown in Sendusu soil
0 - MCP; A - TPS; 0 -' DCPR; M - DCPD - SE of difference between two points 1 Manganese in crop, mg/pot 2 Figure 6.5b 1 ® 1 -SEofdifference betweentwopoints ryegrass growninNalumulisoil The effectofappliedphosphorusontotal manganesein - MCP;
Q -TPS; P applied,mg/pot 78
p -DCPR;
M -DCPD. 79
1 SENDUSU SOIL NALUMULI SOIL.
Cuts 1-8 I
Cuts
1-8
7
6
0
-4 I tter, a m -8 1 Dry
3
2
1
r 0 4 0.75 3 12 0.75 3 12
P applied, mg/pot
Figure 6.6
Effect of applied phosphate on dry matter of ryegrass
0 - MCP; • .A - TPS; - DCPR; m - DCPD SE of difference between two points 8o
TABLE 6.3
Coefficients of linear regressions of manganese concentrations
in ryegrass on amounts of P applied, ppm Mn in dry matter per
mg P applied
Sendusu soil Nalumuli soil Treatment cuts 1-4 cuts 5-8 cuts 1-4 cuts 5-8
Monocalcium phosphate -0.29 2.10 0.07 -0.05
Triple-point solution -0.17 2.13 -0.03 -0.02
Dicalcium phosphate -0.07 2.35 -0.09 -0.53 81
increased yield in Sendusu soil more than MCP or TSP; 12 mg P as DCP
dihydrate did not increase yield more than 3 mg P as DCP residue. , In
Nalumuli soil yield with 3 mgPas DCP residue was a little bigger than
with equivalent MCP or TPS. These results indicate that in both soils
MCP and TPS provided a steady supply of phosphorus throughout crop
growth and that the DCP residue provided little phosphorus initially but
larger amounts were taken up later.
Plant phosphorus. Figure 6.7 shows the effect of the three sources on
concentration of phosphorus in the ryegrass.
Results for concentrations were similar in the two soils. MCP and
TPS increased concentration similarly in cuts 1 to 4 and 5 to 8; neither
form of DCP increased concentration as much as MCP and TPS in cuts 1 to 4
but both were as effective in cuts 5 to 8.
The amounts of phosphorus taken up are shown in Figure 6.8 and the
amounts and percentages of applied P that were recovered are in Table
6.4. More phosphorus was taken up from Sendusu than from Nalumuli soil
in cuts 1 to 4, but amounts were similar in cuts 5 to 8. In both soils
MCP and TPS supplied similar amounts of P in both groups of cuts. Little
phosphorus was taken from either form of DCP in the first four cuts; but
in cuts 5 to 8 more P was taken from DCP residue than from MCP and TPS
in Sendusu soil, and in both soils the amount taken from the dihydrate
approached that from MCP and TPS.
In Sendusu soil about 50% of applied P was recovered from MCP, TPS
and the larger dressing of DCP residue; with the smaller dressing of DCP
residue 72% of the added P was recovered. Only 21% ofPappned as DCP
dihydrate was recovered. In Nalumuli soil, recoveries were more variable 82
0.3 SENDUSU SOIL NALUMULI SOIL
4) 0 sa. 0.2
ter, t ma dry
in
us hor 0.1 Phosp
0 0.75 3
P applied, mg/pot.
Figure 6.7
The effect of applied phosphorus on P concentration in ryegrass
- MCP; A - TPS; ❑ DCPR; - DCPD Phosphorus in ryegrass, mg/pot Figure 10 15 20 0 0.75 6.8
Effect ofappliedphosphorus ontotalPinryegrass 3 SE ofdifference betweentwo points MCP; SENDUSU SOIL
- TPS; 83 P
applied, mg/pot 12 1 1-8 5-81 Cuts
0 -DCPR; - 4 I I 0.75
3 NALUMULI SOIL M -DCPD • 12 Cu ts 84
TABLE 6.4
Recovery of phosphorus from monocalcium phosphate (MCP), triple-point
solution (TPS), dicalcium phosphate residue (DCPR) and dicalcium
phosphate dihydrate (DCPD)
Per cent recovery P recovered, mg of applied P
Treatment P applied cuts cuts cuts cuts cuts cuts mg 1-4 5-8 1-8 1-4 5-8 1-8 Sendusu soil
MCP 3 1.241 0.418 1.654 41 14 55 12 3.428 2.718 6.144 29 23 51
TPS 3 0.960 0.506 1.466 32 17 49 12 3.820 2.285 6.104 32 19 51
DCPR 0.75 0.433 0.109 0.542 58 14 72 3 0.492 1.192 1.682 16 4o 56 DCPD 12 0.501 1.997 2.497 4 17 21
Nalumuli soil
MCP 3 0.964 0.178 1.142 32 6 38 12 3.431 1.747 5.178 29 15 43
TPS 3 1.258 0.460 1.718 42 15 57 12 3.525 1.566 5.091 29 13 42
DCPR 0.75 0.051 -0.071 -0.020 7 -9 -3 3 0.485 0.522 1.007 16 17 34 DCPD 12 0.143 1.067 1.211 1 9 10
SE ±0.2327 ±0.2094 ±0.3175 85
and most were less than in Sendusu soil, averaging 45% for MCP and TPS;
34% of P was recovered from DCP residue and 10% from DCP dihydrate.
The DCP residues that supplied 0.75 and 3 mg P were derived from the
same samples of MCP supplying TPS at 3 and 12 mg P. Figure 6.9 shows the
amounts of P recovered from MCP, and the sum of the amounts of P recovered
from TPS and DCP residue, related to P applied. The Figure shows that
for each group of cuts the points for Sendusu soil are on a straight line
and for Nalumuli soil very close to another straight line. The linear relationship for MCP and for TPS plus DCP residue indicates that the sum
of the effects of TPS and DCPR applied separately was equivalent to P
applied as a single dressing of MCP. The result may be examined in more
detail from Table 6.5 which, for MCP, shows the observed amounts of P recovered from dressings supplying 3 and 12 mg P. TPS supplying these
amounts of P were derived from MCP supplying 3.75 and 15 mg P, assuming
80% of the P transferred to the TPS, and 20% remained in the DCP residue.
The observed recoveries for TPS and DCP residue have been multiplied by
0.8 so that their sums are equivalent to the recoveries from dressings of 3 and 12 mg P as MCP. The proportions of P recovered from the two
derivatives are shown in the right hand columns of Table 6.5; the Table
also shows the recovery from MCP as a percentage of the sum of recoveries from TPS and DCPR.
In both soils, with small amounts of applied phosphorus agreement
between recovery from MCP and the sum of recoveries from TPS and DCPR
was poor, so estimates of the relative amounts taken from each of the
derivatives are unreliable. With bigger dressings the sum of adjusted recoveries from TPS and MCP residue was close to that from MCP: in 86
8 Cuts SENDUSU SOIL 1-8
6
4 1-4 5-8
2
0 0 rn 3.75 12 15
rass, eg ry in d
NALUMULI SOIL 6 1-8 P recovere
4
2 5-8
0 3 3.75 12 15
P applied, mg/pot
Figure 6.9
Phosphorus recovered in ryegrass 0 - from MCP; Q - from TPS + DCP residue 87
TABLE 6.5
Phosphorus recovered from monocalcium phosphate (MCP), and from triple-
point solution (TPS) and dicalcium phosphate residue (DCPR) supplying P
equivalent to that from MCP
P recovered as per cent of recovery P recovered, mg from TPS + DCPR
Treatment P applied cuts cuts cuts cuts cuts cuts mg 1-4 5-8 1-8 1-4 5-8 1-8
Sendusu soil
MCP 3 1.241 0.418 1.654 111 85 103 TPS adjusted to 2.4 0.768 0.405 1.173 69 82 73 DCPR adjusted to 0.6 0.346 0.087 0.434 31 18 27
TPS + DCPR 3 1.114 0.492 1.607 100 100 100
MCP 12 3.428 2.718 6.144 99 98 99 TPS adjusted to 9.6 3.056 1.828 4.883 89 66 78 DCPR adjusted to 2.4 0.394 0.954 1.346 11 34 22
TPS + DCPR 12 3.450 2.782 6.229 100 . 100 100
Nalumuli soil
MCP 3 0.964 0.178 1.142 92 57 84 TPS adjusted to 2.4 1.006 0.368 1.374 96 118 101 DCPR adjusted to 0.6 0.041 -0.057 -0.016 4 -18 -1
TPS + DCPR 3 1.047 0.311 1.358 100 100 100
MCP 12 3.431 1.747 5.178 107 105 106 TPS adjusted to 9.6 2.820 1.253 4.073 88 75 83 DCPR adjusted to 2.4 0.388 0.418 0.806 12 25 17
TPS + DCPR 12 3.208 1.671 4.879 100 100 100 88
Sendusu soil agreement was within 2%, and in Nalumuli soil within 7%, in
all groups of cuts. In cuts 1 to 4, the crop grown in Sendusu soil took 89% of its P from TPS, and 11% from DCPR; the corresponding figures
for Nalumuli soil were 88 and 12%. In cuts 5 to 8 the two sources
differed in the two soils: in Sendusu soil TPS and DCPR supplied 66 and
34% of the P respectively; in Nalumuli soil the proportions were 75% and
25%. The results for both soils show that DCP residue was relatively
unimportant for early growth, but later it contributed more P to the crop.
In the total of eight cuts TPS and DCP residue supplied 78 and 22% of
applied P to ryegrass in Sendusu soil, and 83 and 17% to the crop in
Nalumuli soil. These values are near the proportions of MCP phosphorus
expected in the two derivatives when MCP hydrolyses, indicating that over
the period of the experiment (42 weeks) both derivatives supplied P in
the proportion to the amount of each originally formed.
Discussion
Added phosphate increased yield more in Sendusu than in Nalumuli
soil. Monocalcium phosphate and both its derivatives caused similar
responses, except in cuts 5 to 8 of the crop in Sendusu soil when DCP
residue was better than either MCP or TPS. DCP dihydrate was poorer
than MCP or TPS, especially in cuts 1 to 4.
In the first four cuts, MCP and TPS increased the concentration of
phosphorus in the crop, but the dicalcium phosphates increased the
concentration very little. However, in cuts 5 to 8 all sources increased
the concentration of phosphorus equally. The recoveries of added 89
phosphorus were greater in the crops grown in Sendusu than in Nalumuli soil, perhaps because more added phosphorus was precipitated or adsorbed in the latter soil. The data for recoveries of phosphorus showed that plants took up phosphorus more slowly from DCP residue than from TPS, but over the whole period of the experiment the proportions recovered from the two derivatives were in proportion to the amounts formed from
MCP; this result confirmed that the method of separating the derivatives was satisfactory.
The most important result of the experiment was that both dicalcium phosphates, as well as triple-point solution, increased the concentrations of manganese in the crop, showing that dissolution of soil manganese by triple-point solution was not necessary for monocalcium phosphate to increase manganese in plants. Furthermore, the size and time of the increase were the same for each source of phosphorus, indicating that TPS dissolved very little soil manganese. Triple-point solution dissolves manganese as it moves from the placement site of MCP (Lindsay & Stephenson,
1959a); if this were the cause of greater concentration of manganese in the crop the effect would occur much earlier with TPS than with DCP, and would not be delayed for 21 weeks after the TPS was applied.
The time required to increase the concentration of manganese was much longer than in the first experiment in which the effect occurred within nine weeks (Chapter 4). The reason for the difference is not known: conditions in the pots were the same in the two experiments, except that
200 g soil were used in the first experiment, and 300 g in the second.
The state of soil manganese may have been affected by conditions in the field before sampling. For the first experiment samples were taken in 90
mid-November 1969 after a rainy period when the soil was moist; soils for the second experiment were obtained in August 1970 after-two months
of dry weather when rainfall did not wet the soil below 15 cm. Samples for both experiments were taken from the top 20 cm of soil; they were
air-dried before despatch to England by air. Possibly in the second
samples manganese was in more inert forms, following the dry period,
and became available to the ryegrass only after a long period in moist soil in the pots. 91
CHAPTER 7
POSSIBLE EFFECTS OF MANGANESE IN COTTON
The pot experiments described in Chapter 6 showed that the increased concentration of manganese in ryegrass was associated with phosphorus nutrition, and was not uniquely caused by the effects of the acid triple-point.solution derived from MCP by hydrolysis. This result raised two important questions. First, if excess manganese in the plant, induced by small dressings of triple superphosphate, decreased yield in the field experiments at Namulonge, what inhibited this effect and then increased yield with more fertiliser? Secondly, by what mechanism did manganese damage the plant? To seek answers to these questions cotton was grown in experiments in 'Saxcil' controlled environment cabinets.
Hypothetical causes of the unusual response. Triple superphosphate contains two nutrients, phosphorus and calcium; both are important for cotton at Namulonge. The pot experiments with ryegrass showed that increasing the phosphate available to the plants, and hence its concentration in them, increased the manganese concentration also, possibly to maintain the charge balance of anions and cations. In a calciphilic crop, such as cotton, calcium may be especially important where a potentially harmful cation, such as manganese is abundant. The opposing effects of the phosphate anions and calcium cations may require a Ca:P ratio different from that of triple superphosphate to control the 92
concentration of manganese in cotton. Thus, if the Ca:P ratio is too small, the net effect of the fertiliser may be to increase manganese concentrations until they are limited by the amount of manganese available from the soil; the effect of phosphate will then cease but extra calcium may tend to replace manganese and so lessen its concentration in the plant.
Results from one of the field experiments at Namulonge tended to support this hypothesis. In a multiple regression of plant dry matter yield on concentrations of phosphorus and calcium in cotton leaves, the coefficient for calcium was small but positive; its probability of occurring by chance was 0.13. When I reported this (LeMare, 1968b) I concluded that 'calcium had little effect on yield'. However, it is possible that calcium had a very important effect, but of small size because the triple superphosphate supplied too little calcium.
Further support for the idea that calcium was important in controlling manganese concentration was in an experiment reported by Truong, Wilson and Andrew (1971). They grew white clover (Trifolium repens) in nutrient solutions and showed that increasing calcium supply lessened manganese concentration in the clover, but phosphate increased it.
The effects of excess manganese in the plant. The role of excess manganese in plants is probably associated with its capacity for valency change, and hence its effect on plant constituents that are subject to oxidation.
Kenten and Mann (1950) showed in vitro, and later in vivo (1957), that plant peroxidase preparations, in the presence of certain phenolic 93
substrates, catalyse the oxidation of Mn++. They suggested that because
peroxidase occurs in many higher plants, and catalyses the oxidation of
Mn, the range of compounds oxidised may be greater if the concentration
of Mn in the plant is increased. With excessive manganese the rate of
oxidation of Mn ions may exceed the rate of reduction of 'higher
valency forms of manganese' by plant metabolites and lead to the accumulation of higher oxides of manganese observed in plant tissues by
Kelley (1914), and Bussler (1958b).
The interaction of manganese and iron. In the field experiments at
Namulonge leaves from plants given small dressings of triple superphosphate contained smaller concentrations of phosphorus than leaves grown without added phosphate; yields were also diminished, so the effect on concentration was not caused by phosphorus being diluted in a larger crop.
I suggested (LeMare, 1968b) that the concentration of phosphorus was decreased because less phosphate was absorbed by the plants, or that it was not transported in them. I did not suggest a cause but it may involve oxidation of ferrous iron to ferric, by manganic ions, as proposed by
Somers and Shive (1942) for the mechanism of manganese induced iron- deficiency in pineapples growing in soils supplying little iron. They suggested that excessive manganese oxidised ferrous iron which then precipitated as ferric phosphate or in iron-phosphorus organic complexes in roots and conducting tissues, so depriving leaves of adequate iron.
This mechanism received little support until Kenten and Mann demonstrated that manganese was important in oxidation systems in plants (Hewitt, 1963).
In soils where manganese and iron are plentiful, but phosphate is not adequate, precipitation of ferric phosphate in roots may cause phosphate 94
deficiency in leaves. Thus, by the same mechanism manganese may induce deficiency of iron or of phosphate, depending upon the amounts of these elements, and physical conditions, in different soils.
Auxin oxidation by manganese. An important problem at Namulonge was abscission of flower buds and of bolls. Dale and Milford (1965) found that two growth substances, extracted from cotton bolls, did not alter the rate of abscission, nor the total number of shed bolls. However, manganic ions oxidise indole-3-acetic acid (IAA) in plants (Maclachlan
& Waygood, 1956) so excess manganese at Namulonge may influence crop' yields by disturbing auxin systems. Although there was no experimental
evidence that auxin functions were abnormal at Namulonge, in cotton
grown in a greenhouse in Texas (Morgan, Joham & Amin, 1966) excessive manganese increased IAA-oxidase activity and decreased the activity of an
inhibitor of IAA-oxidase; the authors suggested that manganese toxicity symptoms were an expression of auxin deficiency caused by excessive IAA-
oxidase activity. Later, with Taylor they reported that co-factor and
inhibitor activities of the IAA-oxidase system varied as leaf manganese
concentration varied (Taylor, Morgan, Joham & Amin, 1968). Maximum
inhibitor activity was in leaf extracts of plants grown in solutions
containing 0.5 ppm of Mn; it decreased in extracts from plants grown in
solutions with more and with less manganese. IAA-oxidase activity was
greatest in extracts from plants grown with very little manganese.
Cotton experiments at Rothamsted. The effects of manganese in a Uganda
variety of cotton grown in Namulonge soils and in nutrient solution
culture were investigated in three experiments in 'Saxcil' controlled 95
environment cabinets at Rothamsted. The first cotton experiment was planned to give experience of growing the crop in the cabinets, and to show which treatments were likely to need detailed investigation. The second experiment investigated the interaction of phosphorus, calcium and manganese on the nutrition and some enzyme systems of cotton grown in quartz chips with nutrient solutions. In the third experiment cotton was grown in Sendusu soil treated with a range of phosphate and calcium concentrations in nutrient solutions. These experiments are described in the following four chapters. 96
CHAPTER 8
EFFECTS OF TRIPLE-POINT SOLUTION ON COTTON GROWN IN TWO
NAMULONGE SOILS
This chapter describes an experiment made to gain experience of growing cotton in a 'Saxcil' controlled environment cabinet. It measured some effects of nutrients on the plants growing in soils from
Sendusu and Nalumuli.
The experiment was set up in September 1971 before the second ryegrass experiment had shown how the two derivatives from hydrolysis of monocalcium phosphate (MCP) affected the concentration of manganese in that crop. The cotton therefore received phosphate as triple-point solution, because this seemed likely to increase manganese in crops.
Added ferrous iron was also tested.
Ryegrass did not receive calcium in the basal nutrient solution when the experiments were set up, but calcium nitrate was given after one month. This procedure was also used in the cotton experiment but after seven weeks leaves became chiorotic; to test whether this was because the plants lacked calcium the effects of solutions containing two calcium concentrations were compared.
Experimental
Soils. 300 g of unmanured soils from Sendusu and Nalumuli were used.
They were part of the samples taken in August 1970, which were used in the second ryegrass experiment. 97
Nutrient treatments
Phosphate. Triple-point solution supplying 0, 2, 4 and 6 mg P/pot supplied P; it was derived from 1, 2 and 3 lots each of 10.4 mg of MCP hydrolysed in glass tubes as described in Chapter 6. The corresponding amounts of calcium supplied by the triple-point solutions were approximately 0, 1, 2 and 3 mg Ca/pot.
Iron. Because ferrous iron is readily oxidised in aqueous solutions exposed to air, 5 ml of a suspension containing 0.5 g of saccharated ferrous carbonate per 100 ml water was added to the water in the saucers in which the pots stood. The water was sampled, at first weekly and ++ later on alternate days and Fe concentrations were measured. Freshly ++ prepared ferrous carbonate suspension was added when the Fe concentration was less than 1 ppm.
Basal nutrients.. When the experiment was set up, each pot was given the basal nutrients shown in Table 8.1.
Calcium was not applied before the cotton was sown, but three weeks after sowing, and then weekly until the seventh week, 10 mg Ca and 7 mg N,
as calcium nitrate, were added in solution to the saucers in which the pots stood.
From the eighth week until the experiment ended eleven weeks after sowing, nutrient solutions, instead of water, were given daily. Two
calcium concentrations, 47 and 142 ppm Ca were compared, but all pots received 100 ppm N, 100 ppm K, 24 ppm Mg and 72 ppm S. Two solutions
were used; both contained 220 mg K2SO4 and 244 mg MgSO4.7H20 per litre.
One solution was a mixture of calcium nitrate and ammonium nitrate with
mole ratio Ca:N = 0.167; the other solution contained calcium nitrate 98
TABLE 8.1
Basal nutrients applied before sowing cotton
mg of element Compound used per pot
Nitrogen 50 NH NO 4 3 50 K SO Potassium 2 4 Magnesium 10 MgS0 .7H 0 4 2 Sulphur K SO and MgS0 .7H 0 34 2 4 4 2 Boron 0.50 H BO 3 3 Copper 0.25 CuSO4.5H20
Zinc 0.25 ZnSo4 .7 H 20
Molybdenum 0.05 (NH4)6Mo7024.4H20 99
only, mole ratio 0.5. Leaves of cotton grown in the field at Ukiriguru,
Tanzania, contained calcium and nitrogen in the mole ratio 0.5 (LeMare,
1972) so this ratio was compared with the smaller ratio in the pot experiment described here.
Procedure. The experiment, was in amber glass pots, 10 cm high x 10 cm diameter, as used for the ryegrass experiments. Triple-point solution was applied to the soil by the method described in Chapter G. Cotton seeds (Gossypium hirsutum L. var. BPA68) were treated with concentrated sulphuric acid to remove lint fuzz, washed, and left to soak in water overnight before sowing. Three seeds were sown in each pot.
The pots were covered, placed in glass saucers containing de-ionised water and put in a 'Saxcil' controlled environment cabinet in which the o temperature was 30°C during the 'day' and 25 C at night; these periods were equal.
The pots were uncovered two days after sowing, when the seedlings broke through the surface of the soil-quartz mixture. Germinated seedlings were planted in pots having less than three plants. One plant was removed from each pot nine days after sowing, so there were two plants per pot for the rest of the experiment.
Design. The 413 x 2S x 2Fe treatments, for phosphorus, soils and iron respectively, were arranged in four blocks of eight pots each so that the interaction P" x S x Fe (where P" is the quadratic effect of dressings of phosphorus) was confounded with one degree of freedom for block comparisons. Calcium (Ca) was introduced later so that the S x Fe x Ca, and hence the P" x Ca, interactions were confounded with the two remaining degrees of freedom for blocks. 100
Results
Observations during growth
Within three weeks of sowing the older leaves became chlorotic, the margins curled downwards and later they fell off. Scores for the degree of chlorosis showed that it was associated with the phosphate treatments in both soils, but there was no difference between • soils or between iron treatments.
Chlorosis and leaf fall were not cured by 10 mg Ca and 7 mg N/pot per week, but when nutrient solutions, instead of water, were supplied daily from the eighth week, growth improved, leaves ceased to fall and flowers developed. The improvement was quicker with the solution containing the larger concentration of calcium. After twelve days of treatment with the two calcium solutions, there were chlorotic leaves in only eight pots; seven of these pots contained Sendusu soil, of which six received the solution with the lesser concentration of calcium.
Leaf production. The numbers of leaves per pot were counted at fortnightly intervals during the last six weeks of the experiment; the results are shown in Figure 8.1. In Nalumuli soil phosphate did not significantly affect the number of leaves held by the plants, but there were large effects in Sendusu soil.
Leaf counts on 5 November showed that in Sendusu soil all dressings of triple-point solution significantly decreased the number of leaves held; later, plants grown with 6 mg P/pot had more leaves than those grown with 2 and 4 mg P, but they still held less than plants grown without added P. At harvest, the plants grown with 4 mg P held fewest 101
Figure 8.1
The effect of phosphorus applied as triple-point solution on number of leaves per pot
0 5 November V19 November 7 December A21 December SE of difference between two points; valid for both soils.
SENDUSU SOIL NALUMULI SOIL t
o 20 p r e p s ve lea f o r I be I
Num 10
0
0 2 4 6 0 2 P applied, mg/pot 102
leaves, so the relationship between number of leaves and the amount of
phosphorus applied was similar to the relationship of yield of seed
cotton, and plant dry matter, to triple superphosphate applied in the
field experiments at Namulonge, where small dressings of triple super-
phosphate decreased yield.
The leaves that abscissed before the calcium comparison was
introduced were collected, weighed and analysed.
Dry matter and plant composition
The introduction of a calcium comparison into the experiment
complicated interpretation of the dry matter and plant composition
results. The results for shed leaves were unaffected by the calcium
comparison because all of them were collected before the calcium
treatment was introduced, so in the account that follows the results
• for these leaves are reported first.
Results for shed leaves. The weights, concentrations of phosphorus,
calcium and manganese, and the amounts of these elements in the leaves
that were shed are in Table 8.2.
In Sendusu soil, triple-point solution increased the weight of
fallen leaves per pot, and the concentrations of P, Ca and Mn in them,
so the amounts of these elements lost by leaf fall also increased with
added phosphate. In Nalumuli soil, leaf fall was less and not affected
by triple-point solution. Although triple point solution increased the
concentrations of P, Ca and Mn, the weights of these elements lost in
fallen leaves were not affected.
103
TABLE 8.2
Dry matter and chemical composition of shed leaves
Dry Per cent of dry matter Phosphorus matter P Ca Mn mg/pot P Ca Mn mg mg mg
Sendusu soil
0 0.795 0.052 3.34 0.047 0.431 26.9 0.376 2 0.915 0.062 3.80 0.062 0.592 34.5 0.535 4 0.930 0.078 3.74 0.068 0.721 34.4 0.663 6 1.047 0.075 3.8o 0.067 0.791 39.4 0.746
Nalumuli soil 0 0.587 0.052 3.22 0.040 0.307 18.8 0.232 2 0.610 0.064 3.56 0.046 0.394 21.7 0.281 4 0.615 0.079 3.88 0.050 0.489 23.7 0.298 6 0.580 0.069 3.6o 0.056 0.402 20.6 0.311
SE ±0.1184 ±0.0061 ±0.156 ±0.0062 ±0.116 ±3.72 ±0.1028 104
The concentrations of phosphorus in leaves from both soils were
very small, even with the largest phosphate dressings but concentrations
of calcium and manganese were large, especially in leaves grown in
Sendusu soil.
Results for plants at harvest
Dry matter. Figure 8.2 shows the effects of phosphate treatments
on the amounts of dry matter in roots, stems, and green leaves held by
the plants at harvest.
In Nalumuli soil, dry matter yields were only about half those
from Sendusu soil and the effects of phosphate were small. In the
experiment described in Chapter 6, yields of ryegrass, and response to
phosphate, were also smaller in Nalumuli soil and suggested reactions
between soil constituents and added phosphate lessened the amount of
phosphorus available to the plants. The physical condition of the soil
under ryegrass was not examined but in the cotton experiment differences
between the soils were easily seen. Nalumuli soil developed a crust on
the surface, and the soil in the pots was more compact than Sendusu soil,
so aeration was probably poorer. The surface crust was broken approximately
weekly during the experiment; the two soils were treated similarly, but
the surface of Sendusu soil was always the more friable. The poorer
physical condition of Nalumuli soil may have been a cause of the poorer yield.
The effects of phosphorus on yields of plants in Sendusu soil were
also small but the relationships of yield to the amount of phosphorus
applied were similar to those in the field experiments at Namulonge. 105
Figure 8.2
The effect of phosphorus applied as triple-point solution on dry matter production
A leaves ❑ stems 0 roots V total dry matter J SE of difference between two points
SENDUSU SOIL NALUMULI SOIL 15
10
-P 0
tter, a m
Dry I I
L I 0 I C s 0 0 .------0 1.
0 0 2 4 6 0 2 4 6
P applied, mg/pot 106
With 2 and 4 mg P/pot, dry matter yields of leaves and roots, but not of stems, were less than with no added phosphorus, but with 6 mg P yields improved. Yield of stems with 4 mg P was similar to that without added phosphorus, but yields with 2 and 6 mg P were bigger, and the linear trend, 0.126 ± 0.069 g dry matter per mg P applied, indicated that phosphorus increased yield of stems over the range of dressings.
The relationship between total dry matter and applied phosphorus was similar to the relationships for roots and leaves; the minimum yield was 1.31 ± 0.98 g less than the yield without added P. The effect was small and its probability of being due to chance was 0.20, more than the conventional criterion for statistical significance.
Plant manganese. The concentrations of manganese in dry matter of all parts of the plants are shown in Figure 8.3.
In the stems, unlike other parts of the plant, concentrations of manganese were lessened by extra calcium, but were not affected by phosphate. Concentrations in leaves and roots were smaller in plants grown in Nalumuli soil than in those from Sendusu soil. In roots from both soils, and in leaves from Nalumuli soil, the relationship between concentration of manganese and added phosphate were similar: concentration was diminished by 2 mg P, but then increased with 4 and 6 mg P. In leaves of plants grown with 4 mg P in Sendusu soil, concentration was very large so the relationship between concentration and applied phosphorus had two points of inflexion. The analyses were repeated but no reason for the apparently eratic relationship is known.
Plant phosphorus. The relationship between phosphorus concentration in dry matter and applied phosphorus for each soil is in Figure 8.4. This 4) a) U a) Manganese in dry matter, Figure 8.3 .02 .o6 .05 .03 .04 .01 01 0
A Leaves C:1 on theconcentrationofmanganeseindrymattercotton The effectofphosphorusappliedastriple-pointsolution leaves, stemsandroots 0 SE ofdifferencebetweentwopoints Stems Roots 2 SENDUSU SOIL 4
107 P applied, mg/pot 6 0 NALUMULI SOIL 2 4
S I
- 108
Figure 8.4 The effect of phosphorus applied as triple-point solution on the concentration of phosphorus in leaves, stems and roots of cotton
NALUMULI SOIL A - Leaves O Stems O Roots V - Shed leaves - SE of difference between .20 two points
SENDUSU SOIL
.15 .15
OR r,
tte .10 .10 a m dry in L rus ho Phosp .05
0 0 0 2 4 6 0 2 4 6 P applied mg/pot 109
shows that in all parts the concentration of phosphorus was greater in plants grown in Nalumuli soil, than in those in Sendusu soil. More dry matter was produced by plants in Sendusu soil and although more phosphorus was taken up from this soil its concentration in the plant was less than in Nalumuli, indicating that dry matter production in this soil was not primarily limited by phosphate supply, even without added phosphorus.
Triple-point solution increased the concentration of phosphorus in all parts of the plant but, except in stems grown in Nalumuli soil, maximum concentration was with 4 mg P/pot.
Table 8.3 shows that added iron and calcium did not affect the concentration of phosphorus in any part of the plant.
Plant calcium. The concentration of calcium was greater in all parts of plants grown in Sendusu soil, and also with the nutrient solution having the larger calcium concentration (Figure 8.5).
Phosphate did not affect the concentration of calcium in any part of the plant at harvest.
Discussion
This experiment supported the hypothesis that calcium:phosphorus ratio is important for controlling the concentration of manganese in cotton; it did not however support the idea that manganic ions oxidise ferrous to ferric, which then precipitate phosphate. Although this mechanism is possible, the role of iron was not investigated further because techniques to follow its oxidation state, in experiments with 110
TABLE 8.3
Mean concentrations of phosphorus in leaves, stems
and roots of cotton
Phosphorus, per cent of dry matter
Green Stems Roots Shed leaves leaves
Soils Sendusu 0.142 0.061 0.112 0.067 Nalumuli 0.206 0.076 0.179 0.066
Calcium treatments 47 ppm Ca 0.172 0.065 0.142 142 ppm Ca 0.177 0.072 0.148
Iron treatments None 0.169 0.067 0.145 0.068 Fe added 0.180 0.070 0.146 0.065
SE of difference ±0.0072 ±0.0023 ±0.0040 ±0.0043 111
Figure 8.5
The effect of phosphorus in triple-point solution on the concentration of calcium in dry matter of cotton leaves, stems and roots
Leaves Stems Roots SE of difference between two points
SENDUSU SOIL NALUMULI SOIL
I
2 r, tte ma dry in ium lc
Ca 1
0R
2 6 o 2 P applied, mg/pot 112
growing plants, were not known. It seemed more important to get data on the interactions of calcium, phosphorus and manganese which related to practical aspects of using phosphate fertilisers in soils that supply too much manganese to crops.
The leaf curling and marginal chlorosis of the young plants resembled symptoms of slight manganese toxicity. These conditions, and the weight of leaves that abscissed, were correlated with the amount of added phosphate, and with the concentration of manganese in the leaves.
When the plants were given more calcium the abnormal leaf conditions were eliminated. The effects were greater in plants grown in soil from
Sendusu, and in this soil the relationship between dry matter and added phosphate was similar to that in the Namulonge field experiments.
The chemical compositions of shed leaves, and of green leaves held by the plant, differed greatly. The shed leaves contained much smaller concentrations of phosphorus, and larger concentrations of calcium, than the green leaves at harvest. The difference for calcium is surprising because all shed leaves were collected before the concentration of calcium in the nutrient solution was increased.
The concentrations of manganese in the green and shed leaves were the same without added triple-point solution, but this treatment increased the concentration of manganese in the shed leaves only. In plants grown in Sendusu soil, the relationship between manganese concentration in green leaves and the 'dressings' of triple-point solution was eratic, but in Nalumuli soil a small dressing lessened the concentration. The reason for the greater effect of triple-point solution on manganese in shed leaves, compared with green leaves, may be that the 113
shed leaves grew when little Ca was supplied. The interactions of phosphorus, calcium and manganese in cotton were investigated in the experiment described in the next chapter. CHAPTER 9
INTERACTIONS OF PHOSPHORUS, CALCIUM AND MANGANESE APPLIED
TO COTTON GROWN IN NUTRIENT SOLUTIONS
The pot experiments described in earlier chapters showed that phosphate applied to Namulonge soils increased both the dry matter yield and the concentration of manganese in ryegrass grown in them.
The experiment described in Chapter 6 indicated that similar effects occurred in cotton, and that damage by excess manganese was alleviated by increasing the supply of calcium.
Truong et al (1971) showed that with small concentrations of manganese in a nutrient solution white clover contained 200 to 300 ppm of manganese in dry matter, over a wide range of concentrations of calcium and phosphorus. With more manganese in the nutrient solution the concentrations of manganese in the plants were linearly related to those in solution, but the slope of the relationship depended upon the concentrations of calcium and phosphorus in the solution; phosphorus increased, and calcium decreased it. Truong et al commented that opposing effects of calcium and phosphorus implied that the ratio of
Ca:P in the growing medium might influence manganese toxicity.
The opposing effects of calcium and phosphorus on manganese in plants, seemed important to the problem at Namulonge, where small amounts of triple superphosphate decreased, but large dressings increased, yield. 115
The effects of applying two factors together at various ratios
If a dependent variable y is a linear function of two variables x and x 1 2
Y = A + Bx + Cx + Dx x (1) 1 2 1 2
If the ratio of x to x is constant, 1 2 x 1 say k, then x 2
2 Y = A + (B + Ck)x + Dkx (2) 2 2 and y will have a maximum value if D < 0, or a minimum if D > 0, when
= - B + Ck x (3) 2 2Dk
These arguments may be applied to the linear effects of a feitiliser salt, where x and x represent the amounts of the nutrient ions added. 1 2 Usually a fertiliser is given to supply one ion, the other having little effect, so that the equation reduces to the form y = a + bx. However, when monocalcium phosphate was applied to Namulonge soil, it seemed necessary to relate both calcium and phosphate to the manganese concentration in plants. This chapter describes a pot experiment to show how the concentrations of calcium, phosphorus and manganese in nutrients solutions affected growth and chemical compoition of cotton grown in the solutions; the results were applied to equation (1), to test the hypothesis that the Ca:P ratio in the solution controlled the concentration of manganese in the plants. Chapter 11 describes an 116
experiment that tested the hypothesis with plants grown in Namulonge soil.
Experimental
The plants were grown in quartz chips in amber glass pots placed in saucers containing nutrient solutions.
Treatments. Sixteen nutrient solutions provided all combinations of two concentrations each of calcium and phosphorus, and four concentrations of manganese. The concentrations were:
calcium : 6, 90 ppm (0.15, 2.25 mmol 1-1) Ca (as Ca(NO .4H 0) 3 )2 2 phosphorus : 2, 30 ppm (0.065, 0.968 mmol 1-1) P (as KH2PO4)
manganese : 0.5, 4.5, 8.5, 16.5 ppm (9, 82, 155, 300 pmol 1-1) Mn
(as MnSO4.4H20)
Treatments were replicated five times. Two replicates were in each of 'Saxcil' cabinets C3 and C6; one replicate was divided between the two cabinets so that the highest order interaction was confounded with comparison between cabinets.
Basal nutrients. Because the calcium and phosphorus treatments were supplied as Ca(NO and KH PO the concentrations of N and K varied 3 )2 2 4 correspondingly. Nitrogen and potassium were given as KNO3 in the basal nutrient solution so their concentrations in the four calcium-phosphorus solutions were within the ranges reported to be suitable for nutrient solutions (Hewitt, 1966):
117
Ca
ppm
90 30 105 155 6 30 74 233 90 2 105 120 6 2 74 198
Concentrations of other nutrients in the solutions were:
Element Concentration Compound used ppm
Mg 25 MgS0 .7H 0 S 33 4 2 B 0.5 H BO 3 3 Cu 0.02 CuSO4.5H20 Zn 0.05 ZnSO4.7H20 Mo 0.025 Na Mo0 .2H 0 2 4 2 Fe 0.5 Sequestrene 138 Fe
Sequestrene 138 Fe, containing 6% of Fe as ferric ethylene diamine
di-o-hydroxyphenyl acetate (Fe-EDDHA), was supplied by Geigy (UK) Ltd,
Simonsway, Manchester 22.
Procedure. White quartz chips (grade P16/24, size 0.6-1.1 mm, supplied
by Reed-Harris, London SW6) were put into 10 cm diameter amber glass
pots to make the weight 1200 g. The pots stood overnight in saucers
containing nutrient solutions, and the following day the solutions were
poured onto the surface of the quartz to ensure it was thoroughly wet.
Four cotton seeds (var. BPA68), previously treated with sulphuric
acid to remove the lint fuzz, and soaked overnight, were placed 15 mm 118
deep and covered with quartz. The pots were covered and put into the
iSaxcil' cabinets. 2 The light intensity in the cabinets was 124 Wm of visible 2 radiation from fluorescent lamps plus 50 Wm total radiation from
incandescent lamps. Daylength was 12 hours, 09.00 to 21.00.
For the first three days temperatures were 30°C during the day o and 25 C at night. These temperatures were too great, or the atmosphere
in the cabinets was too dry, for the seedlings to shed the testas easily,
so temperatures were decreased to 25° and 20°C for six days and then
increased to 28° and 23°C for the remaining period of the experiment.
The experiment was sown on 8. March 1972. Four replicates were
harvested on 11 April when the bigger plants overshadowed the smaller ones.
One replicate was maintained until 26 April to observe effects of the
treatments in older plants.
Results
Observations during growth
Several features of abnormal growth that developed in the plants about three weeks after sowing were related to the treatments.
Calcium deficiency. With severe deficiency of calcium, cotton petioles collapse about half way along their length (Cooper & Donald 1949). This condition is rare in the field and did not occur in the first pot
experiment with cotton. In the second experiment, described in this chapter, petioles collapsed only on plants given the solution containing 119
the smaller concentration of calcium and the larger concentration of phosphorus. Collapse was prevented by a large concentration of manganese but much manganese caused 'crinkle leaf' and other effects.
Manganese toxicity. With too much manganese, leaf margins curled downwards and were chlorotic; later, the leaves were severely distorted, with areas of dead tissue. The apical buds died; axillary buds developed but later they also died. These conditions are typical of manganese toxicity in cotton. A plant with severe manganese toxicity is shown in Plate 9.1.
Assessments made by eye on 4 April of the effects of'treatments on the extent of manganese toxicity symptoms are in Figure 9.1. These show that as the concentration of manganese in the nutrient solution increased, plants were more severely damaged, especially when grown with the solution having much phosphorus and little calcium. With the larger concentrations of both calcium and phosphorus, manganese toxicity was conspicuous only with the largest concentration of manganese.
Photographs of the plants on 26 April are in Plate 9.2, and notes made the same day are in Table 9.1.
With the smaller phosphorus concentration plants were small and leaves were dark green, characteristic of phosphorus deficiency in field cotton. With the bigger calcium concentration, phosphorus deficient plants were less affected by excess manganese; but this concentration of calcium improved growth only with, the larger concentration of phosphorus.
Plants grown with the larger phosphorus concentration and little calcium were damaged by 4.5 ppm Mn (pot 71); the larger calcium supply 120
Plate 9.1
'Crinkle leaf': manganese toxicity in cotton 121
15 ' f a le
le k in
r 10 'c
r fo
Score 5
0.5 4.5 8.5 16.5
Manganese in nutrient solution, ppm
Figure 9.1
Estimates of manganese toxicity, assessed as scores for severity of 'crinkle leaf', related to concentration of manganese in the nutrient solution
0 - 6 ppm Ca; 2 ppm P N7 - 90 ppm Ca; 2 ppm P - 6 ppm Ca; 30 ppm P - 90 ppm Ca; 30 ppm P
122
Applied Applied Mn, 0.5 4.5 8.5 16.5 Ca P ppm ppm
6 2
Pot No. 65 69 73 77
90 2 11.°7#1111t
Pot No. 74 78
6 30
Pot No. 67 71 75 79
90
Pot No. 68 72 76 8o
Plate 9.2 Cotton plants grown with sixteen combinations of manganese, calcium and phosphorus concentrations in the nutrient solution; seven weeks after sowing
TABLE 9.1 Characteristics of cotton plants grown with sixteen combinations of calcium, phosphorus and manganese in nutrient solutions
Calcium in Phosphorus Manganese in solution, ppm solution in solution ppm ppm 0.5 4.5 8.5 16.5
6 2 Pot 65: Small plants; Pot 69: Very small Pot 73: All apical Pot 77: Plants very small leaves with some plants; very small growing points dead; small; all apical marginal chlorosis and leaves, with marginal no sympodia growing points dead; crinkling; no sympodia chlorosis; two apical no sympodia growing points dead; no sympodia
90 2 Pot 66: Plants very Pot 70: Plants small; Pot 74: Plants small; Pot 78: Plants small; small; leaves dark leaves green with leaves dark green growing points dead; green; no sympodia slight marginal with slight marginal leaves dark green; chlorosis; no chlorosis; no leaf margins curling sympodia sympodia downwards, with marginal chlorosis; some brown specks near midribs; no sympodia
6 30 Pot 67: Leaves pale Pot 71: Young leaves Pot 75: All leaves Pot 79: As Pot 75 green; lower leaves with marginal curled and crinkled; shedding, but without chlorosis; margins interveinal chlorosis petiole collapse; no curling downwards; on most leaves; very marginal chlorosis but older leaves shedding short internodes; margins tending to but without petiole apical growing points curl downwards; no collapse; no dead; sympodia sympodia sympodia stunted
90 30 Pot 68: Young leaves Pot 72: Young leaves Pot 76: Young leaves Pot 80: Youngest leaves normal; internodes healthy; plant form slight marginal with slight marginal shorter than 72 and normal chlorosis; no curling; chlorosis; all other 76, but longer than 80 very slight necrosis; leaves (on monopodia and brown specks along sympodia) chlorotic and veins of older leaves curling downwards with some dead tissue; inter- nodes short; brown specks along veins of old leaves 124
prevented the damage and these plants were the biggest, and appeared the
healthiest, in the experiment (pot 72). With 8.5 and 16.5 ppm manganese
the harmful effect of manganese was alleviated, but not eliminated, by
extra calcium.
'Dry matter and chemical composition
Dry matter. -Weights of dried leaves, stems and roots of the four
replicates 'harvested' on 11 April, 34 days after sowing, are shown
in Figure 9.2; total weights of plant dry matter are shown in Figure 9.3.
The effects of treatments were similar for all parts of the plants so
only total dry matter results are discussed.
Growth was poor with little phosphorus, and effects of calcium
and manganese were small. With the bigger concentration of phosphorus,
the effects of calcium and manganese were large. With little calcium,
yield decreased linearly as the manganese supply increased. With much
calcium 0.5 ppm Mn produced less dry matter than 4.5 ppm Mn but more
manganese was harmful and yield diminished with 8.5 and 16.5 ppm Mn.
In Figure 9.4 total dry matter is plotted against the mole ratio
of Ca:Mn in the nutrient solution on a logarithmic scale. This shows
that with each concentration of phosphorus, yield was a linear function
log Ca/Mn, at both concentrations of calcium and all concentrations of
manganese except the smallest, which supplied too little manganese at
each concentration of calcium. These results show that over wide ranges
of concentrations of both - nutrients, the balance of calcium and
manganese is very important for cotton.
In pots that received 8.5 and 16.5 ppm Mn with the larger
concentrations of phosphorus and calcium, small brown specks occurred 125
Figure 9.2
The effect of concentration of manganese in the nutrient solution on dry matter of leaves, stems and roots of cotton grown with four combinations of calcium and phosphorus concentrations 10 Leaves
5
0 0 I
I
3 Roots
I
0
0.5 4.5 8.5 16.5 Manganese in nutrient solution, ppm
- Ca 6, P 2 ppm; V - Ca 90, P 2 ppm; ❑ - Ca 6, P 30 ppm - Ca 90, P 30 ppm; j - SE of difference between two points 126
Ca 0 6 2 ppm I SE of V 90 2 ppm difference 15 ❑ 6 30 ppm between two LS, 90 30 ppm points
10 0 o.• rn
tter,
ma O l dry ta
To 5
0
0.5 4.5 8.5 16.5
Manganese in nutrient solution, ppm
Figure 9.3
The effect of concentration of manganese in the nutrient solution on total dry matter of cotton grown with four combinations of calcium and phosphorus concentrations
127
Figure 9.4
The relationship between total dry matter of cotton and the Ca:Mn ratio in the nutrient solution
0 2 ppm P 4.5, 8.5, 16.5 ppm Mn A 30 ppm P 0 2 ppm P 0.5 ppm Mn AL 30 ppm P 3 1 SE of difference between two points
15
ter, t ma l dry ta To 5
0 I I I 1 1 0.5 1 2 10 20 30 100 250 Mole ratio Ca:Mn (Log scale) 128
along the veins of the older leaves (shown in Plate 9.3). The specks appeared under a microscope as spherical nodules, 0.02 to 0.15 mm diameter, in the leaf epidermis; a photomicrograph is in Plate 9.4.
A number of nodules were picked out with a fine glass hook and placed on glass fibre filter paper; when treated with ortho-phosphoric acid and potassium periodate, and warmed, purple colour developed, indicating that the nodules contained manganese. Among the dark nodules there were some swollen cells containing colourless liquid. These were not examined chemically so their composition is not known; possibly they were an early stage in the development of the nodules; some had small nodules within them.
Leaves in all pots were examined with a hand lens (x5). Nodules were found in the plants mentioned, and in those that received 16.5 ppm
Mn with the smaller concentration of phosphorus and the larger concentration of calcium. These results indicate that in plants with large concentrations of manganese in their tissue, much calcium in the nutrient supply causes
manganese to precipitate in older leaves.
Most nodules occurred along the midrib and nearby veins but a few were in the lamina; under a hand lens these were distinguishable from thegossypol glands that occurred throughout the lamina.
Plant manganese. Concentrations of total manganese in the dry matter of
leaves, stems and roots are shown in Figure 9.5a-c. These data are from
plants harvested 15 days before the nodules containing manganese were
noticed in the older leaves so separate estimates of mobile and immobile
manganese were not attempted. 129
a b
Plate 9.3
Parts of mature cotton leaves a) with nodules of a manganese compound near the midrib and veins; from a plant grown in a solution containing 90 ppm Ca, 30 ppm P and 16.5 ppm Mn b) without nodules; from a plant grown in a solution containing 90 ppm Ca, 30 ppm P and 0.5 ppm Mn
(x 2.5 approx) 130
Plate 9.4
Nodules of a manganese compound in a cotton leaf
(x 60 approx) Figure - Ca 90, P30 ppm; I- SE of difference between two points - Ca
Manganese in dry matter, 0 6, 9.5 0.2 0.2 0.3 0 0. 0.1 0 The effectofconcentration ofmanganeseinthenutrient 0. solution onconcentrations ofmanganeseindrymatter 0 combinations ofcalciumandphosphorus concentrations leaves, stems androotsofcotton grownwithfour P 2ppm; 7-Ca 90,P2 ppm; 0- Ca 6,P30 ppm; 1 3 Manganese innutrient solution,ppm 13 1 c) Roots b) Stems a) Leaves I
132
In the roots (Figure 9.5c), concentrations increased rapidly as the supply of manganese increased, except where much calcium and little phosphorus were given; with this treatment increasing manganese in the nutrient solution affected the concentrations in the plant very little.
The concentrations of manganese in the stems (Figure 9.5b) were controlled most by calcium: with much calcium, the concentration of manganese increased little; but with little calcium, it increased greatly as the concentrations of manganese in the nutrient solution increased.
The effect of manganese in the nutrient solution, on manganese concentrations in leaves (Figure 9.5a) was large with all calcium and phosphorus treatments; phosphorus increased the effect but calcium diminished it.
Figure 9.6a-c shows the relationships between concentrations of manganese in the plant and the calcium : manganese ratio in the nutrient solution on a linear scale.
In all parts of the plants concentrations decreased as the Ca:Mn ratio increased. In the stems, the relationship was well defined, and was the same with little and with much phosphorus. The concentration diminished rapidly from 0.275 to 0.07% Mn as the ratio increased from
0.5 to 1.8; then it decreased more slowly to 0.025% when the ratio was about 15. In the leaves and roots the relationship was similar, except in plants given the bigger concentrations of calcium and phosphorus together; with this treatment, concentrations of manganese were larger than with the other calcium-phosphorus treatments.
133
Figure 9.6 Relationship between concentration of manganese in leaves, stems and roots, and the ratio of Ca:Mn in the nutrient solution
A 0.3 s c Roots
0.2 A
0.1
❑0.5 ppm Mn A 0 10 15 20 250 Mole ratio of Ca:Mn
Ca Ca
0 6 2 ppm 6 30 ppm SE of difference
V 90 2 ppm A 90 30 ppm between two points 134
The concentrations of manganese in the leaves were used to examine the hypothesis proposed in the introduction to this chapter. The equations for concentrations of manganese in leaves of plants grown in solutions containing 4.5 and 8.5 ppm Mn were:
= 0.25905 - 0.05295x - 0.06609x + 0.01978x x (4) Y4.5 1 2 1 2
y8.5 = 0.31577 - 0.05541x1 + 0.08871x2 - 0.02294x1x2 (5) where y is per cent Mn in dry matter; x1 and x2 are concentrations
(mmol per 1) of calcium and phosphorus in the nutrient solution.
The relationships between per cent of manganese in the leaf dry matter plotted against applied phosphorus, are shown in Figures 9.7 and
9.8. The thick lines join the experimental data; the thin lines show the interpolated relationships, defined by equations (4) and (5), between leaf manganese concentration and phosphorus, applied with calcium at various ratios.
Figure 9.7 indicates that in plants grown in 4.5 ppm Mn, the concentration of manganese in the leaves decreased as more phosphorus was given, at all ratios with calcium. However, Figure 9.8 indicates that• in plants grown with nearly twice as much manganese in the nutrient solution, the calcium : phosphorus ratio was very important to control the concentration of manganese in leaves. The relationships indicate that within the range of calcium and phosphorus concentrations applied, if the Ca:P ratio was 1:2, manganese concentration in the leaves increased as more Ca and P were given. Calcium and phosphorus applied in equal molar concentrations (Ca:P = 1:1) affected the concentration of manganese in 135
Figure 9.7 The relationships between concentration of manganese in cotton leaves and phosphorus applied to plants grown in solutions containing 4.5 ppm Mn and two concentrations of Ca in solution: -1 -1 0- 0.15 mmol 1 ; C)- 2.25 mmol 1 . interpolated relationships at four mole ratios of Ca:P in 0.3- solution.
a) 0.2 A Ca:P = 1:2 a) 1-) 1: 1
2:1
a) 1) 0.1 rn S
0 0 0.2 0.4 0.6 0.8 -1 P in solution, mmol 1
136
0.4-
Ca:P = 1:2
1:1
0.3
2:1
r, te t
ma 0.2 dry in
se ne a Mang
0.1
0 4 0 0.2 0.4 0.6 0.8 1 P in solution, mmol 1-1
Figure 9.8 The relationships between concentration of manganese in cotton leaves and phosphorus applied to plants grown in solutions containing 8.5 ppm Mn and two concentrations of calcium: -1 -1 0 - 0.15 mmol 1 ; Q - 2.25 mmol 1 . interpolated relationships at four mole ratios of Ca:P in solution. 137
leaves very little; but if Ca:P was greater than 1, the concentration of manganese decreased with increasing concentrations of phosphorus and calcium.
These results support the hypothesis that the Ca:P ratio in the source of nutrients controls the concentration of manganese in cotton plants, but there were not enough experimental data to define the relationships precisely.
Plant phosphorus. Figure 9.9 shows the concentrations of phosphorus in the plants. With little phosphorus all concentrations were small, and they were affected little by calcium or manganese. But with the larger concentration of phosphorus, manganese increased the concentration of phosphorus in the roots, and lessened it in the leaves; these linear trends were significant at P < 0.05 and < 0.01 for roots and leaves respectively. Increasing manganese in the nutrient solution greatly increased the concentrations of phosphorus in the stems of plants given little calcium, but not in those having much calcium.
Plant calcium. The effects of treatments on the calcium concentrations in the plant are shown in Figure 9.10. Plants given the larger amount of calcium contained much more calcium in all parts. Phosphorus increased the concentration of calcium in leaves and roots but not in stems.
Manganese tended to diminish concentration of calcium in the roots and stems. I Figure - SE of difference between twopoints - Ca 0 0 0 A
9.9 6, Phosphorus in dry matter, The effectofconcentration ofmanganeseinthenutrientsolution roots ofcottongrown withfourcombinationsofcalciumand on concentrationsof phosphorusindrymatterofleaves,stems and phosphorus concentrations 0 0.6 0.2 0.2 P 2ppm; V -Ca90,P2ppm; 0Ca 0.4 o.8- 0.2 0 0.6 0.6 0. 0 0.4 4 0.5 •
Manganese innutrient solution,ppm 4.5
13 8 8.5
6, P 30ppm; ACa 16.5 O c) Roots b) Stems a) Leaves 1 I I 6, P 30ppm
139 4.0
a) Leaves A 3.0 A A
V
2.0
1.0
1J - 0 .1
1.0 b) Stems 0 0.8 4) o.6 0.4 • .1-1 0.2 a a ri 0
1.0
c) Roots 0.8
0.6
0.4 I 0.2
0 0.5 4.5 8.5 16.5 Manganese in nutrient solution, ppm Figure 9.10 The effect of concentration of manganese in the nutrient solution on concentrations of calcium in dry matter of leaves, stems and roots of cotton grown with four combinations of calcium and phosphorus concentrations
0 - Ca 6, P 2 ppm; V - Ca 90, P 2 ppm; D - Ca 6, P 3o ppm; - Ca 90, P 30 ppm; I - SE of difference between two points 140
Discussion
In this experiment the characteristic symptoms of calcium and phosphorus deficiency, and of manganese toxicity, occurred in the cotton; but with some concentrations of the three nutrients large, healthy. plants were also produced. The results establish several new principles involving the interactions of calcium, phosphorus and manganese in the nutrition of cotton. ,
Yield was very closely related to the ratio of calcium to manganese in the nutrient solution, for concentrations of manganese bigger than that which caused maximum yield. This result shows that the effects of too much manganese are countered by extra calcium.
In the plants grown in the largest concentrations of all the three nutrients, a manganese compound, possibly Mn0 , was precipitated 2 in the leaves. Kelley (1914) and Bussler (1958a) reported that symptoms of manganese toxicity developed first in the epidermis near veins and
Bussler (1958b) that manganese dioxide was precipitated in roots and hypocotyl of garden bean (Phaseolus vulgaris); no-one seems to have observed nodules in leaves, although Joham and Amin (1967) commented that brown flecking occurred in the leaves of cotton plants grown with 27 ppm of manganese, 31 ppm of phosphorus and 160 ppm of calcium. Foy, Fleming andArminger (1969) reported that cotton is very tolerant of manganese, but they observed differences between species: G. hirsutum and G. barbadense tolerated 3000 and 2000 ppm Mn respectively in leaves plus stems. However, with 2400 ppm of manganese in leaves of a hirsutum cotton grown in
Tanzania many plants died, and smaller concentrations were harmful
(Le Mare, 1972). 141
Probably this precipitation of manganese is the reason why large concentrations are not always harmful. My pot experiment was not adequate to define the conditions for precipitation but the presence of a large concentration of calcium seemed necessary; with the smaller concentration of calcium few nodules occurred and 'crinkle leaf' was severe; manganese may have been mobile in the plants.
In all parts of the plant the concentration of manganese was a linear function of the concentration of manganese in the nutrient solution up to 8.5 ppm Mn, and in the roots and stems it was linear up to 16.5 ppm. The results indicated that in plants grown with 4.5 ppm manganese the ratio of Ca:P in the nutrient was not important. However, in plants grown in 8.5 ppm Mn the concentration of manganese in leaves increased as more calcium and phosphorus were given with Ca:P < 1, but the concentration of manganese diminished with Ca:P > 1. This result implies that in a soil with much manganese available to crops the nature of the phosphate fertiliser is very important. If the soil supplies the crop with too little calcium, a fertiliser with small Ca:P ratio may be harmful because the concentration of manganese in the crop may become too large. Most important, however, is the observation that the differential effects of calcium and phosphorus, on the manganese in the crop, can cause a maximum in the effect of nutrient manganese if concentrations of calcium and phosphorus are small. These observations seemed very important in relation to the effects of calcium and phosphorus, and especially the effect of triple superphosphate (Ca:P = 0.5), in
Namulonge soil. 142
CHAPTER 10
THE EFFECTS OF CALCIUM, PHOSPHORUS AND MANGANESE ON
SOME ENZYME SYSTEMS CONTROLLING AUXINS IN COTTON
At Namulonge, cotton flowers and bolls often shed prematurely; in the pot experiment described in Chapter 8 leaf fall was correlated with the amount of added phosphorus, and with the concentrations of manganese in the leaves that fell. Concentrations of auxins in cotton affect abscission of leaves, flowers and bolls; because oxidation of auxins is affected by concentrations of manganese in plants, the effects 'of calcium, phosphorus and manganese on some enzyme systems that may control auxin concentrations were investigated in cotton grown in the experiment described in the previous chapter.
Manganic ions oxidise the auxin indole-3-acetic acid (IAA) in plants. Peroxidase catalyses oxidation by hydrogen peroxide of some monohydric and m-dihydric phenols, and of some mono-amino and m-diamino aromatic compounds, to give free radicals which oxidise manganous to manganic ions (Kenten & Mann 1950; 1957). The system of cyclic oxidations is shown in Figure 10.1. Morgan et al (1966) investigated the activity, and inhibition, of IAA-oxidase in plants grown in nutrient solutions containing 1, 3, 9, 27 and 81 ppm manganese. The plants grown with 81 ppm Mn had 'crinkle leaf', the typical symptom of manganese toxicity, and the leaves contained much manganese; IAA-oxidase activity was large and inhibition of the oxidation was small. Morgan et al suggested that 'crinkle leaf' (manganese toxicity) was an expression of
143
Figure 10.1
Oxidation of IAA by a system of cyclic oxidations of phenolic and manganese cofactors
oxidised oxidation ++ H2O form of Mn product phenol of IAA
peroxidase
peroxygenic suitable +++ -Dew H 0 IAA substrates 2 2 phenol 144
auxin deficiency, caused by excessive IAA-oxidase activity. Taylor et al
(1968) investigated inhibition and activity of IAA-oxidase in plants grown with 5.0 and less ppm manganese. Inhibition of IAA-oxidase decreased if the solution concentrations of manganese were less than, or more than, 0.5 ppm; IAA-oxidase activity was greatest in plants grown with concentrations of manganese that were too small for good growth.
Taylor et al commented that their results were the first to show that activity of IAA-oxidase was large in plants with too little, as well as with too much manganese for good growth. Morgan et al and Taylor et al implied that IAA-oxidase was a discreet enzyme but recently Morgan and Fowler (1972) suggested that IAA-oxidase is peroxidase based.
In the cotton plants described in Chapter 9, a wide range of growth characteristics were associated with manganese; and the interaction of manganese with calcium and phosphorus was important. Because the main purpose of experiment was to measure the effects and interactions of calcium, phosphorus and manganese on yield of dry matter and on concentrations of manganese in the plants, enzyme activities in the plants could not be investigated thoroughly; however, biochemical assays were made on leaves taken from the plants of one replicate of the experiment seven weeks after sowing, to seek relationships which would help explain the field results at Namulonge. I am indebted to J. M.
Hill, Biochemistry Department, Rothamsted, for the assays. 145
Experimental .
Activities of peroxidase and IAA-oxidase
Procedure. The treatments applied in the experiment caused large variations among the plants (Table 9.1; Figure 9.3) so similar leaves could not be taken from all plants. Where possible fully expanded leaves were taken; from severely deformed plants the healthiest leaves were removed. Leaves were macerated in a glass blender with 2 ml of
0.1M potassium phosphate, buffered at pH 7, per gram of leaf; the homogenate was centrifuged at 3000 g for 15 minutes, and the residue extracted with distilled water and centrifuged again. The supernatent fractions were combined and water was added to make the volume to 10 ml per gram of leaf.
Two stock solutions, A and B, made to 100 ml with distilled water, contained:
A B
Pyrogallol (AR) 0.5 g 0.5 g 0.15M H2 5 ml 2 2 M/5 KH2PO4, pH 6 10 ml 10 ml
The solutions were equilibrated at 30°C for 10 minutes and then 0.1 ml of extract was added to 5 ml of each solution. After 20 seconds, reactions were stopped by adding 2 ml of 2N H2SO4. Purpurogallin, formed by oxidation of pyrogallol by hydrogen peroxide in the presence of leaf peroxidase, was then extracted with 5 lots of 2 ml of diethyl ether, and the final volume made to 10 ml. Optical density was measured at 430 nm. The molecular extinction coefficient of purpurogallin -1 71 was taken as 2.47 . p14 (Chance,& Maehly, 1955). 146
An attempt was made to measure activity of IAA-oxidase in the leaf extracts by adding IAA to the extracts. Two methods of detecting oxidation were tested: the removal of IAA from the system was followed spectro- photometrically, and oxygen consumption was measured with a polarograph.
Both methods showed that the leaf extracts did not oxidise added IAA.
These results indicated that an inhibitor of IAA oxidation was present in the extracts so inhibition of oxidation was investigated in a model
IAA oxidising system.
Inhibition of IAA oxidation
Procedure. Inhibition of IAA oxidation was measured by adding
extracts of cotton leaves to an IAA oxidising system and determining the weights of leaf needed to halve the oxygen uptake with a standard amount of IAA.
The IAA-oxidising system contained:
0.1 M Sodium acetate at pH 4.5 1 ml 0.01 M IAA 0.1 ml 0.01 M MnC1 0.1 ml 2 0.001 M p-chlorophenol 0.1 ml
p-chlorophenol was used because its oxidation product tends not to
polymerise. The volume was made to 3 ml with distilled water and the
system was then equilibrated at 30°C. 10 lig of horseradish peroxidase
was added; the amount of oxygen consumed was measured with an oxygen
electrode operated in a system sealed from air. Known volumes of
extracts of cotton leaves were added. The weight of fresh leaf required to
halve the rate of oxygen consumption was calculated from the amount of extract 147
taken and the actual amount of oxygen consumed. Inhibition of IAA oxidation was thus inversely related to the weights of fresh leaf, needed to halve the rate of oxygen consumption.
Results
Activity of peroxidase. Figure 10.2 shows estimates of peroxidase activity plotted against the concentration of manganese in the nutrient solution. With each concentration of manganese in the nutrient solution increasing the concentrations of calcium and phosphorus decreased
peroxidase activity. Increasing concentrations of manganese in the nutrient solution tended to lessen peroxidase activity, except when
manganese was given in the solution containing both 6 ppm calcium and
30 ppm phosphorus; then, increasing concentrations of manganese greatly
increased the activity of peroxidase.
One result did not fit the relationships described; in plants grown
in the solution containing 90 ppm calcium, 2 ppm phosphorus and 8.5 ppm
Mn, the activity of peroxidase was much larger than in plants given more
or less manganese. The cause of this exceptional activity is not known.
Inhibition of IAA oxidation. Figure 10.3 shows the relationships between
inhibition of IAA oxidation (expressed as weight of fresh leaf plotted
inversely) and concentration of manganese in the nutrient solutions, for
each combination of calcium and phosphorus in the solutions.
Inhibition was greatest in plants given the smallest concentrations
of calcium (6 ppm) and phosphorus (2 ppm) together; in plants grown in
solutions containing 90 ppm of calcium and 2 ppm of phosphorus, inhibition
was a little less. Changing the manganese concentration in the nutrient Figure 10.2 purpurogallin/mg offres hlea f 0.2 0. 0.1 3 0- Ca 0 -Ca I of manganeseinthenutrientsolution Relationship betweenperoxidaseactivityandconcentration 0. 0 - SEofdifferencebetweentwopoints 5
6, 6, Manganese insolution,ppm P 30ppm; P 2ppm; 4.5 0
148 8.5 V
A -Ca90,P30ppm V -Ca90,P2ppm; 16.5 0 t 1 Peroxidase activity increasing
149
Figure 10.3
Relationship between inhibition of IAA oxidation and concentration of manganese in the nutrient solution
0 - Ca 6, P 2 ppm; V - Ca 90, P 2 ppm; ❑- Ca 6, P 30 ppm; A - Ca 90, P 30 ppm
0
ig
j Inhibition of f, IAA oxidation
lea 1 increasing 1000 f o ht ig We
WE
2000 i 0.5 4.5 8.5 16.5
Manganese in solution, ppm 150
solution affected inhibition very little in plants grown in solutions containing 2 ppm phosphorus with each concentration of calcium; but with 30 ppm phosphorus and 6 ppm calcium, inhibition of IAA oxidation increased as manganese in the solution increased up to 8.5 ppm Mn.
Inhibition of IAA oxidation was least in plants grown in solutions containing 90 ppm of calcium and 30 ppm phosphorus, but with them varying the concentration of manganese caused large effects. Inhibition was greatest with 4.5 and 8.5 ppm manganese; it was small with 0.5 ppm Mn, and very small in plants grown in the solution containing 16.5 ppm Mn.
In Figure 10.4 the estimates of inhibition are plotted against the mole ratio of Ca:Mn in the nutrient solution on a logarithmic -scale.
Inhibition of IAA oxidation in leaves of plants grown in the solution containing 90 ppm Ca, 30 ppm P and 16.5 ppm Mn was exceptionally small.
With all other treatments inhibition diminished as the mole ratio of
Ca:Mn in the solution increased from 0.5 to 250, especially in plants grown in solutions containing 30 ppm of phosphorus. The exceptionally small inhibition occurred in the plants whose oldest leaves contained nodules of a manganese compound.
In Figure 10.5 mean yields of dry matter from the four harvested replicates are plotted against inhibition of IAA oxidation in the leaves of the fifth replicate. Yields of plants grown in solutions containing large concentrations of both calcium and phosphorus increased as inhibition of oxidation increased; thus the smaller yields were associated with leaves that permitted most oxidation in the IAA system, so the poorer yields may have been caused by deficiency of auxin. These results are similar to those of Morgan et al who suggested that plants with 'crinkle
151
Figure 10.4
The relationship between inhibition of IAA oxidation and the ratio of Ca:Mn in the nutrient solution
0 0 0
O
rn
f, a
le Inhibition of 1000 f IAA oxidation A increasing
ht o ! ig We
A
2000 I I i t t . 0.5 1 2 10 20 30 100 200 300 Ratio of Ca:Mn in nutrient solution (log scale)
o - 2 ppm P in solution Y = 58 logiox + 111 ±15.6 ±18.7 (6 d.f.)
- 30 ppm P in solution Y = 271 logiox + 247 ±48.3 ±59.9 (5 d.f.)
152
Inhibition of IAA oxidation increasing ---110••■-•
15 A
A 0 in4
10 4, 4, 0 5
H 5 0
2000 1000 0 Weight of leaf, ig
Figure 10.5
The relationship between yield of dry matter and inhibition of IAA oxidation
0 - 6 ppm Ca, 2 ppm P - 90 ppm Ca, 2 ppm P - 6 ppm Ca, 30 ppm P - 90 ppm Ca 30 ppm P
I - SE of difference 153
leaf', grown with much calcium and phosphorus, contained too little auxin. However, in plants grown with small concentrations of calcium or phosphorus, yields increased as inhibition of IAA oxidation diminished, suggesting that poor yield was associated with excess of auxin; some of these plants had 'crinkle leaf'.
Discussion
The peroxidase assays showed that this enzyme was active in the leaf extracts. Activity was related to the concentrations of calcium and phosphorus in the nutrient solutions but the concentration of manganese in the solution affected peroxidase activity only if the ratio of Ca:P in the nutrient solution was small. 4I- IAA is oxidised by peroxidase if a suitable phenol and Mn ions are present; Fowler and Morgan (1972) showed that in cotton grown in solution containing much phosphorus and calcium the activities of peroxidase and IAA-oxidase were correlated. However, IAA added alone to my leaf extracts was not oxidised, although many extracts contained active peroxidase; this indicated that an inhibitor of IAA oxidation was present and assays showed that inhibition was related to the mole ratio of Ca:Mn in the nutrient solution.
Comparisons of the effects of manganese on inhibition in my plants, with the effects in plants grown by other groups of workers are limited
because none of the groups varied the concentrations of calcium and
phosphorus in their nutrient solutions; all used concentrations of
phosphorus similar to my larger concentration, but they used more calcium 154
than I did. Only Morgan et al used concentrations of manganese that were close to the range of concentrations in my experiment: their concentrations were 1, 3, 9, 27 and 81 ppm Mn. Morgan et al found that in young leaves inhibition of IAA oxidation was large; it varied little if the nutrient concentration of manganese was 27 ppm or less. Inhibition- was much smaller in older leaves; it was the same in plants given 1 and
9 ppm Mn, but decreased with more manganese. In my experiment inhibition decreased as concentrations of manganese increased from 4.5 to 16.5 ppm
Mn but inhibition was smaller with 0.5 than with 4.5 ppm Mn. Taylor et al (1968) grew cotton in solutions containing < 0.005 to 5.0 ppm Mn.
They reported that inhibition of IAA oxidation was greatest in leaf extracts of plants grown in solutions containing 0.5 ppm Mn; inhibition decreased in solutions with more or less manganese. These comparisons indicate that in cotton grown with large concentrations of calcium and phosphorus, the concentration of manganese in the nutrient solution is critical for the inhibition of IAA oxidation. However, the relationships from my experiment indicate that the ratio of Ca:Mn in the nutrient medium is also important to control oxidation of IAA in cotton. Further investigations are needed on the effects of varying the concentration of manganese at various concentrations of calcium, and phosphorus, in the nutrient medium.
The nature of the inhibitor was not investigated but it may have been gossypol (Figure 10.6). Taylor et al (1968) stated that gossypol, an ortho-dihydroxybenzene, inhibits oxidation of IAA; Yamazaki and Piette (1963) showed that some o- and p-dihydroxybenzenes inhibited the Mn++-peroxidase system, but monophenols and m-dihydroxybenzenes were activators. Smith 155
HO OH HO OH
CH CH •
CH3 CH3 CH CH 3 3
Figure 10.6
Gossypol: 8:8'-dicarboxaldehyde-1:11 ,6:6',7:7,- hexahydroxy-5:51 -di-isopropy1-3:31 -dimethyl-2:2'- binaphthalene
(Adams, Morris, Geissman et al, 1938; Heinstein, Smith & Tove, 1962) 156
(1961) reported that roots of cotton (with and without gossypol glands in leaves and seed), especially the feeder roots, contain much gossypol.
He found that gossypol was synthesised in excised root tissue, even.of root tips that did not grow when incubated in a nutrient solution; the root dry matter contained 2.315% of gossypol. Bell (1967) reported that glanded and glandless cotton infected with fungal pathogens produced more gossypol than healthy plants; and in plants treated with cupric and mercuric ions synthesis of gossypol increased. These results of Smith and of Bell indicate that synthesis of gossypol continues when cotton growth is disturbed, so many causes of poor yield may act because gossypol modifies auxin systems.
The results presented in this chapter were not sufficient to indicate the mechanisms of the effects observed but they showed that the interactions of calcium and phosphorus with manganese may be important in relation to IAA oxidation, and that results from cotton plants grown in solutions containing only one concentration each of phosphorus and calcium may be misleading. 157
CHAPTER 11
THE EFFECTS OF THE RATIO OF CALCIUM TO PHOSPHORUS IN
NUTRIENT SOLUTIONS ON COTTON GROWN IN NAMULONGE SOIL
The experiment described in Chapter 9 showed that the ratio of calcium to phosphorus in the nutrient solution applied to cotton controlled the relationship between concentrations of manganese in the nutrient solution and the concentrations in the plants. The experiment described in this chapter measured the effects of solutions supplying phosphorus and calcium at different Ca:P ratios on cotton grown without added manganese in soil from Sendusu, Namulonge.
Experimental
The plants were grown in soil in amber glass pots placed in saucers.
50 ml of deionised water were added to 300 g soil and left overnight.
Next day the moist soil was stirred in a Kenwood mixer for one minute;
220 g of this moist soil were weighed and the rest of the soil was mixed with 300 g of white quartz chips. 150 g of the soil-quartz mixture was put into the pot, followed by 220 g moist soil, and another 150 g of soil-quartz mixture; each was pressed before the next addition was put into the pot. Six delinted cotton seeds (var. BPA68), which had soaked overnight were sown on 19 May 1972 and covered with the remaining soil- quartz mixture. The pots were covered and put into 'Saxcil' controlled o o environment cabinets at 25 C during the 12 hour day, and 20 C during
158
the 'night'. The saucers were filled with water. On the 22 May, when
the seeds had germinated, the pots were uncovered, and on 24 May the
saucers were emptied and refilled with nutrient solutions. The poorest
seedlings in each pot were removed on 25 May, to leave four seedlings o o per pot. From 1 June temperatures were 28 C during the day and 23 C
at night; relative humidities were 66% (day) and 84% (night). The
experiment was harvested on 7 July 1972.
Treatments. The treatments, replicated four times, were all combinations
of none and four concentrations of calcium, with none and three
concentrations of phosphorus:
calcium: 0, 0.1, 0.2, 0.4, 0.8 mmol 1-1
phosphorus: 0, 0.2, 0.4, 0.8 mmol 1-1
The ratio of Ca:P varied from 1:8 to 4:1. All solutions contained:
N K Mg -1 2.8 1.5 0.45 0.2 mmol 1
B Cu Zn Mo -1 2.3 0.016 3.8 0.013 ol 1
The compositions of the 20 nutrient solutions are in Appendix X
Each solution was adjusted to the soil's pH, 5.75, with 0.1N NaOH or
0.1N H2SO4.
Results
Presentation of results. In Figures 11.1 to 11.8 the results are shown
by plotting plant dry matter and chemical composition against concentration 159
of phosphorus in the nutrient solution. Points at each Ca:P ratio are joined so that the effects of phosphorus at different ratios can be compared easily. At each concentration of phosphorus the vertical distances between points show the effects of calcium.
Dry matter. Total dry matter yields at 'harvest' are in Figure 11.1.
Without applied phosphorus yields were small, about 3 g/pot. The smallest concentration of phosphorus, 0.2 ppm, increased the mean yield to 10.4 g; with more phosphorus mean yield was nearly 12 g/pot. Calcium had small effects.
At each concentration of phosphorus most yields increased with increases in the Ca:P ratio; however, with Ca:P = 1:1 yields were a little smaller than those with Ca:P = 1:2 but the differences were not greater than the standard error.
The dry weight of leaves that were shed during the experiment was
0.547 g without added phosphorus; the linear increase of dry weight of shed leaved caused by phosphorus was 0.029 ± 0.0012 g/0.1 mmol P/1 in the solution. Calcium did not affect the weight of shed leaves.
Plant manganese. Concentrations of manganese in four parts of the plants are in Figures 11.2 to 11.5. The concentrations decreased in the order shed leaves > leaves at harvest > roots > stems, but the effects of different Ca:P ratios in the nutrient solutions, on the relationships between concentration of manganese in the crop and concentration of phosphorus in the nutrient solution, were similar in each part of the plant.
The effects of different Ca:P ratios were shown well by the concentrations in the leaves at harvest (Figure 11.2). Without added
Figure 11.1 4-) 0) 0
Total dry matter, 10 15 5 0 - I C)- NoaddedCa of Ca:Pinsolution,ontotaldrymattercottongrown The effectofphosphorusappliedatvariousmoleratios Mole ratiosofCa:PQ-4:1; in Sendususoil - SE ofdifferencebetweentwopoints Ca appliedwithoutP,mmol1 a d • • 0
P innutrientsolution, mmol1 Ca:P =4:1 0.2
160 - 1:2; 2:1 0.4
-1 N7- X -1:4; : a-0.1;b0.2; c -0.4;d0.8 2:1; -1
0.8 d- - p -1:1; No addedCa 1:1 1:8 161
Figure 11.2
The effect of phosphorus applied at various mole ratios of Ca:P in solution, on concentration of manganese in leaves at harvest.
Symbols as Figure 11.1
0.15 t n
ce No added Ca r e 0.10 p r, te
t Ca:P
ma 1:8 dry in
e 1:4 es an
ng 0.05 Ma
1:2 T SE of I difference
1:1 4:1 2: 1
0 0 0.2 0.4 0.8
P in nutrient solution, mmol 1-1 162
calcium, increasing concentrations of added phosphorus increased the concentration of manganese in the leaves from 0.024% to 0.103%. With added calcium, the concentration of manganese in theleaves was less, but with ratios of Ca:P = 1:8 and 1:4, for which there were only one and two treatments respectively, applied phosphorus increased the concentration. With ratios of Ca:P = 1:2 and 1:1 there were treatments at all concentrations of phosphorus; with each of these ratios the smallest concentration of phosphorus increased the concentration of manganese in the leaves, but giving more phosphours diminished the concentrations of manganese and with 0.8 mmol P per 1 the concentrations of manganese were less than in plants grown without added phosphorus or calcium. With ratios of Ca:P = 2:1 and 4:1, the concentrations of manganese diminished with increasing concentrations of applied phosphorus. Without added phosphorus, increasing concentrations of calcium in solution decreased the concentrations of manganese in the leaves, but the effects were small.
In the leaves that were shed. (Figure 11.3), the concentrations of manganese were bigger than those in the green leaves at harvest, but the relationships between concentration and applied phosphorus were similar. However, whereas in the leaves at harvest the concentrations of manganese decreased progressively as the Ca:P-ratio in the nutrient increased, in the shed leaves there were two exceptions: concentration was greater with Ca:P = 1:4 than with Ca:P = 1:8; and it was greater with Ca:P = 4:1 than with Ca:P = 2:1; but the differences were small and not significant.
In the stems (Figures 11.4) and roots (Figure 11.5) the concentrations of manganese were much smaller than in the leaves; the relationships Manganese in dry matter, Figure 11.3 0.15 0.05 0 Symbols asFigure11.1 The effectofphosphorus,appliedatvariousmoleratios of Ca:Pinsolution,onconcentrationmanganese shed leaves b,c • a 0 a 0 r
P innutrientsolution, mmol1-1 0.2 t
163
0.4 i
0.8 1 Ca:P No addedCa 1:4 SE of differenc
164
Figure 11.4
The effect of phosphorus applied at various mole ratios of Ca:P in solution, on the concentration of manganese in cotton stems.
Symbols as Figure 11.1
0.03 a)
ca 4i E 9a No added Ca +Ca:P = 1:8 1:4 94 A 1:2 T SE of 0 2: 1 1:1 difference z i 0 0.2 0.4 0.8 -1 P in nutrient solution, mmol 1
Figure 11.5
The effect of phosphorus, applied at various mole ratios of Ca:P in solution, on concentration of manganese in cotton roots.
Symbols as Figure 11.1
No added Ca r, Ca:P = 1:8, 1:2 te t 1:4 ma ISE of 1:1 !difference dry in 0
Mn 0 0.2 0.4 0.8 165
between the concentrations of manganese and applied phosphorus resembled those in the leaves, but the differences between Ca:P ratios were smaller.
Plant phosphorus. The effects of added phosphorus and calcium on the phosphorus concentrations in the plants are shown in Figures 11.6 and
11.7.. The effects of calcium were small and not significant, so concentrations of phosphorus at the various calcium levels were averaged and lines join the mean values. In each part of the plant, concentration of phosphorus increased linearly as the concentration in the solution increased. In roots and leaves at harvest concentration increased from
0.1 to nearly 0.6% P, and in the stems and shed leaves from 0.05 to 0.2% of phosphorus in the dry matter.
Plant calcium. Figure 11.8 shows the concentrations of calcium in the stems and leaves at harvest, and in the shed leaves.
The concentrations decreased in the order shed leaves > leaves > stems > roots. The data for roots are not shown because the concentrations were small and varied little as the ratio of Ca:P in the nutrient solution changed: the mean concentration of calcium in the roots was 0.47% Ca in the dry matter.
In the stems and leaves at harvest (Figure 11.8) the smallest concentration of added phosphorus decreased the concentrations of calcium at all ratios of Ca:P, but the effect diminished as the ratio increased.
With more added phosphorus, concentrations of calcium increased.
Figure 11.1 showed that phosphorus, but not calcium, increased the amount of dry matter produced. Thus extra calcium was not needed to produce
more dry matter and the decrease in concentration of calcium in the plant 166
Figure 11.6
The effect of applied phosphorus, averaged over added calcium, on the concentration of phosphorus in shed leaves and leaves at harvest
4) SE of 'difference 0.5 Leaves at harvest -
r, tte a m dry
in Shed leaves P 0 r. 0 0.2 0.4 0.8 -1 P in nutrient solution, mmol 1
Figure 11.7
The effect of applied phosphorus, averaged over added calcium on the concentration of phosphorus in roots and stems
TSE of 0 0.5 Ldifference
tter, ma dry in P 0 0 0.2 0.4 0.8 -1 P in nutrient solution, mmol 1 Figure 11.8
Calcium, per cent in dry matter 4 1 3 2 0 The effect ofphosphorus appliedat various moleratios ofCa:P in solution, onthe concentration of calcium in shed leaves, leaves at harvest and stems. a d • b a• as c• b• Cs d • d b • • • 0
P innutrientsolution, mmol1 Ca:P =4:1 Ca:P =4 0.2 167
:
1 0.4 SHED LEAVES
STEMS LEAVES ATHARVEST 2:1
Symbols as Figure 11.1 -1 0.8 +1:8 Ca 1:1 1:4 1:1 1:2 No added 1:2 1:8 No addedCa 1:4 'difference ISE of SE of SE of difference difference 168
shows that the concentrations without added calcium or phosphorus were unnecessarily large. However, when the concentration of added phosphorus was more than 0.2mmol per 1, more calcium was taken up and with the largest concentration of added calcium, yields were smallest with the smallest ratio of Ca:P, indicating calcium limited yield; in these plants the concentration of calcium in the leaves was less than 1.5%.
In the shed leaves, concentrations of calcium in plants without added calcium were not affected by added phosphorus. But with added calcium the concentrations increased as more calcium and phosphorus were Wake given. The concentrations of calcium in the shed leaves 30i:a.s much greater than in the leaves at harvest. These results indicate that in cotton, as in other plants (Kirby & de Kock, 1965), little calcium moves out of leaves.
Nutrient ratios in plants
Calcium : manganese ratio. Ratios of calcium : manganese in- the cotton
plants are in Figures 11.9 and 11.10. The sizes of the ratios decreased
in the order stems > leaves at harvest > shed leaves > roots, but the relationships of the ratios to added phosphorus were similar, so only
those in leaves at harvest (Figure 11.9a) are discussed.
Without added phosphorus or calcium the Ca:Mn ratio in the leaves
was 90. Added calcium, without added phosphorus, increased the ratio to
190; applied phosphorus, without added calcium, diminished the ratio to 10.
With 0.2 mmol per 1 of added phosphorus, the Ca:Mn ratio in the
leaves was about 40 without added calcium, and when the Ca:P ratios in
the nutrients were 1:2 and 1:1. But with more calcium, the Ca:Mn ratio 169
Figure 11.9
The effect of phosphorus applied at various mole ratios of Ca:P in solution on the ratio of Ca:Mn in a) leaves at harvest and b) shed leaves
Symbols as Figure 11.1
200 ,a) Leaves at harvest • c •
150 b • Ca:P 4:1 ■ 1:1 2:1
a c Mn :
100 f Ca o io t Ra
50 1:2
1:4 + 1:8 No added Ca 0 0 0.2 0.4 0.8
b) Shed leaves d • 100 a,c • Ca:P = 4:1 b 2:1 ■ 1:1
Mn :
Ca 1:2
f 50
o 1:4, 1:8
io No added Ca t Ra
0 O 0.2 0.4 0.8 -1 P in nutrient solution, mmol 1
170
Figure 11.10
The effect of phosphorus applied at various mole ratios of Ca:P in solution on the ratio of Ca:Mn in a) stems and b) roots
Symbols as Figure 11.1 300 a) Stems d s
C •
250
200
Mn :
f ea 150 o io t Ra
100
50
o
0 0.2 0.4 0.8
50 Mn :
f Ca 1:4
o 1:2, 1:8 No added Ca
tio 0 Ra 0 0.2 0.4 0.8 -1 P in nutrient solution, mmol 1 171
in the leaves increased to 85 and 130 with Ca:P = 2:1 and 4:1 in the solution.
The Ca:Mn ratios in leaves of plants grown with solutions containing
Ca:P = 1:2 and 1:1 were especially important. These solutions diminished the Ca:Mn ratio in the leaves similarly, from 90 to about 40, when they supplied 0.2 mmol P per 1, but when more phosphorus and calcium were given the effects of the two solutions were very different: more phosphorus and calcium at ratio 1:2 increased the Ca:Mn ratio in the leaves very little, but the solution with Ca:P = 1:1 increased the Ca:Mn ratio in the leaves to 140.
Phosphorus : manganese ratios. The ratios of P:Mn in the three plant parts at harvest are in Figures 11.11 and 11.12. The ratios for shed leaves were small and varied little.
Without applied phosphorus or calcium the P:Mn ratio in leaves was 5; in stems and roots it was a little bigger. Added calcium, without added phosphorus, increased the ratio of P:Mn in leaves and roots very little, but it increased the ratio in the stems to 18. Phosphorus given without calcium increased the ratio in the roots to 20, but had no effect in stems and leaves. When phosphorus was given with calcium, the P:Mn ratio in all parts of the plant increased as the Ca:P ratio in the nutrient solution increased. With each Ca:P ratio in the solution, the P:Mn ratio in the plants increased as more phosphorus and calcium were given. In leaves and roots the largest P:Mn ratio was nearly 40; in stems it was 75.
Soil pH. Three times during the experiment the pH of the soil in each pot was measured by putting a glass electrode into the layer of soil in Figure 11.11
Ratio of P:Mn Ratio ofP: Mn 10 10 40 20 20 30 30 40 0 0 Symbols asFigure11.1 The effectofphosphorusappliedatvariousmoleratiosCa:P in solutionontheratioofP:Mna)leavesatharvestand b) roots 0 0
Ca:P =4:1 P innutrientsolution, mmol1 0.2 0.2 a) Leavesatharvest b) Roots
172 0.4 0.4 2:1
-1 0.8 0.8 ÷ 1:8 1:1 1:4 1:4 1:2, 1:8 No addedCa No addedCa 1:2 1:1 173
Figure 11.12
The effect of phosphorus applied at various mole ratios of Ca:P in solution on the ratio of P:Mn in stems
Symbols as in Figure 11.1
80
70
60
50
rl 40 0 •,-10 4) o g 30
20
10
0 , , I. 0 0.2 0.4 0.8 -1 P in nutrient solution, mmol 1 174
the middle of the pot. Table 11.1 shows the mean values for each treatment on 23 June; they resumbled pH values at earlier dates.
The pH of the soil in 0.01M CaC1 at the beginning of the experiment 2 was 5.75. The mean pH of soil in the pots on 23 June was 5.63. The largest difference between individual treatments was 0.27; the mean effects of treatments were very small and the ratio of Ca:P did not affect pH.
Discussion
This experiment showed that, in plants grown for seven weeks in
Sendusu soil, the effect of added phosphorus on the concentration of manganese in the plants depended upon the ratio of Ca:P in the nutrient solution. With ratios less than 1:2, the concentration of manganese in the leaves increased with all concentrations of added phosphorus. With
Ca:P ratios of 1:2 and 1:1; which are those of monocalcium phosphate and dicalcium phosphate respectively, the relationships had maxima with 0.2 mmol P per litre; with more phosphorus at each ratio, and therefore with more calcium, the effect on concentration of manganese in the plants diminished.
The ratios of Ca:Mn in the leaves were also controlled by the Ca:P ratio in the nutrient solution. If more than 0.2 mmol P per 1 was given the critical Ca:P ratio in solution was 1:2. With this ratio of Ca:P, the Ca:Mn ratio in the leaves remained small; but with Ca:P = 1:1, the
Ca:Mn ratio in leaves increased greatly as more phosphorus and calcium were given.
175
TABLE 11.1
pH of soil in pots on 23 June 1972
Added Added calcium mmol 1-1 phosphorus mmol 1 -1 0 0.1 0.2 0.4 0.8 Mean
±0.091 ±0.041
0 5.76 5.65 5.68 5.59 5.75 5.68 0.2 5.52 5.69 5.55 5.59 5.65 5.6o 0.4 5.58 5.49 5.71 5.58 5.68 5.6o 0.8 5.7o 5.66 5.65 5.59 5.61 5.64
±0.046
Mean 5.64 5.62 5.65 5.58 5.67 5.63 176
The effects described occurred within a narrow range of pH, so it is likely that amounts of manganese available to.the plants from the soil varied little between treatments; therefore the effects observed were caused by changes associated with processes affecting uptake of the nutrients by the plant.
They were probably not affected by chemical processes in the soil releasing more manganese, caused by the treatments applied. The results resemble those of the previous experiment; and they support the arguments made in Chapter 9 on the opposing effects of calcium and phosphorus, applied at constant ratio, on manganese uptake by plants.
With ratios of Ca:P = 1:2 and 1:1, the shapes of the relationships between added phosphorus and concentration of manganese in the plants are the inverse of the relationships between triple superphosphate applied at Namulonge and the yields of seed cotton and dry matter, so they are consistent with the possibility that excess manganese decreased yields of crops given small dressings of triple superphosphate. With the ratio of Ca:P = 1:2 in the nutrient solution the relationship between added phosphorus and the ratio of Ca:Mn in leaves was similar to the yield response at Namulonge. The ratio of Ca:Mn may be more important than the concentration of manganese in leaves to explain unusual effects of triple superphosphate on field crops. The results of this experiment, in relation to the field results at Namulonge, and their implications for tropical agriculture, are discussed in Chapter 12. 177
CHAPTER 12
GENERAL DISCUSSION
IMPLICATIONS OF THE WORK FOR TROPICAL AGRICULTURE
The work provided new information on two topics that are important to agriculture: firstly, on how monocalcium phosphate increases manganese in crops; secondly, on the effects of manganese, and its interactions with calcium and phosphorus, in cotton. The experiments indicated the cause of the unusual response to triple superphosphate at
Namulonge and cast doubts on the commonly held theory that sigmoid response to phosphate is due to fixation of fertiliser phosphate into insoluble forms. Now field experiments in Uganda are needed to test the results of the work at Rothamsted.
Review of experiments at Rothamsted
Behaviour of monocalcium phosphate (MCP) in soil. Laboratory experiments at Rothamsted confirmed the findings of TVA workers that small amounts of MCP, equivalent to granules of triple superphosphate, hydrolyse in soil and that a residue of dicalcium phosphate remains at the site where the MCP was placed. The ryegrass experiments confirmed the results of
Larsen (1964) and of Truong et al (1971) that MCP increases the concentration of manganese in crops. But the experiments showed that the extra manganese was not taken up because acid triple-point solution (TPS), formed during hydrolysis of MCP, dissolved manganese from soil constituents, as those 178
workers suggested: dicalcium phosphate left as a residue when MCP hydrolysed, and a prepared dicalcium phosphate, also increased the concentration of manganese in the plants; the effects of these two salts on plant manganese were as big as the effect of TPS.
Manganese nutrition of cotton. The experiments with ryegrass testing
MCP and its derivatives indicated that phosphate increased concentration of manganese in plants independently of reactions between fertiliser and soil. Cotton grown in nutrient solutions (without soil) (Chapter 9) confirmed this and showed that the ratio of Ca:P in the nutrients available to the plants had an important effect in controlling the concentration of manganese in the tissue produced: calcium decreased, and phosphorus increased the concentration of manganese. Truong et al obtained similar results. Interpolated curves from results of the experiment at Rothamsted indicated that if the ratio of Ca:P available in the nutrient medium was too small, the concentration of manganese in the plants increased as more
Ca and P were given in the same ratio; but calcium and phosphorus diminished the concentration of manganese if the Ca:P ratio was large.
Yield of dry matter was well related to the mole ratio of Ca:Mn in the nutrient solution, at each concentration of phosphorus. Furthermore, the Ca:Mn ratio in the solution seemed to control an inhibitor of IAA oxidation, perhaps gossypol. Other workers who investigated the behaviour of IAA in cotton grew their plants in solutions containing much calcium and phosphorus. In my experiment, dry matter yield of plants grown in solutions containing large concentrations of calcium and phosphorus decreased
as inhibition of IAA oxidation decreased, so it is possible that IAA 179
oxidation was excessive. Inhibition of IAA oxidation was very small in plants grown in solutions containing much manganese; this result supported the suggestion by Morgan et al (1966) that manganese toxicity symptoms in cotton were expresions of auxin deficiency, associated with excessive IAA oxidation. However, in plants grown in a solution with very little manganese inhibition of IAA oxidation was also small. Taylor et al
(1968) found that inhibition of IAA oxidation decreased in plants given too little, as well as too much manganese.
Unlike the yields from plants grown with much calcium and phosphorus, dry matter yields of plants grown with little calcium or phosphorus increased as inhibition of IAA oxidation decreased, indicating that an excess of auxin was associated with small yields. The work of Bell (1967) and Smith (1961) showed that gossypol, which is an inhibitor of IAA oxidation, is synthesised as much or more by poorly growing cotton as by healthy cotton, so it is possible that in plants given little calcium or phosphorus inhibition of IAA oxidation was a result of poor growth rather than a cause of it.
Deposits containing manganese occurred in the old leaves of plants grown in solutions containing large concentrations of phosphorus, calcium and manganese together; much calcium seemed necessary for•manganese to precipitate. Young leaves on the same plants had symptoms of excess manganese. Foy et al (1969) grew cotton with much Calcium and found that the plants tolerated up to 3000 ppm of manganese in the dry matter. In
Tanzania cotton with 'crinkle leaf', the symptoms of manganese toxicity, contained less manganese; liming the soil eliminated crinkle leaf
(Le Mare, 1972). It seems likely that cotton is not harmed if excess 180
manganese is precipitated but small concentrations may be harmful if manganese remains mobile in the plant.
Effects of calcium and phosphorus, and their interactions with manganese, in cotton grown in Sendusu soil
Cotton grown at Rothamsted in soil from Sendusu, Namulonge, with solutions containing various mole ratios of Ca:P, confirmed that this ratio controlled both the concentrations of manganese in the plants, and the ratios of Ca:Mn they contained. In plants grown in soil with added solutions containing calcium and phosphorus in the mole ratios of
Ca:P = 1:2, 1:1 and 2:1, the relationships between added phosphorus and the Ca:Mn ratio in the leaves were similar to the unusual yield response at Namulonge. These relationships and the yield responses are shown together in Figure 12.1. In Figure 12.1a the mean yields of two crops of seed cotton plus plant dry matter in each of the experiments C2 and C4
(Le Mare, 1968b) are plotted against the amount of triple superphosphate that was applied before sowing in 1961. Only yields with 0 to 500 kg/ha of triple superphosphate are shown; the form of the complete response with dressings up to 2000 kg/ha of the fertiliser is in Figure 1.1.
Figure 12.1b shows the ratio of Ca:Mn in leaves of plants grown in
Sendusu soil at Rothamsted, plotted against the concentration of phosphorus in the solution added to the soil. The range of soil pH was 5.5 to 5.8; this is not likely to have affected the amounts of manganese available to the plants.
The similarity of the yield response in the field experiments with
the relationships from plants in the pot experiment strongly suggests that
181 4500
Experiment C4 .1 3) 4000
0
3500 Experiment C2
3000
0
0 0 250o 0 0
2000
o 62.5 125 250 500 Triple superphosphate, kg/ha
Figure 12.1a
Relationships between yield of cotton and applied triple superphosphate in two field experiments at Namulonge, 1961-63
150 Ca:P = 2:1 1:1
0 as 100
ri X S 0 c11 50 1:2 0
S
0
0 0.2 0.4 0.8 Phosphorus in nutrient solution, mmo1/1
Figure 12.1b
Relationships between Ca:Mn ratios in leaves of cotton grown in Sendusu soil at Rothamsted, and phosphorus applied at three mole ratios of Ca:P in solution
Ca:P ratios 1F - 2:1; n - 1:1; AL- 1:2 182
the yields at Namulonge were diminished because small dressings of triple superphosphate, which has a mole ratio of Ca:P = 1:2, decreased the ratio of Ca:Mn in the crop.
Cheng and Ouellette (1971a) reported that the ratio of concentrations
of Ca:Mn in leaves of potato plants is normally about 350 and that plants
may have symptoms of manganese toxicity if the ratio is 70. Suitable ratios in cotton are not known but in the leaves of plants grown in
Sendusu soil at Rothamsted, the concentration ratio of Ca:Mn was not
greater than 50 if the mole ratio of Ca:P in the nutrient medium was 1:2; the ratio of Ca:Mn increased greatly if more calcium was given, so 50 may
be too small for good yields. However, the Ca:Mn ratio in plants may be
misleading if it includes manganese that is precipitated; then it may be necessary to determine amounts of mobile manganese instead of total
manganese.
The ratio of concentrations of P:Mn in all parts of the plants
increased as the mole ratio of Ca:P in the nutrient solution increased
(Figures 11.11, 11.12). Increasing amounts of added phosphorus increased the P:Mn ratio in all parts of the plants if calcium was given in the nutrient solution; but without added calcium extra phosphorus did not
increase the concentration ratio of P:Mn in leaves and stems. These results indicate that phosphorus and manganese were taken up together
unless calcium was given. It seems possible that crop failures associated with large concentrations of phosphorus in plants, sometimes attributed
to phosphorus toxicity (Warren & Benzian, 1959; Rossiter, 1951; 1955),
may be caused by excess of a harmful cation taken up together with
phosphorus. Phosphate fertilisers with large Ca:P ratios might prevent
the crop failures. 183
Results of experiments at Rothamsted related to field
results at Namulonge
The results of the experiments at Rothamsted indicated the causes of several hitherto unexplained effects at Namulonge. The field experiment results at Namulonge have been quoted by others to support ill-defined concepts of phosphate fixation, so these are discussed now in relation to the new experimental results.
Phosphate fixation and sigmoid response curves. Willson (1972) stated that phosphate is quickly 'fixed' in acid soils; and that Lindsay and
Stephenson (1969a) had described the mechanism of fixation and shown that large dressings of fertiliser were necessary before appreciable quantities of phosphate were available to plants. Willson commented that the Namulonge experiments confirmed that fixation occurred in acid soils in Uganda. Kellogg and Orvedal (1969) made a general statement that the response curve with phosphate fertiliser is commonly sigmoidal rather than parabolic, and quoted the Namulonge experiments to support their statement. They added that many tropical soils tend to fix added phosphate in unavailable forms which, they said, gives added emphasis to precise placement in relation to the seed or plant.
In my second paper (Le Mare, 1968b) I reported (p 277) estimates of soluble phosphate, measured as pH2PO4 , in soils sampled, two years after triple superphosphate was applied. The relationship between soluble phosphate in the soil and applied triple superphosphate was similar to the relationship between yield and added fertiliser. However, I showed that soluble phosphate did not account for the effect of triple superphosphate 184
on yield. The evidence for this was that yield of plant dry matter was linearly related to pH PO on plots without applied fertiliser and on 2 4 those with all dressings except the two smallest amounts of fertiliser,
62.5 and 125 kg/ha. I considered this result was very important: it indicated that diminished yields with the smallest dressings were not caused by applied triple superphosphate decreasing the solubility of native phosphate in the soil. I suggested that with small dressings of triple superphosphate crops were not able to absorb enough phosphate or_ that adequate phosphate was not transported in the plants, perhaps because an excess of another element interferred. The Rothamsted experiments supported this speculation and showed that manganese was the element most likely to have caused the field results.
The reason why concentrations of soluble soil phosphate were less with small dressings of triple superphosphate than with none, is not known. However, the standard error (7 d.f., not previously reported) indicated that it may have been a chance result.
My third paper (Le Mare, 1968c) reported that two years after triple superphosphate was applied, the solubility of the phosphate added to the soil was similar to that of the native soil phosphate. The comparisons made then were between plots with and without added phosphate; there were no comparisons within plots before and after applying phosphate. Hence the experiment did not provide conclusive information on the effect of triple superphosphate on the native soil phosphate, as
Willson implies by linking the results at Namulonge with the work of
Lindsay and Stephenson. There is no chemical evidence that addition of a phosphate salt diminishes the solubility of soil phosphate; nor is there
any chemical theory to support such a reaction. 185
Kellogg and Orvedal, like Willson, also ignored my reference to movement of phosphate in plants, and my suggestion that phosphate was not the only nutrient element associated with diminished yield; they assumed the effect was wholly related to reactions of phosphate in the soil.
Other workers (Steenbjerg, 1954; Burns, Bouldin & Black, 1963;
Kafkafi & Putter, 1965) have suggested that placement of fertiliser granules in soil may cause sigmoidal response curves, but few experimental results support the idea. The cotton experiments at
Rothamsted showed a real cause of a sigmoidal response: if two independent factors have opposing effects, their combined effect when applied together depends upon their ratio and the amounts of them applied. The theoretical basis for this concept is simple; it was described in Chapter 9, and the effect was demonstrated by applying calcium and phosphorus at various ratios to cotton growing in Sendusu soil (Chapter 11). It is possible that sigmoidal responses elsewhere were also caused by the opposing effects of two ions applied together in a fertiliser, rather than by t fertiliser placement or 'fixation' wctions in soil. The results from experiments on the Namulonge soils show how important it can be to consider the effects of both ions, and their ratio, in a fertiliser.
Phosphorus concentration in plants. In the field experiments the concentrations of phosphorus in leaves of cotton were less with small dressings of triple superphosphate than with none. In cotton grown with adequate phosphorus, in nutrient solutions without soil at Rothamsted, phosphorus concentration decreased in leaves and increased in roots as concentrations of manganese in the nutrient solution increased. This 186
result suggests that in the field excess manganese in the plant, caused
by small dressings of triple superphosphate, hindered movement of
phosphate to the leaves. The suggestion made in Chapter 7 that iron,
oxidised by manganic ions, may immobilise phosphorus in roots was not
demonstrated.
The effect of calcium in triple superphosphate. In my second paper on
the field experiments I discussed the effect of the calcium applied in
triple superphosphate. A multiple regression of yield on concentrations
of phosphorus and calcium in leaves indicated that the effect of calcium
was small; from the regression equation, and from the relationship
between phosphorus in leaves and yield of plant dry matter, I concluded
that the major effects of triple superphosphate were caused by phosphorus•
and I disregarded calcium. The experiments at Rothamsted showed this
was a mistake: the results indicated that although the effect of calcium
was small, it was very important; and that triple superphosphate did not
contain enough calcium for cotton grown in Sendusu soil. Because calcium
and phosphorus were applied together in the field, and the effects of
phosphate on yield were greater than the effects of calcium, the results
of regression analyses were misleading. Large dressings, 500 to 2000
kg/ha of triple superphosphate, linearly increased-yield of, plant dry
' matter: results of experiments at Rothamsted indicate that the cause of this
continuing effect of large dressings of the fertiliser was more likely to
be associated with calcium than with phosphorus. 187
Soil physical conditions. In reviewing work on availability of manganese in soils Cheng and Ouellette (1971b) emphasised that any factor that affects the oxidation-reduction potential in soil and plants alters manganese relationships. Some physical conditions of soils at Namulonge may affect the amount of manganese in the soil solution.
Waterlogging often increases the amount of manganese available to plants. Grant, Roschnik, Hughes and Nduku (1966) found that temporary waterlogging increased the concentrations of soluble manganese in red clays and clay loams in central Africa, more manganese being released if the soil was air dried before being waterlogged. Temperature alters the amounts of manganese taken up by crops; for example, concentration of o o manganese in oats was greater in a crop grown at 38 C than at 21 C (Cheng,
Bourget & Ouellette, 1971); soy beans grown in manganese-deficient soil o o at 27 C contained more than plants grown at 15 C (Mederski & Willson,
1955). At Namulonge in 1961-63 soils were rarely dry because those years were very wet. In all pot experiments at Rothamsted the soils were kept . wet to simulate field conditions, and to ensure that the crops did not 0 0 lack water; for cotton, temperatures were maintained at 23 C and 28 C during the night and day respectively. In these conditions manganese is likely to dissolve and remain in solution.
Because manganese is more available to crops in wet soils, measures to conserve water may cause harmful concentrations of soluble manganese in the soil during wet years. At Namulonge crops were grown on ridges until recently. To conserve water and prevent soil erosion the ridges were 'tied' at five-foot intervals by soil scraped from the furrow. At
Ukiriguru, Tanzania, ridge-tying increased crop yields in mostyears; at 188
Namulonge tying increased yield very little, and sometimes diminished it, although the rainfall in the cotton season was similar to that at
Ukiriguru (Le Mare, 1954). It is possible that ridge-tying increased soluble manganese in Namulonge soils, and that this was harmful to crops, especially when triple superphosphate was used. Recently, most fields at
Namulonge were limed, and crops are not now grown on ridges because other soil conservation methods are used, so crops are less likely to be damaged by manganese in future.'
Effects of other fertilisers. Fertilisers other than those supplying phosphorus and calcium affect the concentration of manganese in crops.
Potassium chloride and other salts increase the concentrations of manganese in crops grown in acid soils (Jackson, Westerman and Moore,
1966; Le Mare, 1972). Westerman, Jackson and Moore (1971) considered the standard oxidation potentials of halides and hydrous oxides of manganese and suggested that, theoretically, Cl from potassium chloride, could reduce manganese oxides in acid soils, but they did not verify this; with other salts, the effects were related to the pH changes they caused. In Tanzania, potassium chloride applied to acid soils was harmful because it increased the concentration of manganese in cotton, but after the soils were limed, potassium chloride increased yields.
The effect of lime was ascribed to raising soil pH but extra calcium may have decreased the concentration of manganese in the plants (Le Mare,
1972).
Beer, Durst, GrUndler et al (1971) reported that anions increased the amounts of manganese taken up from aqueous solutions of manganese salts by oats, maize, peas and beans, in the order NO > P0 4> Cl > CO > 3 3 50 Calcium inhibited uptake of manganese and its translocation from 4' 189
root to shoot. These results suggest that nitrate and phosphate fertilisers should be used especially carefully in manganese-rich soils, and that ample calcium must be present with the anions. Sulphate had least effect on the amount of manganese taken up so single superphosphate may be less harmful, per unit of phosphorus applied, than triple superphosphate.
Because a fertiliser with a large ratio of Ca:P seems necessary in manganese-rich soil, dicalcium phosphate is likely to be a better fertiliser than triple superphosphate, despite its small solubility in water, because its Ca:P ratio is twice that of triple superphosphate.
Phosphates with larger Ca:P ratios, such as tricalcium phosphate (Ca:P
1.5:1) or apatites (rock phosphates) (Ca:P = 1.67:1) have the disadvantage of very small solubility in water. However, some of the soft rock phosphates increased yields in tropical soils, and released phosphate over a long period. Scaife (1971) found that Minjungu phosphate, a collophane (poorly crystaline hydroxyapatite) from northern Tanzania, was a satisfactory long term source of phosphorus for cotton and cassava in Tanzania. An apatite containing iron from Tororo, Uganda, did not increase yields in Uganda (Manning & ap Griffith, 1949) or Tanzania
(Le Mare, 1959).
Differences in the effects of manganese on various crops and their implications for fertilising at Namulonge
Three crops were grown in the experiments at Namulonge - cotton, beans (Phaseolus vulgaris) and maize (Zea mays). The responses of cotton and beans to triple superphosphate were similar; the unusual effect occurred in yield maize grain sampled before harvest, but not in harvested yield. 190
Many workers have investigated the manganese nutrition of legumes.
The experiments by Truong et al have been discussed in earlier chapters; the experiments with cotton at Rothamsted supported their results.
Ouellette and Dessmeaux (1958) investigated tolerance of alfalfa to manganese. The more tolerant plants contained more water-soluble calcium, and increasing calcium in the nutrient medium diminished manganese toxicity; calcium in the plant seemed to increase precipitation of manganese in roots. Robson and Loneragan (1970) found that increasing the calcium concentration in a nutrient solution, or decreasing the pH' of the solution, decreased concentration of manganese in Medicago species. They suggested that calcium acted by decreasing manganese sorption and that diminishing pH decreased sorption of manganese and its transport to the tops. Vose and Jones (1963) found that manganese decreased the numbers of nodules, and their volume, on Trifolium repens; calcium countered these effects.
Wallace, Frolich and Lunt (1966) suggested that plants may require calcium to prevent injury from excess of other cations, especially of magnesium, copper, iron, manganese and zinc. They reported that maize (and tobacco) grew well in a solution containing less than 3 ppm Ca if its ratio with other ions was carefully adjusted; the concentration of manganese in their solution was less than 0.5 ppm Mn.
Maize normally contains much less calcium, and the ratio of concentrations of Ca:P is much less, than in cotton and legumes. Data reported by Fried and Broeshart (1967) indicate that the ratio of Ca:P in Phaseolus beans is about 6; in maize it is about 0.8. Data for cotton indicate the ratio of Ca:P is about 5 (Berger, 1969) and 6 (Cooke, 1972).
Because fertilisers with small Ca:P ratio increase uptake of manganese by 191
plants, it seems especially important on manganese-rich soils to use fertilisers with a large Ca:P ratio for crops which normally require calcium and phosphorus in large Ca:P ratios. Maize requires little calcium, so triple superphosphate may be a satisfactory fertiliser in more soils for this crop than for cotton, and for legumes which require much calcium for nitrogen-fixing nodules. The relatively small amount of calcium normally needed by maize may be the reason why triple superphosphate was not as unsatisfactory for this crop as it was for cotton and beans at
Namulonge.
Recently Jones (1972) discussed the effects of fertilisers at
Namulonge. He-showed that ground limestone increased yields of cotton and beans but he did not report effects of limestone on maize yields; in an earlier experiment lime did not increase the yield of maize to which it was applied but it increased yield of a later cotton crop (Le Mare,
1962). Before lime was applied in the experiments reported by Jones, soil pH was 5.5 to 5.8; one year later, lime had increased pH to the range 6.2 to 6.7. The pH range before liming shows that the soils were not severely acid; the demonstration at Rothamsted that calcium is important as a nutrient for cotton grown in the soils of pH 5.5 to 5.8 from Namulonge suggests that the lime improved yields of cotton and beans at Namulonge because it supplied calcium, rather than by changing the pH of the soil. Jones showed that without lime a compound fertiliser containing nitrogen, phosphorus, sulphur, potassium and calcium, with
Ca:P in the weight ratio 3.6:1 (mole ratio 2.8:1), increased yields of all crops. It seems likely that the large Ca:P ratio in the compound fertiliser was important for cotton and beans and that with it the crops 192
were able to use the other nutrient3supplied,to produce yields that were much larger than were obtained earlier when the fertiliser applied contained little calcium.
The source of manganese in Namulonge soil
The results of pot experiments at Rothamsted indicated that the unusual response at Namulonge occurred because the ratio of Ca:P in triple superphosphate was too small, and allowed excessive amounts of mobile manganese to accumulate in the plants; this may have disturbed the auxin balance so that although yield of young plants was not diminished, later growth and boll production were decreased. Field experiments are needed, to test these ideas; they should be carefully sited to test the hypothesis that the effects at Namulonge occurred because the field experiments in 1960-62 were affected by manganese from the quartz breccia ridge that is the southern boundary of Namulonge
Research Station.
King (1942) and Pallister (1959) reported that the quartz breccia ridges in Buganda are rich in manganese, so the soils below them probably contain much manganese. The ridges were formed by tear-faults
(Hepworth, 1956); where they were cut by rivers flowing across them, the ridges eroded and valleys with slow-flowing streams or swamps developed along the lines of the faults. The ridges and associated valleys are shown in the map in Appendix II. The Nasirye river, which is the northern boundary of Namulonge Research Station, flows in the longest valley that formed along the line of a tear-fault. 193
Many soils in Buganda contain much manganese but only in relatively small areas (where the soil is known as 'lunyu') is available manganese sufficient to cause toxicity symptoms or very poor growth. These soils occur low on the pediment slope near the swamp, and occasionally on ridges. The results of my work show that manganese is potentially harmful in soils below the breccia ridges, especially if they are fertilised with triple superphosphate.
The comparison of soils from Sendusu and Nalumuli in the early experiments at Rothamsted provided information that may help us understand the soils at Namulonge. Sendusu soil was chosen because it came from an area that had not been cultivated for over 20 years. Nalumuli was the area chosen to take samples of a cultivated soil because in 1969 it was the only site where land had been cultivated for a long period without added manures or fertilisers; if there had been a similar site nearer
Sendusu samples would have been taken from there, because the. field experiments in 1960-62 were at or near Sendusu. The sites from where samples were taken, and of the field experiments are shown in Figure 2.2.
The ryegrass experiments and the first cotton experiment at Rothamsted showed that cultivated soil from Nalumuli supplied less manganese to crops than soil from Sendusu. Soil from Sendusu came from below the main quartz breccia ridge at Namulonge, and the field experiments were on soils below this ridge. Nalumuli ridge lies between two tear-faults: the northern fault line is the Nasirye valley; the southern one is in the Katabusolo valley at the head of which is the dam below the laboratory buildings.
Thus, although Nalumuli is very close to two fault liAps which may be rich in manganese, it is above them so soils at Nalumuli are probably losing 194-
manganese by leaching, whereas soils at Sendusu will tend to accumulate manganese from the ridge above them. Thus the topographical positions of the sample sites, in relation to the breccia ridge and other tear- faults, may explain why ryegrass took up more manganese, and why the effect of added phosphorus on manganese in the plants was greater, in
Sendusu than in Nalumuli soil.
The map in Appendix II shows that there are many breccia ridges to the north and few to the south of Namulonge. The agricultural experiment station at Kawanda, about 16 km south-west of Namulonge, is close to a small ridge. It seems possible that triple superphosphate decreased cotton yields at Kawanda (Stephens, 1966) because the soils below the breccia ridge are manganese-rich.
The importance of manganese in Buganda soils, especially in relation to crop response to fertilisers, should be investigated throughout the area of breccia ridges. Field experiments should be sited near to and remote from the ridges. Chemical analyses of the soils and crops are needed to show how manganese affects yields. A fuller knowledge of soil manganese and its effects on crops may help to explain variation of crop yields between sites that have apparently similar soils and weather. 195
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APPENDIX I
Observations on the phosphate potential of some tropical soils. 7th Int. Congr. Soil Sci., Madison, Wisc., USA, 1960, 3, 600-603.
Ib Cotton Research Memoir, No. 70
Experiments on the effects of phosphate applied to a Buganda soil. J. agric. Sci., Camb. (1968), 70, 265-270; 271-279; 281-285.
I Pot experiments on the response curve
II Field experiments on the response curve to triple superphosphate
III A chemical study of the soil phosphate, the fate of fertiliser phosphate and the relationship with iron and aluminium 7TH INTERN. CONGRESS OF SOIL SCIENCE, MADISON, WISC., U.S.A., 1960 I V.75 Reprint Transactions Vol. III
OBSERVATIONS ON THE PHOSPHATE POTENTIAL OF SOME TROPICAL SOILS by P. H. LE MARE Cotton Research Station, Namulonge, Uganda
In studying the phosphate status of soils a chemical estimate of plant available phosphate giving a good correlation with plant uptake of phos- phorus is necessary. Schofield (1) has suggested using the chemical potential of monocalcium phosphate, expressed as -i-pCa p1-121304 as an index of the availability of soil phosphate to plants; Aslyng (2) has described the method for the determination of the 'phosphate potential' and has presented data for temperate soils which show that the phosphate potential can be of great value for studying the nature of soil phosphate. The present paper summariz- es data obtained in preliminary studies of the phosphate potential of some tropical soils in East Africa.
PHOSPHATE POTENTIAL AND PHOSPHORUS UPTAKE BY COTTON AT NAMULONGE. The relationship between phosphate potential and phosphorus content of cotton plants has been examined using samples taken from fifteen fields on the estate of the Cotton Research Station, Namulonge, on which the soil is a red clay loam. The treatment of the fields from which the samples were taken had differed only in their cropping during the ten years 'since the first land was cleared when the Research Station was opened. None of the fields had received manure, fertilizer or lime at any time. The samples were from fields as varied as land recently cleared from natural vegetation (largely Elephant grass, Pennisetum purpureum) to land which had been under arable cultivation for periods of up to ten years, often carrying two crops per year. Samples of surface soil (0-8 inches) were taken from a plot 120 square yards in area within each field. The range of values for ipCa pH2PO4, determined according to the method of Aslyng (2), in these samples was 8.16 to 6.59. At the time of soil sampling cotton plant samples were also taken from the same plots and their total phosphorus content determined. All the plants were of the same age when sampled and were in the early fruiting stage, with a few small bolls. Linear regressions have been calculated for total phosphorus uptake by the plants (mg-P per plant) on soil phosphate potential, and for concentra- tion of phosphorus (percent P) in the plant on phosphate potential. The regression of total plant phosphorus on phosphate potential was significant at probability P < 0.02. However, the coefficient of determina- tion was low (r2 = 0.37) indicating that only 37 % of the total variation in phosphorus uptake by the plants was accounted for by variations in soil phosphate potential. The low coefficient of determination was largely due to two plant samples with relatively low phosphate uptake being associated with soil samples having high phosphate status (low numerical values of ipCa p1-121304). For the remaining thirteen samples the regression was highly significant (P < 0.001) and the coefficient of determination was high (r2 = 0.79). 600 I V.75
Under field conditions there is always the possibility of factors other than that under study being limiting to plant growth and thus limiting the total uptake of the facthr of interest. This appears to have been so in the present investigation because in the two • instances mentioned the plants had a high concentration of phosphorus, indicating that the soil phosphate was readily available to the plants. The regression, using all 15 samples, for phosphorus concentration (percent P) in the cotton plants on phosphate potential was highly significant (P < 0.001) and the coefficient of determina- tion was high (r2 = 0.72). Considering the data as a whole it may, therefore, be concluded that the phosphate potential has provided a good estimate of plant available soil phosphate.
RELATIONSHIP BETWEEN PHOSPHATE POTENTIAL AND OTHER SOIL CHARACTERISTICS. An examination of phosphate potential in relation to pH for Namulonge, soils indicated a linear relationship between the two. In addition there was evidence of a linear relationship between phosphate potential and percent organic carbon in the soil, as determined by the Walkley-Black method. To investigate these relationships the multiple regression of phosphate potential (Y) on pH (X1) and percent organic carbon (X2) has been calculat- ed for 34 soils. These included the 15 soil samples used in the investigation described above and 19 other samples chosen as representative of the soils of Namulonge. None had received any fertilizer, lime or manure. The range of pH, which was determined in 0.01 M calcium chloride, was 4.38 to -6.20, the range of percent organic carbon was 0.90 to 2.84, and the range of phos- phate potential was 8.57 to 6.59. The regression equation for the 34 soils is Ye = 11.31 — 0.56X1 — 0.28X2 ± 0.145 ± 0.141 The equation is very highly significant (P < 0.001). The partial regression coefficient for pH is also very highly significant, but that for organic carbon just fails to reach significance at P = 0.05. Thus there is no doubt of the relationship with pH, but there is some doubt about that with organic carbon. Closer examination revealed that the two soils noted in the previous investigation had low organic carbon values relative to the phosphate potential. These two were apart from the otherwise linear grouping of phosphate potential plotted against organic carbon and appeared to be giving considerable weight to the regression. The regression equation recalculated for 32 samples is Ye = 10.68 — 0.40X, — 0.38X2 ±0.133 ±0.126 This equation is also very highly significant and in it hoth partial re- gression coefficients are significant at P < 0.01. It appears, therefore, that for the majority of soils examined both pH and organic matter have an important influence on the value of the phosphate potential.
CONCLUSIONS The data presented indicate that the phosphate potential provides a good index of soil phosphorus availability to cotton plants at Namulonge. In addition it is relevant to mention some work by the author, to be published elsewhere, relating to soil fertility studies at Ukiriguru, Tanganyika, where soil phosphate availability limits crop production. There the phosphate 601 IV.75 potential has provided a better estimate of soil phosphate for correlation with crop yield than has been provided by various estimates using dilute acid or alkali extractants. In East Africa, and also in Ghana (3), the responsiveness of a soil to phosphate is often related to the length of time since the land was last under a resting crop. It seems likely that in tropical soils a proportion, perhaps a fairly large proportion, of the phosphate taken up by crops is derived from readily mineralizable organic sources. In this respect the partial dependence of phosphate potential on soil organic matter which has been demonstrated at Namulonge is of particular interest. The dependence of phosphate potential on soil pH is probably indicative of the influence of iron and aluminium on phosphate solubility. The level of organic matter may be important in this respect also since it has been shown (4) that organic substances may play an important part in preventing precipitation of phosphate by iron and aluminium because of their ability to form stable complexes with these metals. In the two multiple regression equations relating phosphate potential to pH and organic carbon the coefficients of determination (R2) were 0.50 and 0.45 for 32 and 34 samples respectively, indicating that only about half the variation in phosphate potential has been accounted for bij variation in the other two variates. It is pertinent, therefore, to enquire into the likely sources of unexplained variation. The determination of phosphate potential presents difficulties in soils of low phosphate status (ipCa pH2PO4 numerically greater than about 8.0 in acid soils) and this source of error probably accounts in part for the low coefficient of determination. It is likely that the method for estimating the part played by organic matter is not entirely appropriate; a study of the role of organic phosphorus and its mineralization is likely to be rewarding, and a closer examination of the apparently anomalous nature of the two soils discussed above may disclose important factors influencing the phosphate potential. pH measurements may be made with precision so that there is little scope for reduction of error; improvement is likely to be gained by a study of other soil factors, such as iron and aluminium, whose effect on the solubility of phosphates is linked with soil pH.
REFERENCES 1. SCHOFIELD, R. K., Can a precise meaning be given to 'available' soil phos- phorus? 1955. Soils and Fertilizers. 18, 373. 2. ASLYNG, H. C., The lime and phosphate potentials of soils; the solubility and availability of phosphates. Royal Veterinary and Agricultural College, Copenhagen, Yearbook 1954. Reprint pp. 1-50. 3. NYE, P. H. and M. H. BERTHEUX, The distribution of phosphorus in forest and savannah soils of the Gold Coast and its agricultural significance. 1957. J. Agric. Sci. 49, 141. 4. BRADLEY, D. B. and D. H. SIELING, Effect of organic anions and sugars on phosphate precipitation by iron and aluminium as influenced by pH. 1953. Soil Sci. 76, 175. SUMMARY Results of studies of the phosphate potential of soils at Namulonge, Uganda, are reported. The phosphate potential has provided a good index of plant available soil phosphate, as indicated by the phosphorus uptake and phosphorus concentration in cotton plants. The phosphate potential is shown to be significantly and linearly related to soil pH and, in the majority of soils examined, to soil organic matter also. 602 I V.75
These results are discussed in relation to the effects of iron and aluminium on soil phosphate availability, and to the probable importance of phos- phorus released from readily mineralizable organic sources.
RESUME Cette communication donne des resultats d'etudes, effectuees a Namu- longe (Uganda) sur le pOtentiel phosphatique des sols. Ce potentiel constitue un excellent indice de la quantite de phosphore du sol disponible aux plantes, comme la quantite de phosphate adsorb& par les plants de coton, et la concentration de cet element dans leurs paties vege- tales l'indiquent. 11 est inontre que le potentiel phosphatique est relie lineairement et de maniere significative au pH et a la matiere organique de la plupart des sols examines. Ces resultats sont discutes en tenant compte des effets du fer et de l'aluminium sur la disponibilite des phosphates du sol et de l'importance probable du phosphore libere a partir de matieres organiques aisement mineralisees.
ZUSAMMENFASSUNG
-Ober Ergebnisse von Studien, das Phosphatpotential von Boden aus Namulonge, Uganda, betreffend, wird berichtet. Das Phosphatpotential hat sich als eine gute Messzahl des den Pflanzen zuganglichen Bodenphosphates erwiesen, wie durch die Phosporaufnahme und Phosphorkonzentration in Baumwollepflan.zen erwiesen wurde. Das Phosphatpotential steht in deutlichem linearem Verhaltnis zu dem Boden pH und, bei der Mehrzahl der untersuchten Boden, zu der organischen Substanz im Boden ebenfalls. Diese Resultate werden in Beziehung zu dem Einfluss von Fe und Al auf die Benutzbarkeit des Bodenphosphats, sowie zu der wahrscheinlichen Bedeutung von Phosphor aus leicht minerali- sierbarefi 'organischen Stoffen auszullisen, besprochen.
603 COTTON RESEARCH CORPORATION
RESEARCH MEMOIRS
No. 70 —Experiments on the Effects of Phosphate Applied to a Buganda Soil Parts 1, 2 and 3
By P. H. Le MARE
1968 Cotton Research Corporation 12 Chantrey House Eccleston Street LONDON, S.W.1
J. agric. Sci., Camb, (1968), 70, 265-270 265 With 4 text-figures Printed in Great Britain
Experiments on the effects of phosphate applied to a Buganda soil I. Pot experiments on the response curve
By P. H. LE MARE Cotton Research Corporation, Cotton Research Station, Namulonge, Uganda*
(Revised MS received 12 December 1967)
SUMMARY In a pot experiment providing for rapid growth of sorghum plants in the red clay loam soils of the Cotton Research Station, Namulonge, in the Buganda Province of Uganda, large yield increases were obtained from added phosphate. In an uncultivated soil which had carried undisturbed natural vegetation (Pennisetum purpureum) for at least 10 years the response curve had negative curvature throughout. In an arable soil, yields without added phosphate were very poor and the response curve was sigmoid in form. With the arable soil, liming did not modify the form of the response curve for added phosphate but at the heaviest dressing lime decreased yield.
In early field experiments in Buganda, the pro- derived from the accumulated fertility of this fallow vince of Uganda round the northern shores of period. But as arable cultivation continued, crop Lake Victoria, there was no response, or only a yields decreased, recovering only partially and small one, to phosphate. Manning & ap Griffith temporarily after a grass ley. The survey men- (1949) reported a series of experiments designed to tioned above showed that the decline of productivity test phosphates mined or prepared in East Africa with extended arable cultivation was associated for use in local agriculture. Although many of with increasing soil acidity and decreasing soil their experiments did not compare a well-known organic matter. This suggested that in soils fertilizer, such as superphosphate, with the new recently cleared from uncropped indigenous vegeta- materials the results suggested that the soils of tion, enough phosphate might be released from Buganda had enough available phosphorus. Sub- organic sources for good, though perhaps not sequent field experiments at Namulonge supported maximum, yields of arable crops. But as cultivation this, the response to 21 cwt per acre of triple continues, increasing acidity and depleted organic superphosphate being small. However, a survey of matter may cause fertilizer phosphate to be correlations between the phosphate content of precipitated as iron and aluminium phosphates. In cotton leaves and soil phosphate potential at these circumstances the response curve may be Namulonge (Le Mare, 1960) indicated that cotton sigmoid, having positive curvature with small production might be limited by inadequate phosphate dressings followed by a point of in- phosphorus uptake. Furthermore, the phosphate flexion and then negative curvature with the potential was correlated with soil pH and organic larger dressings. If this occurs large dressings of matter; as these become less, there will be less phosphate, and perhaps liming, may be necessary phosphate for crops and responses to fertilizer to obtain an appreciable yield response to applied phosphate will be more likely. phosphate and could account for the small re- The soils at Namulonge are red-brown clay- sponses hitherto observed. loarns underlain by red clay often containing This hypothesis was tested using uncultivated quartz and ironstone pebbles; they have been and arable soils in pot experiments followed, in the classified by Radwanski (1960) into his Buganda- arable soils, by field experiments and associated Mirambi catenary complex. Before the research laboratory studies on the fate of fertilizer phos- station was opened, the land at Namulonge had phorus and the nature of the phosphate compounds reverted from peasant cultivation to the climax involved. (Pennisetum vegetation, largely elephant grass EXPERIMENTAL purpureum) and for some years yields were * Present address: Western Research Centre, TJkiri- The pots used were cylinders of 500-gauge iurt,.., Mwanza, Tanzania. polyethylene tubular film, sealed at the bottom;
266 P. H. LE MARE they were 5 in. in diameter by 8 in high and had with the phosphate at varying rates. To compen- four small holes at the bottom. Two kilograms of soil, sate for this when the experiment was set up a placed on top of 400 g of coarse quartz sand were solution of ammonium nitrate was applied in used. In the centre of the pot was placed a 1 in amounts for each treatment necessary to bring the diameter glass tube, open at both ends and with total nitrogen level up to that supplied by the four holes about 1 in from the bottom, which monoammonium phosphate at the 160 mg P level. extended about half way down the soil layer. This The greatest rate of P application necessarily had tube was used to ensure that the basal nutrient twice this amount of nitrogen in combination with solutions and water reached the body of the pot the phosphate but the assumption was made that without flooding the surface of the soil. this was unnecessarily high, especially where no P was applied and at the lower P applications. Experiment I Nitrogen was, however, given in the nutrient Two soils were used and had the characteristics solution applied during the growth of the crop. shown in Table 1. Throughout the experiment the moisture status Soil 925 came from a field which had been under was maintained at about 50 % of the water- arable cultivation for 10 years, carrying two crops holding capacity of the soil by adding nutrient per year; 926 was from an uncultivated site which solution or water to bring the pot to the necessary had carried elephant grass (Pennisetum purpureum) pre-determined value. The amounts added varied, for at least 10 years. being dependent upon actual water used and hence The experiment was designed to examine with on the rate of crop growth, which in turn was a precision the form of the curve with small dressings function of the amount of phosphorus given. Thus of phosphate and at the same time to obtain the it was possible to avoid gross differences, between maximum possible yield using very heavy dressings. treatments, in the ratio of phosphorus to other To achieve this in an experiment of manageable nutrients except, for the reasons given above, to size, whilst providing adequate replication of nitrogen at the greatest level of applied P. This treatments, the rates of application adopted were procedure ensured that the fast growing plants in geometric progression. received the water and nutrients they required, Phosphorus, as reagent grade monoammonium whilst the slow growing plants did not suffer from phosphate, was applied at 0, 5, 10, 20, 40, 80, 160, excessive amounts of water or salts. Initially all and 320 mg P per kilogram of soil (ppm). The water was applied as a nutrient solution but in the required amount was mixed with a small quantity later stages of the experiment rain water, collected of soil in a closed tube and then sprinkled over the in a concrete tank from a tile roof, was used. bulk of the soil on a sheet of polyethylene ; the whole The nutrient stock solution had the following was well mixed on the sheet before being poured composition: into the pot around the glass tube. The soil was g/litre g/litre thoroughly wetted by standing the pots in rain MnSO4. 41120 0.446 KNO3 100 water until the surface was moist, and allowing it CuSO4.5H20 0.050 MgSO4.7H20 74 to drain overnight before the seed was sown. ZnSO4.7H20 0.058 NaNO3 23 H3B03 0.372 Treatments were replicated eight-fold and arranged (NH4)6M070244H20 0.007 in randomized blocks. Sorghum was the test crop. To prevent fungal growth on the seedlings, the Calcium was included by using a saturated solution seed was treated for 1 min in 0.1 % mercuric of plaster of Paris, containing about 3 g CaSO4. chloride and after washing to remove any surplus i-1120 per litre, instead of water to prepare the salt, was soaked in water overnight. Next day stock solution. The nutrient stock solution was twelve seeds were sown in each pot; 10 days later diluted 1:200 for use. all except four seedlings were removed. The crop was grown for 9 weeks and then the The use of any salt involves the application of aerial parts of the plants were removed, dried at equivalent amounts of cations and anions and so, 95 °C and weighed. None of the plants flowered. in this experiment nitrogen, as NH4, was given After harvesting the first crop the soil was removed,
Table 1 Org C (Walkley- P pH Black, iu x106 Soil (in 0.01 Tvi uncorrected) (in 0.01 M Total P no. CaC12) (%) /pea pH2PO4 pH2PO4 CaC12) (ppm) 925 5.19 1.0 7.33 6.18 0.79 299 926 5.92 2.5 6.94 5.74 245 462 Effects of phosphate applied to a Buganda soil. I 267
Table 2. Total and residual variances after fitting curves to data for Experiment I Dry matter 1st crop 2nd crop % P 1st crop P uptake 1st crop Variance % of total Variance % of total Variance % of total Variance % of total • Soil 925 Total 1287.12 100 811.39 100 0.02477 100 5794.49 100 Residual after fitting Linear 295.97 22.99 126.78 15.63 0.00845 34.11 177.33 3.06 Type I 51.22 3.98 21.62 2.66 0.00054 2.18 64.63 1.12 Type II 32.47 2.52 1.84 0.23 0.00043 1.74 261.41 4.51 Gombertz - - 570.37 9.84 Soil 926 Total 884.05 100 2342.90 100 0.01346 100 7658.42 100 Residual after fitting Linear 151.78 17.17 305.04 13.02 0.00124 9.21 404.17 5.28 Type I 36.39 4.12 68.96 2.94 0.00069 5.13 150.17 1.96 Type II 50.30 5.69 96.42 4.12 0.00078 5.79 226.76 2.96
crumbled, returned to the pots and a second crop In the graphical presentation, the curve drawn grown for 9 weeks. The phosphorus concentration in each case is that which provided the best fit to the and total phosphorus in the aerial parts of the first data, i.e. accounted for the greatest proportion of crop were determined but only dry matter yields the variance in the dependent variable. Total and were obtained for the second crop. residual variances are presented in Table 2. Dry matter. The curves for yield of dry matter Experiment II reveal a marked contrast in the behaviour of the Using the more acid soil, no. 925, the effect of crops in the two soils towards applied phosphate. calcium hydroxide on the response to phosphate For the uncultivated soil, 926, with the higher pH was examined. Laboratory reagent calcium hydrox- and organic matter and with greater total P and ide was applied at 0, 0.5, 1, 2 and 4 g per kg of soil; lower phosphate potential (indicating greater phosphorus was applied at 0, 10, 20, 40 and 80 mg availability of phosphorus to the crop) type I P per kg of soil, using KII2PO4. The 25 treatment curves fitted the experimental points better than combinations were arranged in six blocks of a type II curves. For the arable soil, 925, type II 5 x 5 lattice square. The calcium hydroxide was curves provided the better- fit; not only did the first mixed with the soil, followed by the addition type I curves have larger residual variances but of the phosphate, in the manner described for the they also indicated negative values for dry-matter previous experiment. Nutrients other than phos- yield without added phosphate. The curves for the phate were given using the nutrient solution two crops are shown in Fig. 1. In addition to the described above. One crop of sorghum was grown. soils providing different types of curve, they also show other differences. The acute phosphate RESULTS deficiency of the arable soil is shown by the very small yield without added phosphate and with Experiment I small dressings; and although the bigger dressings The results are presented graphically and for all caused a large relative increase in yield, the maxi- sets of data two types of curve in addition to the mum yield of the first crop was only about linear have been examined. Both types assume an three-fifths of that in uncultivated soil; further- asymptotic maximum value for the dependent more, in the former there was little extra yield with variable, y, with large dressings of applied phos- dressings greater than 80 ppm P, whilst in the latter phate, x, but one (type I) has negative curvature there was an appreciable increase with the larger throughout, whilst the other (type II) is sigmoid, dressings. The difference between the soils in their in which the curvature is initially positive changing response to large dressings was particularly at a point of inflexion to negative curvature. The marked in the second crop. The curves show that type I curve has the general equation y = a 4-be-kx, in both soils the second crops responded to the where a, b and k are constants. The sigmoid curve, residual phosphate in patterns similar to the first type II, has the general equation y = a/(1 e-Mx-W) crops. In the uncultivated soil, however, the first in which a, b, and m are constants. crop had evidently so depleted the available phos-
268 P. H. LE MARE
50 - First crop 0.24 40 0.22
t) o 30 • %) 0.20 /p 20 ( 0.18 (g 10 tter 0.16 tter F ma 0.14 ma
50 dry 0.12 in dry
t 40 0.100-: P 30 0.081," Plan 20 0.06.1 10 010 40 160 5 20 80 320 0 10 40 160 P applied (ppm) 5 20 80 320 P applied (ppm) Fig. 2. Effect of applied phosphate on the phosphorus Fig. 1. Effect of phosphate on dry-matter production of concentration in plant dry matter, Experiment I, first sorghum in Experiment I. crop. 0-0 Soil 925: • -e Soil 925: first crop: 0.256 29.3 Ye 1 + exp{- 0.0107(x - 86.2)} Ye = 1 exp{ - 0.077(x- 44.2)} second crop: 0-0 Soil 926: 25.3 ye = 0.252 - 0.145exp( - 0.0053x). Ye = 1 i exp{- 0.046(x-70.6)} 0-0 Soil 926: first crop: 120
ye = 50.4 - 29.8 exp( - 0.0111x); 100 t) second crop: y, = 58.9- 52.0 exp( -0.0084x). o /p 80
phate in the untreated soil, and with the smaller (mg
ke 60 dressings, that the second crop yields were much ta less than those of the-first crop; but with the two 40 heaviest applications adequate phosphate evidently P up remained for growth of the second crop to be 20 slightly more than that of the first. In the arable ti soil, second crop yields were consistently rather rl ke lower than those of the first crop, although with 010 40 160 80 320 small amounts of applied P the difference was 5 20 applied (ppm) negligible. P Plant phosphorus Phosphorus concentration in Fig. 3. Effect of applied phosphate on the phosphorus the dry matter of the first crop, expressed as a taken up by plant, Experiment I, first crop. percentage is shown in Fig. 2. A curve of type I 9----e Soil 925: fitted the phosphorus concentration data from the ye = 120.3 -122.1 exp( - 0.0034x); uncultivated soil, whilst a sigmoid, type II, curve 0-0 Soil 925: was slightly better with the arable soil, although ye = 58.70 exp { - exp(1.651 - 0.030x)}. the difference between the residual variances after fitting the curves was very small in each case. 0 0 Soil 926: Phosphorus concentration was markedly less in ye = 142.1 - 119.4exp( - 0.0045x). plants grown in the arable soil with the smaller dressings but the curves converge and with the greatest dressing of applied phosphate there was of type I for each soil and although this supports little difference between the values obtained from the results already presented for soil 926, it is the two soils. contrary to those for 925. However, despite the Phosphorus uptake, expressed as mg P per pot, superior fit of the type I curve, suggesting a is shown in Fig. 3. Here the closer fitting curve was decreasing gradient throughout, the experimental
Effects of phosphate applied to a Buganda soil. I 269 points for P uptake from soil 925 clearly indicate reproduce the apparent increasing gradient in the an increasing gradient between 0 and 80 ppm P. lower region, it indicates a negative value for P In addition to the failure of the type I curve to uptake in the absence of added phosphate, im- plying a loss of phosphorus from the plant to the soil. The possibility that the failure for the type II ...- /.".'" x curve to have the better fit was due to its symmetry 25 about the mean of the maximum and minimum values / of the dependent variable was examined by fitting an asymmetrical sigmoid (Gompertz) curve having ./ o / ..._ --I- the general equation y = k exp - [exp(a + bx)]. -34c) 20 - 1:1 ./. This fitted the points up to and including those for —6—.0 80 ppm P extremely well but for 160 and 320 ppm k CO P the fit was poor and the residual variance for the -..-,' 15 a curve as a whole was larger than for the sym- /. metrical sigmoid curve. Although none of the curves examined fitted all 10 ///. • the experimental points satisfactorily, there appears a / ,i / • to be little doubt that the points in the lower part P-1 o / / of the curve have a positive curvature; those in the // ./. upper part are such that neither of the sigmoid . / ./ curves examined are satisfactory and this may be ..- due to a `luxurious' uptake of phosphorus by the i I t I crop, so that the total phosphorus content was in 0 5 10 20 40 80 excess of that required for normal growth processes. P applied (ppm) In Fig. 3 the Gompertz curve is shown as a full line to demonstrate the extremely good fit in the lower Fig. 4. Effect of applied phosphate on dry-matter pro- portion; the type I curve is shown as a broken line. duction of sorghum in soil 925 in the absence and presence of lime. Experiment II Experiment II Dry matter. As with soil 925 in the earlier 0-0 Lime absent: experiment, the curve representing the response to 23.2 phosphate was sigmoid; liming with calcium hydroxide had no effect on the response to phos- = 1 -I- exp{- 0.080(x - 32.5)} phate at the smaller applications but the largest •—• Lime applied at 4 g per kg soil: dressing reduced crop yields. The data are pre- 16.3 sented in Fig. 4. To avoid congestion of the four Ye 1 + exp{ - 0.089(x - 36.9)}. similar curves relating to the smaller lime applica- tions only three curves are shown; those for the - - - + Mean of all lime treatments: absence of lime, the greatest lime application 21.1 and the mean of all lime additions. For comparison Ye = 1 -1-exp{- 0.082(x - 32-9)} • the corresponding section of the curve for soil 925 in Exp. I is also shown. All the curves shown are of Experiment I type II, which provided a better fit than linear in x--- x Lime absent: each case; total and residual variances are given in 29.3 Table 3. Curves of type I were not fitted because ye = 1+ exp{- 0-077(x- 44.2)}. the initial curvature is clearly positive.
Table 3. Total and residual variances after fitting curves to data for Experiment II Lime (g/kg soil) 0.0 4.0 Mean all dressings Variance % of total Variance % of total Variance % of total Total 326.26 100 172.51 100 278.98 100 Residual after fitting Linear 6.70 2.05 2.89 1.68 5.77 2.07 Type II, sigmoid 1.21 0.37 0.26 0.15 1.35 0.49
17 Agri. Sci.
270 P. H. LE MARE uncultivated soil so that in the arable soil factors DISCUSSION other than phosphate deficiency must have The first experiment showed that under heavy limited crop growth. The physical state of the soils cropping in pots, where the amount of soil available differed considerably, the arable soil having poorer for exploitation by the plants is severely limited, structure and probably inferior aeration. But the supply of phosphate from both soils was despite the differences in yield level the experi- inadequate for maximum growth and added phos- ments together provided data which supported the phate caused large increases in dry matter. In each hypothesis that at Namulonge phosphate supply case the yield with the greatest level of applied may limit productivity and that small quantities of phosphate was close to the asymptotic value of the added phosphate may be rapidly precipitated in fitted curve, indicating that the amounts given the soil. The second experiment showed that were adequate for maximum yield and complete during the short period of a single crop grown in a response curves were obtained. The uncultivated glasshouse liming did not affect the form of the soil provided no evidence of a sigmoid curve, but response to phosphate. in the arable soil this form of response curve was demonstrated conclusively for yield in both experi- I thank G. J. S. Ross of Rothamsted Experi- ments. In the first experiment, the maximum yield mental Station, Harpenden, for fitting curves to the level from the arable soil was less than that from the experimental data.
REFER ENCES LE MARE, P. H. (1960). Observations on the phosphate RADWANSKI, S. A. (1960). The soils and land use of potential of some tropical soils. Trans 7th. int. Congr. Buganda. A reconnaissance survey. Mem. Res. Div. Soil Sci. (Madison, Wisc., U.S.A.) 3, 600-603. Dep. Agric. Uganda. Ser. 1. Soils 4. MANNING, H. L. & AP GRIFFITH, G. (1949). Fertilizer studies in Uganda soils. E. Afr. agric. J. 15, 87-97.
J. agric. Sci., Camb. (1968), 70, 271-279 271 With 12 text ;figures Printed in Great Britain
Experiments on the effects of phosphate applied to a Buganda soil II. Field experiments on the response curve to triple superphosphate
BY P. H. LE MARE Cotton Research Corporation, Cotton Research Station, Namulonge, Uganda*
(Revised MS. received 12 December 1967)
SUMMARY A sigmoid curve for response to superphosphate has been demonstrated in field experiments in a Buganda red clay loam. There was evidence that the curve was abnormal; with small dressings of triple superphosphate yield of cotton and beans, and their phosphorus uptake, were decreased, but these characters were increased with larger dressings. Heavy dressings, from 4 to 16 cwt triple superphosphate, increased yield of these crops. Similar effects were observed with maize during growth and maturity was advanced, but final yield of grain was not affected by applied phosphate. The suggestion is made that in a long growing period the maize was able to absorb adequate phosphorus, even though the phosphate intensity in the soil was low, to achieve its potential yield.
The first of these papers (Le Mare, 1968 a) described. incomplete blocks of six plots each (Cochran & Cox, pot experiments on the form of the response curve 1957); the experiment was started in February 1961 for phosphate applied to a Buganda soil. In an un- on a field next to that from which soil 925 had been cultivated soil the curve had negative curvature taken for the pot experiments and which had been throughout; in an arable soil the curve was sigmoid. cultivated for the same period, 10 years. Experi- This paper describes two field experiments, on sites ment C 4 had fivefold replication in complete blocks ; which had been cultivated for several years in which it was started in June 1961 on land which had been response curves were obtained for four crops of in cultivation for a shorter period. Irrigation faci- cotton, two of maize and one of beans. lities were available on this site and were used when soil moisture conditions, estimated by plaster of Paris covered nylon-stainless steel electrical resis- EXPERIMENTAL tance units (Farbrother, 1957) indicated the need The rates of phosphate adopted in the field ex- for additional water to supplement rainfall. periments were, as in the pot experiment, in geo- In Buganda there are two crop seasons per year. metric series to obtain precision on the form of the Annual food crops are grown mainly during the curve with small dressings and also to determine first half of the year, from February to May or the response with large dressings of phosphate. June ; cotton is always grown in the second half of Granular triple superphosphate was used. The the year and is sown at Namulonge in June; rates in one experiment, C2, were 0, I, 1, 2, 4, 8 picking starts in December. and 16 cwt superphosphate per acre, supplying The cropping sequence and the effects observed 0, 10, 20, 40, 80, 160 and 320 lb P per acre; in the in the experiments are shown in Table 1. other experiment, C4, the cwt dressing was ex- A basal dressing of nitrogen fertilizer was given cluded. The experiments were on ridged land; the to maize and cotton but not to beans. In 1961 a superphosphate was placed in the furrows by hand mixture of Chilean sodium nitrate and ammonium and the existing ridges were split to form new sulphate was used; in 1962 this was replaced by ridges over the fertilizer. Seed was sown by machine ammonium sulphate nitrate. Rainfall was very so that it lay about 1 in below the crest of the ridge heavy during 1961 and up to 150 lb N was given and about 6 in above a broad band of the fertilizer. to each crop, in three or four dressings; smaller In C 2 treatments were replicated sixfold in seven dressings were given in 1962. The first dressing was * Present address: Western Research Centre, Ukiri- given to each crop about six weeks after sowing. guru, Mwanza, Tanzania. Details are in Table 2. 17-2 272 P. H. LE MARE
Table 1 Experiment C2 Experiment C4
Superphosphate Superphosphate Year Season Crop effect observed Crop effect observed 1961 First Maize Direct None None var. K 8 Second Cotton 1st residual Cotton Direct var. NC 61 var. NC 61 1962 First Beans 2nd residual Maize 1st residual var. K 8 Second Cotton 3rd residual Cotton 2nd residual var. NC 62 var. NC 62
Table 2. Date, form and amount of nitrogen applied N, lb/acre as
Ammonium Date N Ammonium sulphate Total N Year Crop applied Chile nitrate sulphate nitrate lb/acre Experiment C2 1961 Maize 2 Mar. 12.5 12.5 27 Mar. 12.5 12.5 125 6 Apr. 37.5 37.5 Cotton 26 Aug. 12.5 12.5 2 Oct. 37.5 37.5 150 15 Nov. 50.0 1962 Beans 0 30.01 Cotton 13 Aug. 90 24 Oct. 60.01 Experiment C4 1961 None Cotton 19 July 12.5 12.5 28 Aug. 12.5 12.5 150 5 Oct. 25.0 25.0 14 Nov. 50.0 1962 Maize 6 Apr. 25.0 25 Cotton 30.01 9 Aug. 90 16 Oct. 60.01
RESULTS Results with cotton The crop yields, and chemical data on the crops, Effects on early growth. At the end of July 1961, are presented for each crop individually. The great- 6 weeks after sowing, twelve sample plants were est amount of information is on cotton followed by taken from each plot of Exp. C4 to determine the maize and beans. direct effect of superphosphate on early dry-matter Plant population was determined for all cotton production and phosphorus content of the plants. and maize crops; in no case was population affected The data obtained are presented in Fig. 1. Dry significantly by treatments. Where the regression matter was increased steadily by superphosphate coefficient for yield on plant population, based on applications up to 4 cwt per acre but the heavier the ' error ' terms in the analyses of variance and co- dressings gave no further production. Percentage variance, was significant, the yields were adjusted phosphorus in the plants increased throughout the to the mean plant population of the experiment. range of applications; total uptake of phosphorus In all figures the vertical line indicates the mag- increased very rapidly with dressings of super- nitude of the standard error for treatment means, phosphate up to 4 cwt, but, thereafter, the ratefo i.e. for the individual points in the curve. uptake was less. Effects of phosphate applied to a Buganda soil. II 273
(d) 1200 I S.E.± 1000 (c)
re) 800 Mean lb/ac 0 0 ( 600 tton o d c
e 400 (b) S. E.± Se ■ 111/11111.- Triple superphosphate (cwt/acre) 200 -"r S.E.± (a) Fig. 1. Dry matter (a), P uptake (b) and percent P (c) of cotton plant samples in relation to applied triple superphosphate; Exp. C4, July 1961. 0 11 2 4 8 16 Triple superphosphate (cwtfacre)
Cotton yield and dry-matter production. At the end Fig. 2. Yield of seed cotton in relation to applied triple of the season, crop production was measured in two superphosphate. (a) Direct effect: Exp. C4, 1961-2. forms : as seed cotton and as plant dry matter (b) First residual effect: Exp. C2, 1961-2. (c) Second remaining after all the seed cotton had been re- residual effect: Exp. C4, 1962-3. (d) Third residual moved. Although seed cotton is the commercial effect: Exp. C2, 1962-3. commodity for which the crop is grown, it may not be, in some circumstances, a suitable index of pro- ductivity for soil fertility investigations, because it is subject to loss from many causes unrelated to -3 4000 the nutrition of the crop. Some of these, such as loss 0 due to insects, may be controlled but other causes .0 of loss, for example, those associated with climate, may have an unknown and uncontrollable effect. to 3000 In 1961-2 yields of seed cotton were poor through- out Uganda, associated with abnormally heavy rainfall and probably with low temperature and 2000 light intensity also, and the two experiments des- cribed here averaged less than 300 lb seed cotton per acre. Yields in Exp. C4 may have been affected 1000 also by damage from an impurity in the DDT in- 0;1 2 4 8 16 secticide used (Perry, 1962), which caused plant Triple superphosphate (cwtfacre) deformities similar to those due to 2-4D, to which Fig. 3. Yield of cotton plant dry matter in relation to cotton is very sensitive. applied triple superphosphate. (a) Direct effect: Exp. The response curves at the bottom of Fig. 2 show C4, 1961-2. (b) First residual effect: Exp. C2, 1961-2. little effect of superphosphate on seed cotton yield (c) Second residual effect: Exp. C4, 1962-3. (d) Third in 1961-2 and although the linear trend due to residual effect: Exp. C2, 1962-3. first residual effects in C 2 was significant (P < 0.001), the mean rate of increase was only 6-5 lb seed cotton per hundredweight of superphosphate. residual effects on seed cotton, provided by Exps. In 1962-3, yields were very much better, although C 4 and C 2 respectively in 1962-3, reached a maxi- not large, and the response curves were more clearly mum with 4 cwt superphosphate per acre and defined. The curves for both experiments illustrate averaged about 150 lb of seed cotton per acre. a feature which subsequently proved to be a charac- The data for plant dry matter are presented in teristic of both experiments; there was a decrease Fig. 3. The greatest yield of dry matter and the in yield associated with the smaller rates of super- steepest slope of the response curve were given by phosphate, followed by an increase exceeding the the crop to which the superphosphate had been yield without the fertilizer. The second and third applied, followed by those using first, second and
274 P. H. LE MARE third residuals of superphosphate. All curves except Factors affecting the yield of cotton. The, data pre that for the second residual effect show the de- sented for cotton show that application of super- pression in yield with and 1 cwt superphosphate, phosphate influenced both yield and phosphorus followed by an initial rapid increase with 2 and content but the possibility that calcium, as well as 4 cwt superphosphate; thereafter, the rate of in- phosphorus, influenced yield needs to be examined. crease declines but in all cases the maximum yield Cotton, unlike most other annual crops, has a large of dry matter, unlike seed cotton, was given by concentration of calcium in the leaf. In another 16 cwt. The curves for the mean values of the four experiment at Namulonge (Le Mare, unpublished) sets of data are also shown in Figs. 2 and 3. liming increased the leaf concentrations of calcium Relationship between plant phosphorus and applied and phosphorus, as well as increasing the yield; at superphosphate. For all crops the concentration Ukiriguru, Tanzania, cotton yield is related to ex- in the second full leaf below the terminal bud was changeable calcium in some soils (Kabaara, 1964). determined in October, about 4 months after Thus there is evidence of the importance of calcium sowing. as a nutrient for cotton in two areas of East Africa In three of the four crops leaf phosphorus de- and this leads to consideration of the importance of creased where the smaller dreSsings of superphos- calcium supplied by the superphosphate in the ex- phate had been given; it recovered with 2 cwt to periments at present under review. Leaf calcium the value associated with the absence of super- was determined for both crops in 1962-3. The mean phosphate and was increased with the heavier concentration for Exp. C2 was 1.79 ± 0.05 and for dressings. Seed phosphorus was determined for two C4 2.69 + 0.05 % Ca; in neither experiment was crops and gave similar results. Data for P concen- there evidence that superphosphate affected the tration in the plant frame showed the depression concentration of calcium in the leaves. with small amounts of superphosphate and a re- The dry-matter yields for Exp. C2 in 1962-3 have covery with 4 cwt in two of three crops examined, been related to leaf concentrations of phosphorus but in no case was there an increase with larger and calcium in a multiple regression and the fol- amounts of superphosphate. lowing equation obtained: An estimate of the phosphorus in the plant frame and in the seed of Exp. C2 in 1961-2 at the time of Y, = 2464(% P) 295(% Ca) - 216, picking is presented in Fig. 4. The data exhibit an S.E. (33 D.F.) ± 596 ± 182. initial decrease in phosphorus uptake with a mini- This equation accounts for 35 % of the variation mum at 1 cwt superphosphate; with larger dressings in the dependent variate, but in the experiment there was an initial rapid increase in seed phos- yields were affected by random variation in plant phorus to the 4 cwt point, followed by a slower population and the inclusion of this term increases increase, but there is no evidence that the maxi- mum uptake was reached with 16 cwt of super- the accountable variation to 68 %, and provides phosphate. the equation: Y, = 2196(% P)-F 204(% Ca)± 0.1143 (population) - 1814, S.E. (32 D.F.) ± 428 ± 131 ± 0.0198. These equations indicate that yield of dry matter was related to leaf phosphorus concentration but
) that calcium had very little influence on yield. In Fig. 5 yields of dry matter are plotted against
lb/acre the phosphorus concentration of the leaf samples ( taken from the corresponding crops earlier in the ke
ta season. Each point represents the mean value for all replicates receiving a particular treatment. All up
P crops provided a positive linear relationship be- tween leaf phosphorus and dry matter yield. The relationships for seed cotton were less well defined, although for both crops of Exp. C2 the correlation coefficients were significant, despite little yield 0 4-1 2 4 8 16 variation in 1961-2. Triple superphosphate (cwt/acre) Boron and molybdenum may limit cotton yield Fig. 4. P Taken up by (a) cotton plant frame and under some conditions. Boron deficiency is not (b) cotton seed in relation to applied triple superphos- likely to have occurred because in addition to am- phate. First residual effect, Exp. C2, 1961-2. monium sulphate part of the nitrogen was given to Effects of phosphate applied to a Buganda soil. II 275
4500 Results with maize Maize was the first crop in experiment C2 and 4000 the second in C 4, so that the two experiments provide information on direct and first residual 3500 effects of superphosphate, albeit at different sites and in different years. Each year plant samples 3000 were taken about 6 weeks after sowing for the determination of dry matter, phosphorus concen- tration and total P uptake. The data are presented 2500 in Figs. 6 and 7. The direct effect of superphosphate ti on early dry-matter production was essentially g 2000 linear and with 16 cwt superphosphate the yield was twice that obtained without phosphate; the curve for total phosphorus uptake was similar, but 1500 that for phosphorus concentration was of the form described above for the cotton crops, having a small 1000 depression associated with and 1 cwt superphos- phate and thereafter a steady rise with greater 500 dressings. The dry matter and P uptake data for the samples from C 4 also provided this type of curve, although dry matter was not greatly in- 0 I I 0 3 0 4 0.5 0.6 creased by the larger amounts of superphosphate and a maximum occurred with 8 cwt; P concen- Leaf P (% of n.ic.) tration was unaffected by 1 and 2 cwt but in- Fig. 5. Relationship between yield of cotton plant dry creased with larger amounts of superphosphate. matter andleaf phosphorus concentration. + — , Exp. Nine weeks after sowing C 2 in 1961 the number 04, 1961-2: b = 8603+2344 (0.02 < P < 0.05); 0-0, of plants with tassels was counted; the data, ex- Exp. C2, 1961-2: b = 3952+892 (0.001 < P < 0.01); pressed as percentages of the total number of x — x , Exp. C4, 1962-3: b = 5156+3047 (0.05 < plants, are given in Fig. 8. The linear trend through P < 0.1); 111-0, Exp. C2, 1962-3: b = 2699±613 all the points is highly significant, indicating that (0.001 < P < 0.01). phosphate had promoted early tasselling; however, the data also show that there was little effect with dressings up to 2 cwt but a large increase between the 1961-2 crops as Chilean sodium nitrate, which 2 and 4 cwt; there was no greater effect with contains appreciable amounts of boron. dressings heavier than this amount. Molybdenum enters the plant as the molybdate In May 1961 samples of cobs were taken from anion and behaves in the soil in a manner similar to the phosphate anion; Mulder (1954) has shown that on molybdenum deficient soils phosphate dressings may have the same effect as an application 0.40 of sodium molybdate, releasing molybdenum by (c) 035 anion exchange. Liming, which has increased cotton yield in the field at Namulonge (Le Mare, 1962), t 0-30 .5 t; may make molybdenum more available in acid soils. A 0-25 The soils on which the present experiments were conducted are such that molybdenum deficiency 0-20 may occur and the larger dressings of superphos- 15 phate might have supplied molybdenum as an im- 1. g 10 purity, or made it available by anion exchange, and g could have influenced yield. This possibility was 5 examined in 1962-3. The plots of C2 were split and 1 lb per acre of ammonium molybdate was applied 0 21 2 4 8 16 to one of each pair of sub plots. The effect averaged over all superphosphate treatments, was to reduce Triple superphosphate (cwt/acre) yield of seed cotton by 56 + 25 and dry matter by Fig. 6. Dry-matter production (a), P uptake (b) and 117 ± 72 lb per acre ; there was no interaction be- P concentration (c) of maize plant samples in relation tween molybdate and rates of superphosphate. to applied triple superphosphate; Exp. C2, 1961.
276 P. H. LE MARE
0.40 0.35 0.30 0.25/25 125 '7 20 1 00 00 15 75
10 50 La 5 25 Pi 0 0 1 2 4 8 16 Triple superphosphate (cwt/acre) Fig. 7. Dry-matter production (a), P uptake (b) and P concentration (c) of maize plant samples in relation to applied triple superphosphate; Exp. 04, 1962.
0 +1 2 4 8 16 Triple superphosphate (cwt/acre)
Fig. 8. Percentage of maize plants with tassels 9 weeks 0 2 4 8 16 after sowing in relation to applied triple superphosphate, Triple superphosphate (cwt/acre) Exp. C2, 1961. Fig. 10. Yield of bean seed in relation to applied triple superphosphate; second residual effect, Exp. C2, 1962.
1"3 crease in weight of grain due to superphosphate 100 and only a very small decrease at the cwt point. 0 ,,`" 90 The effects of superphosphate observed in the May samples were due to its influence on maturity: 2" 80 grain on the plots which received heavy super- .73'6 70 phosphate dressings matured earlier, but final yields of maize, unlike those of cotton and beans, 0 +1 2 4 8 16 were unaffected by superphosphate at all levels in Triple superphosphate (cwt/acre) both experiments. The mean yields of dry grain were 3932 ± 31 in Exp. C2 and 2285 ± 51 lb per acre Fig. 9. Dry grain per cob in relation to applied triple in C4. superphosphate, Exp. C2, 1961. (a) Samples taken in May. (b) At harvest in June. Results with beans The third crop in Exp. C2, grown in the first rainfall season of 1962, was beans and provided each plot of C2 and the mean weight of dry grain information on second residual effects of super- per cob was determined. These data are shown to- phosphate. Yields were not large but were in- gether with the data for grain per cob at final harvest creased by superphosphate at the higher rates and in Fig. 9. In the May data the positive linear trend the linear trend for all dressings was significant; in for all the superphosphate dressings was significant this crop, as with cotton but unlike maize, there but there was evidence of a reduction in weight of was an indication that yields associated with and grain with to 2 cwt of superphosphate. The har- 1 cwt superphosphate (Fig. 10) were smaller than vest data, however, showed no evidence of an in- those without superphosphate. Effects of phosphate applied to a Buganda soil. II 277
Soil phosphate measurements and 2000 their relation to cotton yields Previous work at Namulonge (Le Mare, 1960) 1600 showed that the potential of monocalcium phos- phate in the soil, -ipea pH2PO4, was correlated with the phosphorus content of cotton grown in 5.2 untreated soils and so this method was used in one 5.4 - (b) -x ___--x------_ of the present experiments to investigate the re- O 5.6 , (c) 800 lationship between soil phosphate and cotton yield 58 data 2 years after the superphosphate was applied. m 6.0 400 After the second crop of 1962 had been removed, surface soil samples, 0-8 in. deep, were taken from 6.2 I S.E. ± six centre rows of two plots of each treatment. The 64 0 00 2 4 8 16 plots sampled were in the centre three blocks of the Triple superphosphate (cwt/acre) incomplete block design; they were selected with- out reference to crop data. The samples were air Fig. 11. Yield of cotton plant dry matter (a), seed dried and the phosphorus concentration determined cotton (b) and soil pH2PO4 (c) in relation to applied after shaking for 4 days in 0.01m CaC12, using a triple superphosphate 2 years after application. soil:solution ratio of 1: 2-5. Phosphorus concen- tration was determined by the method of Watanabe 2200 & Olsen (1962), with modification to suit the equip- (a) 16 ment available. 4 The mean pH of all samples, determined by glass 1800 n 0 electrode in 0.01 m-CaC12, was 4-94 + 0-04. The cal- 2 0 cium concentration was constant for all the soil 1400 0 1 samples and so the term pCa has been omitted; o in the present work pH2PO4 is related to crop data. (b) Fig. 11 shows the curves relating mean pH2PO4 1000 values, and crop yields, to the rate of superphos- 2 16 18 0 phate applied; the crop data are means for the two 1 • . plots from which soil samples were taken and so 600 sr- i2i Ii lilt are comparable with the pH2PO4 data. These crop 6.4 6.2 6.0 5.8 5.6 5.4 yield curves may be compared with the corres- p1-1,130, ponding data for the whole experiment presented in Figs. 2 and 3. The form of the curves is very Fig. 12. Yield of cotton plant dry matter (a) and seed similar but dry-matter yields from two plots only cotton (b) in relation to soil 0141304 2 years after application of triple superphosphate. Numerals against were slightly greater than the mean for all plots. points refer to the amount (cwt/acre) of triple super- There was a close similarity between seed cotton phosphate applied. yields for two plots and for the whole experiment, except at 8 cwt superphosphate, which gave rela- tively low yields on the two plots sampled. range 6.3-5.3, representing a tenfold increase of The values for pH2PO4 are plotted negatively so phosphorus concentration in the calcium chloride that a drop in the curve represents a reduction in extract, the figure shows a steady increase in yields the intensity of phosphate supply to the crop, and of dry matter and seed cotton. The points for and vice versa. In Fig. 11 the p1121)04 curve is very 1 cwt lie well below the relevant regression line, similar in form to those for the crop data, even at especially for dry matter, showing an abnormally the point representing 8 cwt, but whereas the crop low yield for the value of pH2PO4. As the yield in data curves show minima at 4- and 1 cwt super- the experiment was linearly related to leaf phos- phosphate the pH2PO4 curve has its lowest point phorus concentration for all superphosphate dres- at 2 cwt. Whilst the points for 1 and 2 cwt are sings (Fig. 5) this suggests that, although yield was below those for no applied phosphate, the values influenced by soil phosphorus, the depression in for 4 and 8 cwt are little different from that for no yield associated with low rates of superphosphate phosphate applied, as with the corresponding yield was due not only to a low intensity of supply but data. also to a failure of phosphorus to be absorbed by In Fig. 12 the cotton yields are plotted against the plant or to be transported within it, perhaps pH2PO4. With the exception of those for -I. and because of a relatively high level of another element 1 cwt, the points lie on a straight line and over the in the plant. 278 P. H. LE MARE mined in samples from duplicate plots of each treat- DISCUSSION ment 2 years after application of superphosphate, The experiments described in Part I (Le Mare, also showed a depression associated with small 1968a) showed conclusively that for sorghum grown dressings. Soil samples were not taken from indi- in pots in an acid, low organic matter Buganda red vidual plots before the experiment was laid down clay loam, the response to phosphate followed a so that there is no conclusive evidence for the sigmoid curve. In field experiments with cotton, possibility that the depression was an effect of, maize and beans sigmoid curves have been obtained rather than a fortuitous association with, the small but in addition a complex group of effects has been dressings of superphosphate, but that the result demonstrated. With directly applied triple super- should have occurred by chance at both sites seems phosphate neither cotton nor maize showed evi- unlikely, especially as at neither site was it shown dence of a sigmoid curve for dry matter in samples by early growth of the crop to which the fertilizer taken during early growth. With residual super- was directly given. phosphate the curve for corresponding samples of That the depression was an effect of the super- maize was sigmoid, but it also indicated a reduction phosphate is supported by similar results on cotton in early dry-matter production associated with in the south-eastern States of the U.S.A., reported 1 cwt superphosphate. Yields of seed cotton, cotton by Terman (1960). He observed that a `slight to plant dry matter and bean seed all showed a reduc- marked turn-down occurred in the response curves tion with and 1 cwt per acre of superphosphate, at relatively low rates (of superphosphate) in 10 of either as a direct or as a residual effect. In general, the 34 experiments'. Terman used concentrated with 2 cwt of superphosphate yield recovered to superphosphate at 0, 20, 40, 60 and 80 lb P2O5, of the level given in its absence. Seed cotton yields which the first two dressings are comparable with reached their maximum with 4 cwt but cotton the smallest dressings used in the Namulonge ex- plant dry matter and bean seed production were periments. Terman was unable to explain the effect increased by dressings up to 16 cwt. The residual but suggested that it may have been due to seedling effect of heavy dressings decreased with time. injury or unbalanced nutrition. In one of the Maize behaved differently from the other two crops. Namulonge experiments, C 2, the yield effects at all Final yield of mature grain was not affected by levels of superphosphate were closely associated superphosphate, despite the effect on early growth, with leaf phosphorus concentration; in the other but maturity was hastened and cob samples taken experiment the relationship was less conclusive. In 6 weeks before harvest indicated a yield response the former experiment the yields were correlated curve similar to that for cotton and beans. P con- with soil pH2PO4 only for 0, 2, 4, 8 and 16 cwt centration of cotton leaves was reduced by small superphosphate; at and 1 cwt the yields of seed dressings of superphosphate, but increased with the cotton and plant dry matter were smaller, by about large dressings ; a similar effect occurred with young I50 and 400 lb respectively, than those expected on maize plants. the basis of the relation between yield and pH2PO4. The results have thus indicated three distinct Thus, with and 1 cwt of superphosphate soil P aspects of the use of triple superphosphate in the intensity was evidently adequate for a greater acid, Buganda red clay loam. First, the depression yield but insufficient phosphorus was absorbed by of yield and plant phosphate associated with and or translocated within the plant to realize the po- 1 cwt of superphosphate; secondly, in cotton and tential yield. Although this would account for the beans the beneficial effect of heavy dressings and, yield effect and support Terman's suggestion of thirdly, the failure of superphosphate to affect final unbalanced nutrition, the mechanism of the be- maize yield despite the effects with light and heavy haviour with the small dressings of superphosphate dressings, similar to those with cotton and beans, in the soil remains unexplained. in early growth. The effect of heavy dressings of superphosphate The yield depression on cotton and beans This occurred at one or more stages of growth The response curves indicate that with cotton and in every crop and was also shown by plant phos- beans little or no yield improvement may be ex- phorus data at both experimental sites, and in both pected in old arable soils with less than about 4 cwt years, but at neither site was it demonstrated in triple superphosphate as an initial dressing, and early growth of the crop to which the superphos- this result is in agreement with earlier work at phate had been applied. At both sites it first Namulonge in which the effect of 21 cwt was gener- appeared after the fertilizer had been in the soil ally small. In beans the response curve continued for 2-3 months. In some cases the depression was to rise up to 16 cwt, as it did for the direct and statistically significant. In one experiment soil phos- first residual responses of cotton plant dry matter, phate intensity as measured by 0121304, deter- though not with seed cotton. Nevertheless, yield Effects of phosphate applied to a Buganda soil. II 279 levels in general were only moderate and the phos- obtained rather earlier in the presence of heavier phate responses were small in relation to the amount dressings. Thus the maize variety grown was able, applied indicating that strong soil-phosphate re- over a longer period of time, to obtain adequate actions occurred so that large phosphate dressings phosphate to achieve its potential yield even though are necessary to be effective; this is supported by the intensity of soil phosphate supply was low; in the data on soil pH2PO4 for which only the heaviest this case the yield was nearly 4000 lb dry grain per dressing caused a major change. Young & Pluck- acre. Possibly a longer term variety with a greater nett (1966) have indicated that in order to achieve potential yield would have shown a response to heavy yields in Hawaiian latosols very heavy initial phosphate in final production of dry grain. In most dressings of phosphate are necessary to quench high areas of East Africa where maize is grown the ces- phosphorus fixation, followed by occasional smaller sation of rainfall at the onset of the dry season dressings. The Buganda soils may require similar causes growth to cease and possibly in such areas treatment. the effect of phosphate on maturity is particular important and is ultimately shown as a yield The effect of superphosphate on maize increment. The behaviour of maize in its response to phos- The soil phosphate status is important for the phate appears to be contrary to much experience, whole agricultural system. The double cropping as maize is a responsive crop in many areas of system, in which cotton follows maize, involves a East Africa where the plant available soil phosphate tight schedule and sometimes maize has been har- status is small. The reason may be associated with vested prematurely or cotton sowing postponed. the effect of phosphate on time of maturity in This has led to recent changes in cropping pro- relation to the length of growing season. In one of grammes at Namulonge, but the maize-cotton the experiments the heavier dressings of applied sequence may be successful on soils with high phosphate advanced maturity and there were early phosphate intensity, using a maize variety which indications of a yield effect, but soil moisture was matures suitably. Although it is doubtful if fertilizer adequate for growth to continue so that in the phosphate is economically worthwhile for individual absence of applied phosphate, and with small crops, on a longer term basis it must contribute to dressings, the final yield achieved was equal to that the viability of intensive agriculture.
REFERENCES COCHRAN, W. G. & Cox, G. M. (1957). Experimental MuLnEn, E. G. (1954). Molybdenum in relation to Designs. New York: John Wiley and Sons, Inc. Ch. 11. growth of higher plants and micro-organisms. Pl. Soil FARBROTHER, H. G. (1957). On an electrical resistance 5, 368-415. technique for the study of soil moisture problems in PERRY, D. A. (1962). Phytotoxic symptoms on cotton the field. I. Performance, design and installation of after spraying with DDT. Emp. Cott. Grow. Rev. 39, nylon/stainless steel resistance units. Emp. Cott. 203-5. Grow. Rev. 34,71-89. TERMAN, G. L. (1960). Yield response in experiments KABAARA, A. M. (1964). Progr. Reps. Exp. Stns Emp. with phosphorous fertilizers in relation to : I. Mean- Cott. Grow. Corp. Tanganyika Lake Regions, Season ingful differences among sources on acid soils of the 1962-63, p. 12. south eastern States. Proc. Soil Sci. Soc. Am. 24, LE MARE, P. H. (1960). Observations on the phosphate 356-60. potential of some tropical soils. Trans. 7th int. Congr. WATANABE, F. S. & OLSEN, S. R. (1962). Colorimetric Soil Sci. (Madison, Wisc. U.S.A.) 3, 600-3. determinations of phosphorus in water extracts of LE MARE, P. H. (1962). Progr. Rep. Exp. Stns Emp. soil. Soil Sci. 93, 183-8. Cott. Grow. Corp. Uganda, Season 1961-62, p. 23. YOUNG, 0. R. & PLUCECNETT, D. L. (1966). Quenching LE MARE, P. H. (1968a). Experiments on the effect of the high phosphorus fixation of Hawaiian latosols. phosphate applied to a Buganda soil. I. Pot experi- Proc. Soil Sci. Soc. Am. 30, 653-5. ments on the response curve. J. agric. Sci., Camb. 70, 265-70.
J. agric. Sci., Camb. (1968), 70, 281-285 281 With 2 text-figures Printed in Great Britain
Experiments on the effects of phosphate applied to a Buganda soil III. A chemical study of the soil phosphate, the fate of fertilizer phosphate and the relationship with iron and aluminium
By P. H. LE MARE Cotton Research Corporation, Cotton Research Station, Namulonge, Uganda*
(Revised MS received 12 December 1967)
SUMMARY Solubility products (pK„) for iron and aluminium hydroxides and phosphates have been determined in a Buganda acid clay loam. Significantly greater values, i.e. lower solubility products occurred when determined at a soil : solution ratio of 0.4 than at 0-8. In the most acid soils, pH 4.8-5-0, values of pK„ were consistent with the possi- bility that iron and aluminium concentrations were controlled by goethite and gibbsite. Above pH 5 the data indicated that these minerals were not effective. The maximum pK„ value for iron phosphate was too small to be consistent with that for strengite, indicating that this mineral did not affect phosphate concentration in the soil; the maximum value for aluminium phosphate was consistent with that for variscite but the occurrence of this mineral cannot be deduced from the data. Triple superphosphate applied at 8 cwt per acre or less was converted within 2 years to a very insoluble form, having a solubility similar to that of variscite; with 16 cwt per acre the phosphate con- centration was much greater than with the smaller dressings after this period and the soils appeared supersaturated with variscite.
The first two of these papers (Le Mare, 1968a, b) those for strengite and variscite may be obtained, describe pot and field experiments which provided these do not provide sufficient evidence for the evidence for a sigmoid response curve for phosphate existence of these minerals in the soil (Bache, 1964). in an acid clay loam in Buganda. This paper gives Another criticism of the use of solubility product the results of a chemical investigation on the nature constants for sparingly soluble compounds is that of the soil phosphate and of the fate of fertilizer unique values are difficult to achieve, because of phosphate. the slow rate of dissolution and hence the difficulty Lindsay & Moreno (1960) suggested that in acid of ensuring equilibrium. Furthermore, experimental soils the hydroxide minerals, goethite and gibbsite, values for the constant may be dependent upon control iron and aluminium concentrations and particle size and degree of crystallinity of the that these concentrations in turn control the compound. phosphate concentration in accordance with the The work described here was undertaken in 1963 solubility products of strengite and variscite. Chak- to examine the possibility that the suggestion made ravarti & Talibudeen (1962) have presented evi- by Lindsay & Moreno (1960) was applicable to the dence that these phosphate minerals occur in acid Buganda clay loam but the results are discussed in tropical soils, but not in temperate soils; Wright the light of subsequent published work. & Peech (1960) have also used solubility criteria to characterize phosphate products in acid soils. How- MATERIALS AND METHODS ever, Bache (1963) suggested that solubility equi- libria in soils are not relevant for this purpose. His Soils work indicates that thermodynamic solubility con- (1) Fourteen surface soil samples (0-8 in.) were stants are not maintained at pH values greater taken in January 1963 from duplicate plots of an than 1.4 and 3.1 for strengite and variscite respec- Exp. (C2) to determine the form of the response tively; surface reactions appear to be more impor- curve to superphosphate in field 318 at Namulonge tant. Although solubility constants of the order of (Le Mare, 1968b). Granular triple superphosphate * Present address: Western Research Centre, Ukiri- at 0, 1, 1, 2, 4, 8 and 16 cwt per acre had been guru, Mwanza, Tanzania. applied in February 1961 since when the land had 282 P. H. LE MARE carried one maize crop, two cotton crops and a sodium acetate followed by 1 ml of 1 % hydro- beans crop. The fertilizer was applied by hand in quinone were added and then 0.5 ml of 0.25 % the furrows of ridged land and then covered by 1:10 phenanthroline. The solution was made up to splitting the old ridges to form new ones. In June 10 ml and the optical density read at 510 mit. A 1961 these were rebuilt in preparation for the next Unicam SP 600 spectrophotometer was used to crop; in March 1962 the field was ploughed, disced measure optical densities in all determinations. and ridged for the third crop and in June that year the ridges were rebuilt for the fourth crop. The soil Calculation, of solubility constants samples taken in January 1963 were from the ridges For the hydroxides Fe(OH)3 and Al(OH)3 the after the fourth crop had been harvested. The field negative logarithm of the solubility product con- had been cultivated since 1950, carrying two crops stant, pK„, may be written per year. (2) The soils used in the pot experiment to deter- pK„ = pM3+ - 3pH+ + 42.06 (1) mine the form of the response curve (Le Mare, For the phosphates, FePO4.2H20 and A1PO4. 1968a) were also examined: soil 925 was from field 21120, 317 which had been cultivated for 10 years and had not received any fertilizer or manure during that pK„ = pM2+ +01,1304 - 2pH+ + 28.04 (2) time; soil 926 used in the pot experiment came from near field 317 and had carried elephant grass pA13+ was determined from total aluminium con- (Pennisetum purpureum) for at least 10 years. centrations and corrected for hydrolysis and ac- (3) Soil samples from fields 153 and 154 were tivity coefficients according to Lindsay et al. (1959); also examined. These fields had been cultivated pH2PO4 was computed from total P concentration since 1952; field 153 had received no fertilizer; using tables published by Aslyng (1954). For the field 154 had 2 tons of ground limestone and 4 cwt calculation of pFe2+ from the molar concentration of triple superphosphate per acre 1 year before of iron determined experimentally, the following sampling in 1963. expression was used : Fe3+ Fe0H2+ Fe(OH)2÷ (total Fe) = „ „ EXPERIMENTAL f f All soil samples were air dried and passed through where the terms in the numerators represent the a 2 mm sieve. activities of the ions concerned and the terms in The soils were shaken with 0.01 m-CaC12 solution the denominators the corresponding activity co- for 88 h at 25 ± 1.5 °C. All samples were examined efficients. This may be written at the soil: solution ratio 0.4; some were also ex- (total Pea+) amined at the ratio 0.8. To obtain these ratios 80 Fe3+ - 2 1 Kh"Kh' , (4) and 160 g of soil were shaken with 200 ml CaCl f,+ f„ + solution in polyethylene bottles on a horizontal 1-1+ f"'. (13+)2 shaker; 1 ml of chloroform was added to prevent where and Ku” are the first and second hydro- microbiological activity. lysis constants for ferric iron. The constants used One hour after removal from the shaker pH was were those given by Sillen (1959): determined using a Pye glass electrode pH meter standardized at pH 4 and pH 7. The glass electrode Ku' = 8.1 x 10-4, Ku" = 5.50 x 10-4. was immersed alone in the soil suspension for 20 s before the reference electrode was immersed in the Values for f„ the activity coefficient of the ion clear supernatant solution and the pH read im- concerned, were computed from the simplified mediately. The solution was then filtered through Debye-Hiickel equation an 18.5 cm No. 542 Whatman filter paper; the first _ Aza /1 log fi = (5) 10-20 ml was returned to the filter and the sub- 1 + sequent filtrate analysed for phosphorus, alumin- ium and ferric iron. where A is a constant having the value 0.508, z is Phosphorus in the CaC12 extract was determined the valency and I the ionic strength, defined by by the method of Watanabe & Olsen (1962), modi- I = (6) fied slightly to suit the apparatus available. Alu- minium was determined according to the method in which ci is the molar concentration of the ion. of Lindsay, Peech & Clark (1959). For determina- The concentration of (Ca2+ +Mg2+), determined by tion of ferric iron, 5 ml of the final hydrochloric titration with EDTA using an EEL photo-electric acid solution obtained in the procedure for alu- titrator, was used to compute the ionic strength, minium determination was taken; 0.7 ml 3 assuming equivalent concentrations of anions. Effects of phosphate applied to a Buganda soil. III 283
RESULTS for the other samples, they were very closely cor- related with pH, as shown in Fig. 1. For the iron Effect of soil: solution ratio hydroxide, in the pH range 4.8-6-2, the correlation Table 1 shows the mean values of pK„ for the coefficient was - 0.988, and for the aluminium hy- hydroxides and phosphates determined at the two droxide, for pH values 4-8-5.2, the coefficient was ratios, 0.4 and 0-8. At the larger ratio the values - 0.959. Neither relationship in Fig. 1 shows points were smaller than for the lesser ratio thus showing lying parallel to the axis of pH and thus there is that unique values were not obtained for the solu- no evidence of constancy of pK„ within the pH bility product, and that at the larger ratio the ranges examined. solubilities of the compounds were greater. Iron and aluminium phosphates Values of plc, at soil: solution ratio 0.4 For Exp. C 2 the values of pK„, presented in All samples were examined at the ratio 0.4 but Table 2, showed no significant variation due to only six of the fourteen samples from Exp. C 2, treatment with superphosphate up to and including together with all the other samples, were examined 8 cwt per acre; the mean values were 33-4 + 0.05 at the 0.8 ratio; for this reason the variation in and 30.4 + 0.10 for the iron and aluminium phos- pK„ in relation to soil conditions and fertilizer phates respectively, but where 16 cwt triple super- treatment is examined only at the smaller ratio. phosphate had been given pK„ was significantly Table 2 shows the mean ionic concentrations and smaller by about one unit, the corresponding values values of pK„, determined at the 0.4 ratio, for the being 32.6 + 0.11 and 29.3 + 0.23. series of superphosphate treatments in the experi- For iron phosphate there was close linear cor- ment, and for the other soil samples. relation (r = 0.969) between pK„ and pH21304 (Fig. 2) and, as with the hydroxides, no evidence Iron and aluminium hydroxides of pK„ being constant. For aluminium phosphate The mean values of pK„ for these compounds in a linear relationship was not obtained and Fig. 2 the soil samples from Exp. C2 were 36.4 ± 0.06 and shows a cluster of all points except that for 16 cwt 33.4 ± 0.11 respectively. There were no significant around the mean value of 30.4. Nevertheless, these differences between the values for different phos- points do not provide evidence that a constant phate treatments, but taken together with the data value of pK„ had been achieved.
Table 1. The effect of soil :solution ratio on solubility product PK8„
No. of samples Ratio 0.4 Ratio 0.8 S.E. Ferric hydroxide 10 36.12 35.82 +0.030 Aluminium hydroxide 7 33.36 33.21 + 0.053 Ferric phosphate 10 33.34 32.92 +0.034 Aluminium phosphate 7 30.35 30.08 + 0.055
Table 2. Mean ionic concentrations, pH and solubility product constants in 0.01 M -CaCl2 extracts Ionic concentration pKs,, P Fe Al Field Field treatment six 10° PH FePO4.2H20 A1PO4. 2H,0 Fe (OH), Al (OH), 318 0 1.25 4.75 4.50 4.84 33.14 30.36 36.33 33.56 (Exp. C2) 14} 0.82 2.60 3.19 4.87 33.54 30.65 36.56 33.67 0.69 3.20 2.56 5.10 33.51 30.43 36.23 33.16 2 0.66 3.32 4.31 5.07 33.54 30.29 36.28 33.02 4 cwt TSP 1.28 4.15 3.80 4.87 33.16 30.36 36.37 33.58 8 0.85 3.02 3.40 4.80 33.45 30.65 36.56 33.77 16 6.30 2.82 4.18 5.06 32.60 29.32 36.35 33.08 S.E. ±0.211 ±0.601 ±1.210 ±0.112 ±0.114 ±0.227 ±0.147 ±0.273 n.F. 7 7 7 7 7 7 7 7 317 (S925) 0 0.79 3.40 1.92 5.19 33.43 30.32 36.10 33.00 Lukuba (S926) 0 2.15 3.94 5.92 32.94 35.30 153 0 0.50 4.00 3.89 5.19 33.56 30.22 36.04 32.70 154 Lime+TSP 1.06 4.29 6.23 33.28 35.00 TSP = triple superphosphate. Standard errors are available only for data from field 318.
284 P. H. LE MARE a 37.0
N OV 6=-1.07±0.057 36.5 33.5 v4 (c) X.11, S.E.± S.E.± 14 (a)
0 •(d) ( 0 11, 2
"6" 36.0 .
(c) 4 33.0 PO Fo 35.5 32.5
35 0 32 0
34.0 31.0 ■13 b = -130 ±0.257
ov S.E.± 33.5 30.5 S.E.± O (a) • • VD • • (c) 0 33.0 (?)" 30.0 71'
32.5 29.5 •
I 29.0 4 5 5.0 5.5 6.0 6.5 5.0 5.5 6.0 6.5 pH pH2PO4 Fig. 1. Relationships between observed pK„ and pH Fig. 2. Relationships between observed pK„ and for iron and aluminium hydroxides. Symbols. Triple pH2PO4 for iron and aluminium phosphates. superphosphate (cwt/acre); None, 0 ; V; 1, A ; Symbols as for Fig. 1. 2, • ; 4, • ; 8, 1=1; 16, n. soil 925, • (a); soil 926, Y (b); field 163, • (C) ; field 154, • (d). tively indicate that pK„, was a continuous func- tion of the independent variable, and the absence of points parallel to its axis indicates that the data DISCUSSION were not derived from unique compounds. How- The results of the comparison of pK„ values at ever, the maximum values obtained with the 0.4 the two soil : solution ratios indicate that true values soil:solution ratio were within the ranges quoted were not obtained for any of the compounds, and by Chakravarti & Talibudeen (1962) for the hydr- that the values determined at the 0.4 ratio are oxide minerals goethite (35.5-38.7) and gibbsite probably greater, by up to 0.4 pK unit, than the (31.7-33.8), and the mean value obtained for alu- values at the greater soil:solution ratio. This im- minium phosphate, excluding plots treated with plies that at the smaller ratio equilibrium was not 16 cwt per acre triple superphosphate, agreed well reached and that incomplete solubility of the com- with Bache's (1963) value of 30.5 for variscite. The pounds had occurred; possibly even at the larger maximum value obtained for iron phosphate, 33.5, ratio the true solubility products were not achieved. was too mall to be consistent with Bache's value In the absence of evidence that true values were of 34.3 for strengite. obtained suggestions concerning the nature of the Although the maximum values obtained may compounds must be tentative. have been overestimated at the 0.4 soil:solution The relationships of pK„ with pH and pH2PO4 ratio the true values for the hydroxides would prob- for the hydroxides and for iron phosphate respec- ably fall within the values quoted for the minerals. Effects of phosphate applied to a Buganda soil. III 285 Hence, at the lowest soil pH values examined, 4.8- phate dressing had been given; but they provide 5.0, the data are consistent with the possibility insufficient evidence to confirm this and Bache's that iron and aluminium concentrations may have (1963) work suggests that variscite is unlikely to been controlled by goethite and gibbsite; but the occur at pH greater than 3.1. Although the data linear relationship between pK„ and pH indicates do not establish the nature of the phosphate com- that above pH 5 these compounds are unlikely to pounds concerned they have shown that at dres- have been operative. sings of 8 cwt per acre or less triple superphosphate For the phosphates the data show that strengite is converted, within two years of its application did not affect the phosphate concentration in any to the acid Buganda clay loam, to a very insoluble of the samples, and the soils behaved as though form, having solubility of the same order as vari- supersaturated with strengite. The data are consis- scite ; but 16 cwt provides a much greater phosphate tent with the possibility that variscite controlled concentration two years after its application, and the concentration except where the heaviest phos- the soils appear supersaturated with variscite.
REFER ENCES ASLYNG, H. C. (1954). The lime and phosphate poten- on the response curve to superphosphate. J. agric. tials of soils; the solubility and availability of phos- Sci., Comb. 70, 271-79. phates. Yb. R. vet. agric. Coll., Copenhagen, pp. 1-50. LINDSAY, W. L. & MORENO, E. C. (1960). Phosphate BACHE, B. W. (1963) Aluminium and iron phosphate phase equilibria in soils. Proc. Soil Sci. Soc. Am. 24, studies relating to soils. I. Solution and hydrolysis of 177-82. variscite and strengite. J. Soil Sci. 14, 113-123. LINDSAY, W. L., PEECH, M. & CLARK, J. S. (1959). BACHE, B. W. (1964). Aluminium and iron phosphate Solubility criteria for the existence of variscite in studies relating to soils. II. Reactions between phos- soils. Proc. Soil Sci. Soc. Am. 23, 357-60. phate and hydrous oxides. J. Soil Sci. 15, 110-16. SILLtist, L. G. (1959). Quantitative studies of hydrolytic CHARRAVARTI, S. N. & TALIBUDEEN, 0. (1962). Phos- equilibria. Q. Rev. chem. Soc. 13, 146-68. phate equilibria in acid soils. J. Soil Sci. 13, 231-40. WATANABE, F. S. & OLSEN, S. R. (1962). Colorimetric LE MARE, P. H. (1968a). Experiments on the effects of determinations of phosphorus in water extracts of phosphate in a Buganda soil. I. Pot experiments on soil. Soil Sci. 93, 183-8. the response curve. J. agric. Sci., Camb. 70, 265- WRIGHT, B. C. & PEECH, M. (1960). Characterisation of 70. phosphate reaction products in acid soils by the appli- LE MARE, P. H. (1968b). Experiments on the effects of cation of solubility criteria. Soil Sci. 90, 32-43. phosphate in a Buganda soil. II. Field experiments
18 Agri. Sci. COTTON RESEARCH CORPORATION 12 CHANTREY HOUSE, ECCLESTON STREET, S.W.1.
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PROGRESS REPORTS FROM EXPERIMENT STATIONS. The reports for the seasons up to and including 1950-51 were published in seasonal volumes; the following are still in print and available at 3s. per volume. 1923-25 (one volume), 1926-27, 1935-36, 1942-43, and 1945-46 to 1950-51. From 1951-52 the reports from each territory were published individually each season. A few of the early reports are out of print, but all others are available at 2s. 6d. per report or £1 for the seasonal set.
ANNUAL REPORTS. The Annual Reports of the Corporation and booklets describing its work are available on application, free of charge. BOOKS. The Insect Pests of Cotton in Tropical Africa. E. 0. Pearson. 1958. Obtainable from Commonwealth Institute of Entomology, price 40s. (Joint publication C.R.C. and C.I.E.) The Application of Genetics to Cotton Improvement. Sir Joseph Hutchinson. 1959. Obtainable from Cambridge University Press, price 15s. (Joint publication C.R.C. and C.U.P.) The Wild Species of Gossypium. J. H. Saunders. 1961. Obtainable from Cotton Research Corporation, price 15s. (Joint publication C.R.C. and O.U.P.)
RESEARCH MEMOIRS. This series comprises reprints of major scientific papers published by the staff of the Corporation since the war, as listed below. All except those marked with an asterisk as out of print are available at 2s. 6d. each. • NO. 1. Breeding Cotton Resistant to Blackarm Disease. R. L. Knight. 1946. * No. 2. Notes on the Classification and Distribution of Genera related to Gossypium. J. B. Hutchinson. 1947. • No. 3. The Role of Major Genes in the Evolution of Economic Characters. R. L. Knight. 1948. • No. 4. The Development of Internal Boll Disease of Cotton in Relation to Time of Infection. E. 0. Pearson. 1948. * No. 5. Observations on the Development of the Cotton Boll, with Particular Reference to Changes in Susceptibility to Pests and Diseases. R. C. Rainey. 1948. LIST OF PUBLICATIONS
No. 6. Experimental Methods with Cotton-Sulphuric Acid Treatment of Cotton Seeds. Effects of Gap-filling on Development and Yield. D. MacDonald, W. L. Fielding and D. F. Ruston. Ginning Percentage and its Determination in Variety Trials. Determination of Staple Length in Single Plant Selections and Variety Trials. W. L. Fielding. 1948. * No. 7. Jassid Resistance and Hairiness of the Cotton Plant. F. R. Parnell, H. E. King and D. F. Ruston. 1949. No. 8. Response of Cotton to Leaf-Curl Disease. J. B. Hutchinson, R. L. Knight and E. 0. Pearson. 1950. * No. 9. Confidence Limits of Expected Monthly Rainfall. H. L. Manning. 1951. No. 10. The Distribution of Wild Species of Gossypium in the Sudan. R. L. Knight. The Cytological Relationships of Gossypium somalense: Giirke. H. Douwes. 1951. No. 11. The Evolution of Blackarm Resistance in Cotton. R. L. Knight and J. B. Hutchinson. 1951. No. 12. Intra-Specific Differentiation in Gossypium hirsutum. J. B. Hutchinson. 1951. No. 13. The Genetics of Withering or Deciduous Bracteoles in Cotton. R. L. Knight. Colchicine Treatment of Young Cotton Seedlings as a Means of Inducing Polyploidy. H. Douwes. 1952. No. 14. The Genetics of Jassid Resistance in Cotton. I. The Genes H1 and H2. R. L. Knight. 1952. No. 15. Bacterial Blight of Cotton. G. M. Wickens. 1953. No. 16. The Cytological Relationships of Gossypium areysianum Deflers. H. Douwes. 1953. No. 17. (a) A Cotton Stainer (Dysdercus superstitiosus Fabr.) as a Potential Pest of Sorghum. (b) Studies of Lygus vosseteri Popp. (Heteroptera, Miridae). A Pest of Cultivated Cotton in East and Central Africa. (c) The Sorghum Midge Contarinia sorghicola (Coq), in East Africa. Q. A. Geering. 1953. No. 18. The Genetics of Jassid Resistance in Cotton. II. Pubescent T611. III. The Kapas Purao, Kawanda punctatum and Philippines Ferguson Group. IV. Transference of Hairiness from Gossypium herbaceum to G. barbadense. R. L. Knight and J. Sadd. 1954. No. 19. New Evidence on the Origin of the Old World Cottons. J. B. Hutchinson. 1954. No. 20. The Biology of Red Bollworm, Diparopsis watersi (Roths.), in Northern Nigeria. Q. A. Geering and A. F. H. Baillie. 1955. No. 21. Cotton Breeding in the Sudan. R. L. Knight. 1955. No. 22. Botanical Studies in Cotton Quality. J. P. Evenson. 1956. No. 23. The Statistical Assessment of Rainfall Probability and its Application in Uganda Agriculture. H. L. Manning. 1956. * No. 24. Vascular Infection of Cotton by Xanthomonas malvacearum (E. F. Smith) Dowson. G. M. Wickens. 1956. No. 25. On the Estimation of Water Losses by Evaporation under Equatorial Conditions. Sir Joseph Hutchinson and H. G. Farbrother. 1956. LIST OF PUBLICATIONS
No. 26. Yield Improvement from a Selection Index Technique with Cotton. H. L. Manning. 1957. No. 27. On an Electrical Resistance Technique for the Study of Soil Moisture Problems in the Field. H. G. Farbrother and L. E. Harrisson. 1957. No. 28. Studies of Crop Loss following Insect Attack on Cotton in East Africa. K. S. McKinlay, Q. A. Geering and T. H. Coaker. 1958. No. 29. Treatment of Cotton Seed against Bacterial Blight (Xanthomonas malvacearum (E. F. Smith) Dowson). G. M. Wickens. 1958. No. 30. On the Characterization of Tropical Rainstorms in Relation to Runoff and Percolation. Sir Joseph Hutchinson, H. L. Manning and H. G. Farbrother. 1958. No. 31. Bacterial Boll Rot of Cotton (Xanthomonas malvacearum (E. F. Smith) Dowson). I. A Comparison of Two Inoculation Techniques for the Assessment of Host Resistance. C. Logan. 1958. No. 32. Some Effects of Feeding by Lygus vosseleri Popp. (Heteroptera Miridae) on the Stem Apex of the Cotton Plant. J. E. Dale and T. H. Coaker. 1958. • No. 33. Crop Water Requirements of Cotton. Sir Joseph Hutchinson, H: L Manning and H. G. Farbrother. 1959. No. 34. Studies in Wild Species of Cotton. 1. Variation within Gossypium anomalum. J. H. Saunders. 1959. , No. 35. Cotton Research and the Development of the Commercial Crop in the Sudan. M. F. Rose. 1960. No. 36. Investigations on Heliothis armigera (Hb.) in Uganda. T. H. Coaker. 1960. No. 37. The Use of a Selection Index Technique in the Analysis of Progeny Row Data. J. T. Walker. 1960. No. 38. Some Effects of the Continuous Removal of Floral Buds on the Growth of the Cotton Plant. J. E. Dale. 1960. No. 39. The Effects of Different Plant Foods on the Fecundity, Fertility, and Development of a Cotton Stainer, Dysdercus superstitiosus (F.). Q. A. Geering and T. H. Coaker. 1960. No. 40. Intraseasonal Variation in Boll Characters in African Upland Cotton. J. P. Evenson. 1960. No. 41. Some Effects of Gibberellic Acid on Cotton. M. Dransfield. 1961. No. 42. A Record of Cotton Breeding for the Lake Province of Tanganyika : Seasons 1939-40 to 1957-58. J. E. Peat and K. J. Brown. 1961. No. 43. The Mechanism of Hairiness in Gossypium. 1. Gossypium hirsutum. J. H. Saunders. 1961. No. 44. Rainfall Conservation and the Yield of Cotton in Northern Nigeria. D. A. Lawes. 1962. No. 45. Observations on Carcelia evolans (Wied.) (Diptera, Tachinidae), a Parasite of Diparopsis watersi (Roths.) (Lepidoptera, Noctuidae), in Northern Nigeria. W. Reed and M. A. Choyce. 1962. No. 46. The Effect of Ridging on the Cotton Crop in the Eastern Province of Uganda. P. D. Walton. 1962. No. 47. The Biology and Control of the Sudan Bollworm, Diparopsis watersi (Roths.), in the Abyan Delta, West Aden Protectorate. J. H. Proctor. 1962. LIST OF PUBLICATIONS
No. 48. The Yield Responses of Rain-grown Cotton, at Ukiriguru in the Lake Province of Tanganyika. I. The Use of Organic Manure, Inorganic Fertilizers, and Cotton-seed Ash. II. Land-resting and Other Rotational Treatments Contrasted with the Use of Organic Manure and Inorganic Fertilizers. J. E. Peat and K. J. Brown. 1963. No. 49. The Control of Bacterial Blight in Rain-grown Cotton. I. Breeding for Resistance in African Upland Varieties. M. H. Arnold. 1963. No. 50. Realized Yield Improvement from Twelve Generations of Progeny Selection in a Variety of Upland Cotton. H. L. Manning. 1963. No. 51. The Genetics of Bacterial Blight Resistance in Cotton. Further Evidence on the Gene Bum. J. H. Saunders and N. L. Innes. 1963. No. 52. Water Requirements of Irrigated Maize in Nyasaland. J. M. Munro and R. A. Wood. 1964. No. 53. Resistance Conferred by New Gene Combinations to Bacterial Blight of Cotton. N. L. Innes. 1964. No. 54. Nitrate Studies in a Fertilizer Experiment in Sukumaland, Tanganyika. A. M. Kabaara. 1965. No. 55. Modal Selection in Upland Cotton. J. T. Walker. 1965. No. 56. The Mechanism of Hairiness in Gossypium. 2. Gossypium barbadense-the Inheritance of Stem Hair. 3. Gossypium barbadense -the Inheritance of Upper Leaf Lamina Hair. 4. The Inheritance of Plant Hair Length. J. H. Saunders. 1965. No. 57. The Empirical Relation between Solar Radiation and Hours of Bright Sunshine near Kampala, Uganda. D. A. Rijks and P. A. Huxley. 1965. No. 58. Inheritance of Resistance to Bacterial Blight of Cotton. 1. Allen (Gossypium hirsutum) derivatives. 2. Intra-herbaceum crosses. N L. Innes. 1965. No. 59. The Control of Bacterial Blight in Rain-grown Cotton. II. Some Effects of Infection on Growth. and Yield. M. H. Arnold. 1965. No. 60. 1. Sakel Strains of Cotton Highly Resistant to Bacterial Blight. 2. Resistance to Bacterial Blight of Cotton: the Genes B9 and Bio. N. L. Innes. 1965. No. 61. Heliothis armigera (Hb.) (Noctuidae) in Western Tanganyika. 1. Biology, with Special Reference to the Pupal Stage. 2. Ecology and Natural and Chemical Control. W. Reed. 1965. No. 62. The Use of Water by Cotton Crops in Abyan, South Arabia. D. A. Rijks. 1966. No. 63. Genetics of Hairiness Transferred from Gossypium raimondii to G. hirsutum. J. H. Saunders. 1966. No. 64. Inheritance of Resistance to Bacterial Blight of Cotton. 3. Herbaceum resistance transferred to tetraploid cotton. N. L. Innes. 1966. No. 65. Cotton and Cotton Research in Africa. J. M. Munro. 1966. No. 66. Reclamation of Ancient Agricultural Soils in Wahidi, South Arabia. A. B. Hearn, D. A. Rijks and D. E. Wilcox. 1966. No. 67. The Distribution of Empoasca lybica (de Berg.) (Hemiptera, Cicadellidae) on Cotton in the Sudan. D. E. Evans. 1966. No. 68, Water Use by Irrigated Cotton in Sudan. I. Reflection of Short-wave Radiation. D. A. Rijks. 1967. No. 69. On the Biology and Ecology of Lygus vosseleri (Heteroptera : Miridae) with Special Reference to its Hostplant Relationships. G. 0. Stride. 1968, LIST OF PUBLICATIONS
No. 70. Experiments on the Effects of Phosphate Applied to a Buganda Soil. I. Pot Experiments on the Response Curve. II. Field Experiments on the Response Curve to Triple Superphosphate. III. A Chemical Study of the Soil Phosphate, the Fate of Fertilizer Phosphate and the Relationship with Iron and Aluminium. P. H. Le Mare. 1968.
MISCELLANEOUS. A very large number of papers written in whole or part by members of the staff have not been reprinted as Memoirs, and are not in general available for sale. A complete list of such papers published in 1964 to 1967 is printed below. The Corporation's journal is referred to as "Cotton Growing Review" throughout, since the volumes continued to be numbered consecutively when the former title "Empire Cotton Growing Review" was changed.
1964. Brown, A. G. P. Field trials of three Fusarium wilt resistant cotton selections in Tanganyika. Cotton Growing Review, Vol. 41, No. 3, p. 194. Cross, J. E. Breeding for resistance to a more pathogenic variant of Xanthomonas malvacearum. Cotton Growing Review, Vol. 41, No. 1, p. 38. Cross, J. E. Field differences in pathogenicity between Tanganyika populations of Xanthomonas malvacearum. Cotton Growing Review, Vol. 41, No. 1, p. 44. Cross, J. E. and Hayward, A. C. Relationship between pathogenicity and phage type in Xanthomonas malvacearum. Cotton Growing Review, Vol. 41, No. 1, p. 49. Dransfield, M. Development of commercial seed dressing in Northern Nigeria. Cotton Growing Review, Vol. 41, No. 4, p. 261. Dransfield, M. Phytotoxicity of liquid mercurial seed dressings to cotton. Cotton Growing Review, Vol. 41, No. 4, p. 266. Dransfield, M. Pre-emergence mortality of cotton in cold weather. Cotton Growing Review, Vol. 41, No. 4, p. 273. Evans, D. E. Status of American bollworm on cotton at Sennar, Sudan. Cotton Growing Review, Vol. 41, No. 3, p. 202. Innes, N. L. Sudan strains of cotton resistant to bacterial blight. Cotton Growing Review, Vol. 41, No. 4, p. 285. Morris, D. A. Variation in the boll maturation period of cotton. Cotton Growing Review, Vol. 41, No. 2, p. 114. Morris, D. A. Capsule dehiscence in Gossypium. Cotton Growing Review, Vol. 41, No. 3, p. 167. Reed, W. Problems posed by early sowing of cotton in Lake Region, Tanganyika. Cotton Growing Review, Vol. 41, No. 4, p. 255. Saunders, J. H. Genetics of hairiness transferred from Gossypium anomalum to G. barbadense. Cotton Growing Review, Vol. 41, No. 1, p. 16. Stride, G. 0. Studies on the chemical basis of host plant selection in the genus Epilachna (Coleoptera, Coccinellidae). I. A volatile phagostimulant in Solanum campylacanthum for Epilachna fulvosignata. Journal of Insect Physiology, Vol. 11, No. 1, p. 21. White, M. H. Technical Assistance. Cotton Growing Review, Vol. 41, No. 3, p. 220. LIST OF PUBLICATIONS
Wickens, G. M. Fusarium wilt of cotton : seed husk a potential means of dissemination. Cotton Growing Review, Vol. 41, No. 1, p. 23. Wickens, G. M. Methods for detection and selection of heritable resistance to Fusarium wilt of cotton. Cotton Growing Review, Vol. 41, No. 3, p. 172.
1965. Anthony, K. R. M. and Moormann, F. R. Agricultural problems and potentialities of a hill tribe area in Thailand. Tropical Agriculture, Vol. 42, No. 2, p 97. Brown, K. J. Response of three strains of cotton to flower removal. Cotton Growing Review, Vol. 42, No. 4, p. 279. Dale, J. E. Abscission of cotton pedicels following removal of the boll and bracts. Cotton Growing Review, Vol. 42, No. 1, p. 52. Dale, J. E. and Milford, G. F. The role of endogenous growth substances in the fruiting of Upland cotton. New Phytologist, Vol. 64, No. 1, p. 28. Evans, D. E. Jassid populations on three hairy varieties of Sakel cotton. Cotton Growing Review, Vol. 42, No. 3, p. 211. Jackson, J. E., Schultz, L. R., and Faulkner, R. C. Effect of jassid attack on cotton yield and quality in the Sudan Gezira. Cotton Growing Review, Vol. 42. No. 4, p. 295. Lee, B. J. S. Man, plant breeding and cotton in Africa. World Crops, Vol. 17, No. 4, p. 75. Morris, D. A. Photosynthesis by the capsule wall and bracteoles of the cotton plant. Cotton Growing Review, Vol. 42, No. 1, p. 49. Mound, L. A. Effect of leaf hair on cotton whitefly populations in the Sudan Gezira. Cotton Growing Review, Vol. 42, No. 1, p. 33. Mound L. A. Effect of whitefly (Bemisia tabaci) on cotton in the Sudan Gezira. Cotton Growing Review, Vol. 42, No. 4, p. 290. Rijks, D. A. and Owen, W. G. Hydro-meteorological records from areas of potential agricultural development in Uganda. Ministry of Mineral and Water Resources, Uganda Government. Stride, G. 0. Chemical phagostimulation in Epilachna fulvosignata (Coleoptera, Coccinellidae). Summarized in Proceedings of XIIth International Congress of Entomology, London, 8-16 July 1964, p. 554. Wallace, M. E. and Gunn, R. E. Affinity in cotton. Heredity, Vol. 20, No. 2, p. 305.
1966. Anthony, K. R. M. and Jones, A. J. A cotton research programme in Thailand. Cotton Growing Review, Vol. 43, No. 4, p. 257. Brettell, J. H. Eleven years work in Abyan (South Arabia) by entomologists of the Corporation. Cotton Growing Review, VoL 43, No. 4, p. 286. Brown, K. J. An analysis of vegetative vigour in three strains of cotton. Cotton Growing Review, Vol. 43, No. 2, p. 107. Choyce, M. A. Pest and disease control of cotton in Northern Nigeria. Proceedings of the Science Association of Nigeria, Vol. 6, 1963, p. 53. Dransfield, M. The fungal air-spora at Samaru, Northern Nigeria. Transactions of the British Mycological Society, Vol. 49, No. 1, p. 121. Hearn, A. B. Cotton breeding in Abyan, 1958 to 1965. Cotton Growing Review, Vol. 43, No. 3, p. 196. LIST OF PUBLICATIONS
Hughes, L. C. Factors affecting numbers of ovules per loculus in cotton. Cotton Growing Review, Vol. 43, No. 4, p. 273. Innes, N. L. Performance under irrigation of medium staple cotton varieties introduced into the Sudan. Cotton Growing Review, Vol. 43, No. 1, p. 9. Innes, N. L. Promising selections from Albar A(57)12 in the Sudan. Cotton Growing Review, Vol. 43, No. 4, p. 263. Jones, G. B. and Anthony, K. R. M. Corporation Summer Meeting, 1966. Cotton Growing Review, Vol. 43, No. 4, p. 300. Lee, B. J. S. Corporation Summer Meeting, 1965. Cotton Growing Review, Vol. 43, No. 1, p. 56. Lee, B. J. S. Cotton in Western Nigeria. 1. History and recent developments Cotton Growing Review, Vol. 43, No. 2, p. 85. Munro, J. M. Trends in work on cotton at some research centres in the United States. Cotton Growing Review, Vol. 43, No. 1, p. 1. Munro, J. M. Cotton expansion in Kenya. Cotton Growing Review, Vol. 43, No. 2, p. 140. Perry, D. A. Multiplication of Xanthomonas malvacearum in resistant and susceptible cotton leaves. Cotton Growing Review, Vol. 43, No. 1, p. 37. Saunt, J. E. Effects of two harvesting methods on yield and quality of cotton in New South Wales. Cotton Growing Review, Vol. 43, No. 1, p. 22.
1967 Anthony, K. R. M. and Johnston, B. F. Notes on a visit to Mexico, September 1966. Cotton Growing Review, Vol. 44, No. 2, p. 100. Arnold, M. H. and Brown, S. J. A simple growth cabinet with controlled air temperatures. Cotton Growing Review, Vol. 44, No. 4, p. 284. Dransfield, M. and McDonald, D. The influence of cropping practice and application of farm yard manure on the soil microflora at Samaru, Northern Nigeria. Nigeria Agricultural Journal, Vol. 3, No. 2, p. 42. Hayward, J. A. Cotton in Western Nigeria. 2. Entomological problems. Cotton Growing Review, Vol. 44, No. 2, p. 117. Jackson, J. E., Faulkner, R. C. and Schultz, L. Razoux. Studies on the sowing date of cotton in the Sudan Gezira. III. The effect of sowing date on yield and quality under different fertilizer and spraying treatments. Journal of Agricultural Science, Vol. 69, No. 3, p. 329. Low, A. A preliminary study of introduced early maturing Russian varieties of Gossypium hirsutum. C.S.I.R.O. Plant Introduction Review, Vol. 3, No. 3, p. 39. Rijks, D. A. Optimum sowing date for yield: a review of work in the BP52 cotton area of Uganda. Cotton Growing Review, Vol. 44, No. 4, p. 247. Riggs, T. J. Response to modal selection in Upland cotton in Northern and Eastern Uganda. Cotton Growing Review, Vol. 44, No. 3, p. 176. Saunt, J. E. Sowing date, development and yield of cotton in the Murrumbidgee irrigation areas, New South Wales. Cotton Growing Review, Vol. 44, No. 1, p. 2. Saunt, J. E. Spindle picker and stripper harvester performance in New South Wales : a further comparison. Cotton Growing Review, Vol. 44, No. 3, p. 190. LIST OF PUBLICATIONS
Schultz, L. Razoux, Jackson, J. E. and Faulkner, R. C. Studies on the sowing date of cotton in the Sudan Gezira. IL The relationship between sowing date of cotton and the incidence of insect pests. Journal of Agricultural Science, Vol. 69, No. 3, p. 317. Walker, J. T. and Rijks, D. A. A computer programme for the calculation of confidence limits of expected rainfall. Experimental Agriculture, Vol. 3, No. 4, p. 337. Watson, J. S. Cotton Research Corporation Summer Meeting, 1967. Cotton Growing Review, Vol. 44, No. 4, p. 298. 204
APPENDIX II
THE GEOLOGY OF SOUTH MENGO DISTRICT, BUGANDA, WITH
SPECIAL REFERENCE TO SOURCES OF MANGANESE
Pallister (1959) described the geology of South Mengo district,
Buganda, and reported that manganese dioxide occurs (in economically
unimportant amounts) at several places. It was found in stream
concentrates from the Mate river and traced to brecciated quartz veins that occur in nearby hills (King, 1942); Wayland (1921) reported that
manganese was common in lateritic ironstones. These reports led me to
study the published Geological Survey, unpublished records in the
Geological Department at Entebbe, and the breccia ridge at Namulonge,
during a visit to Uganda in May 1972.
Namulonge (0° 31' N, 32° 37' E) is about 40 km north of Lake
Victoria, in Kyadondo county of Mengo. The physiography of Mengo was
described in nine divisions by Pallister (1957); of these, two are
important in relation to the field results at Namulonge. In the southern
area, round the north of the lake, there is a repeating pattern of flat-
topped hill, pediment slope and swamp-filled valley. In the central
area, in which Namulonge lies, rounded hills with well developed pediment
slopes are typical; there are very few flat-topped hills; many valleys
grow grass but papyrus occupies the larger water courses. Quartzite
hills mark the northern boundary but within the area steep sided narrow
ridges of brecciated quartzose rock strike northwest-southeast. 205
The three major features of the landscape of southern Mengo - the flat-topped and the rounded hills, and their valleys, were formed from peneplains by three major erosion cycles (Pallister, 1959). The flat- topped hills (of which there are about 400) are the remnants of mid- tertiary dissection of the Buganda peneplain, at 1300-1400 m, preserved by the thick resistant 'duri-crust' that formed its surface. Central and northern Mengo are on the Tanganyika surface formed from a lower peneplain. Its base level at 1130-1200 m is represented by scattered platforms of laterite or, as at Namulonge, by coarse well rounded gravels.
Lateritic ironstone occurs frequently, but is not as prominent as on the
Buganda surface: it is often covered by soil but on the lower slopes of the pediments a lateritic crust often emerges.
The Acholi surface, which occurs extensively further north in Acholi district, represents a valley floor peneplain at 1040 m and in Mengo this surface is now one of accumulation, not erosion.
Hepworth (1956) described the geology of the area round Namulonge.
Most of the area is on biotite-hornblende granitic gneiss with soda- potash felspar of the basement complex, and granites. There are also zones of schists impregnated by granitic material. All these rock types occur at Namulonge, which lies in an area where the crush breccia
(brecciated quartzose rock) is also common.
The crush breccia ridges indicate a group of tear-faults. They are composed of white and green quartz rock under the red breccia which, in the longest and widest ridge at Sempa, is between 1200 and 1300 m above sea level. Where the breccia ridges end near north flowing rivers, their direction is often continued by the course of swamps, streams or rivers, 206
probably following lines of easy erosion in the main fault direction; the Nasirye river, which is the northern boundary of Namulonge Research
Station, is the longest example of such erosion. The white quartz rock is almost pure but the green quartz contains chlorite and epidote. The breccia, with very few exceptions, lies over the quartz, its redness inceasing upwards; it contains finely divided haematite, limonite and pyroluSite. The distribution of crush breccia ridges and the swamp and river valleys are shown in Figure 11.1.
There are few remnants of the Buganda surface in the area so
Hepworth had difficulty correlating the breccia ridges with them, but the altitudes of Sempa (1300 m) and Nyakabango (1267 m) are similar to those of two high level laterites at 1290 m at Migade and Degeya, so the breccias probably derived their iron and manganese from laterites of the Buganda surface. Where laterite occurs at lower altitudes, on the slopes of pediments, it is associated with the Tanganyika surface and unrelated to the breccias.
207
Figure 11.1 Distribution of crush breccia in Mengo• district
(after Pallister, 1959) Scale 1:150,000
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,4 •9`.. i • r. y
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Nia•ux:, •••• )%1 lit 1I c=" I, It1I ;•--\c• n N. 1 '1 c_-...... ,. 1 1 i %. -....‘11 1., ‘ \ v,...... Ai %.., . ,....._, , ,....., •4 ,,;..... ---1 1 1 r ‘,..... , \., / •• -A , -, ..- l' I I / „..-...... -.I
....,, I
ts-A. :. ...."-.1,1 \ ■...... „,„ ia X 11 1 AYAZ/14-;,‘, \ t-=7.2.-0,-...1 s. • 1I1 /;"./.11 I cl.„.-2 I //, h brecc /,‘S. -5.. I \%%. 1 Ili j \ ( A I C)rk( ‘%\''& Crus
It I 1 • '.."?‘. (--/- • • , NN
i V , — 1 k ZT-s1 11 I I //'/S-, II •S / V ••‘. I / • I "• 208
APPENDIX III
Analysis of crop samples
0 Approximately 0.5 g of dry matter was ashed at 450 C for 2 hours;
5 ml of 6N hydrochloric acid was added.to the ash, digested for 15 minutes and then evaporated to dryness, another 5 ml of 6N HC1 was added, digested and evaporated; the residue was taken up in 5 ml 6N HC1 and made up to
100 ml with water; the solution was then filtered through No. 40
Whatman paper.
Phosphorus in the solution was determined by the method of Fogg and Wilkinson (1958), with a Technicon AutoAnalyzer; calcium (Salt,
1967) was determined in the early samples by flame emission on an
Unicam SP900 spectrophotometer, and later on an SP90 atomic absorption
spectrophotometer; manganese was determined by atomic absorption on
equipment built at Rothamsted (Rawson, 1973). 209
APPENDIX IV
Yields of dry matter, and concentrations of phosphorus
and manganese in crops
Ryegrass Experiment 1 (See Chapter 4)
Dry matter, g/pot
P applied, mg/pot
0 1 2 3 4 Mean
Cut 1
Soil 1 0.75 0.80 0.90 0.91 1.02 0.88 2 1.00 0.93 1.02 0.97 1.00 0.98 3 0.80 0.87 0.82 0.92 1.00 0.88 SE ±0.032 ±0.014 Mean 0.85 0.87 0.91 0.93 1.00 0.91 SE ±0.019
Cut 2 Soil 1 0.68 0.73 0.82 0.92 0.93 0.82 2 0.94 0.92 0.95 0.95 0.92 0.94 3 0.69 0.73 0.77 0.80 0.81 0.76 SE ±0.030 ±0.013 Mean 0.77 0.80 0.85 0.89 0.89 0.84 SE ±0.017
Cut 3 Soil 1 0.90 0.94 0.93 0.92 0.96 0.93 2 1.21 1.21 1.31 1.27 1.12 1.22 3 0.54 0.62 0.73 0.86 0.88 0.72 SE ±0.053 ±0.024 Mean 0.88 0.92 0.99 1.02 0.98 0.96 SE ±0.031
Total of 3 cuts soil 1 2.32 2.48 2.64 2.76 2.91 2.62 2 3.15 3.07 3.28 3.19 3.04 3.14 3 2.03 2.22 2.32 2.59 2.68 2.37 SE ±0.061 ±0.027 Mean 2.50 2.59 2.75 2.84 2.88 2.71 SE ±0.035 210
APPENDIX IV CONTINUED
Phosphorus in dry matter, %
P applied, mg/pot
0 1 2 3 4 Mean Cut 1
Soil 1 0.140 0.163 0.197 0.210 0.217 0.185 2 0.215 0.229 0.254 0.251 0.245 0.239 3 0.161 0.170 0.205 0.209 0.222 0.193 SE ±0.0080 ±0.0035 Mean 0.172 0.187 0.218 0.223 0.228 0.206 SE ±0.0046
Cut 2
Soil 1 0.119 0.131 0.141 0.151 0.167 0.142 2 0.246 0.259 0.285 0.295 0.301 0.277 3 0.122 0.139 0.153 0.175 0.178 0.154 SE ±0.0050 ±0.0023 Mean 0.162 0.177 0.193 0.207 0.215 0.191 SE ±0.0029
Cut 3
Soil 1 0.083 0.079 0.077 0.080 0.081 0.080 2 0.102 0.108 0.116 0.111 0.109 0.109 3 0.084 0.070 0.070 0.072 0.077 0.074 SE ±0.0029 ±0.0013 Mean 0.089 0.085 0.088 0.088 0.089 0.088 SE ±0.0016 211
APPENDIX IV CONTINUED
Manganese in dry matter, ppm
P applied, mg/pot
0 1 2 3 4 Mean Cut 1
Soil 1 153 158 158 147 152 153 2 145 138 134 136 151 141 3 147 151 150 154 150 150 SE ±5.6 ±2.5 Mean 148 149 147 146 151 148 SE ±3.2
Cut 2
soil 1 422 450 444 454 459 446 2 296 287 284 294 287 290 3 327 359 351 370 379 357 SE ±12.2 ±5.4 Mean 348 365 360 373 375 364 SE ±7.0
Cut 3
Soil 1 167 196 219 238 259 216 2 191 199 176 196 225 197 3 182 190 213 223 211 204 SE ±12.2 ±5.4 Mean 180 195 203 219 231 205 SE ±7.1 212
APPENDIX V
Yields of dry matter, and concentrations of phosphorus
and manganese in crops Ryegrass Experiment 2 (See Chapter 6)
Dry matter, g/pot
P applied, mg/pot None MCP TPS DCPR DCPD
Cut . Soil 0 3 12 3 12 0.75 3 12
1 Sendusu 1.06 1.03 1.05 1.00 1.02 0.91 0.91 0.93 Nalumuli 0.86 0.83 0.86 0.90 0.81 0.77 0.69 0.63 SE ±0.048 2 Sendusu 1.26 1.36 1.30 1.36 1.34 1.38 1.41 1.31 Nalumuli 0.95 0.98 1.03 0.94 1.03 0.94 1.03 1.03 SE ±0.046
3 Sendusu 1.13 1.19 1.27 1.18 1.26 1.20 1.19 1.19 Nalumuli 1.07 1.14 1.18 1.14 1.16 1.10 1.13 1.11 SE ±0.029
4 Sendusu 1.11 1.17 1.21 1.15 1.26 1.10 1.10 1.13 Nalumuli 1.02 1.14 1.23 1.14 1.18 1.03 1.12 1.09 SE ±0.030
5 Sendusu 1.18 1.23 1.39 1.19 1.35 1.17 1.24 1.26 Nalumuli 1.03 1.06 1.25 1.25 1.24 1.00 1.13 1.14 SE ±0.034 6 Sendusu 1.08 1.13 1.40 1.12 1.37 1.06 1.26 1.29 Nalumuli 1.02 1.02 1.15 1.05 1.16 1.00 1.11 1.15 SE ±0.033
7 Sendusu 0.68 0.72 0.86 0.72 0.84 0.68 0.83 0.83 Nalumuli 0.70 0.69 0.68 0.69 0.78 0.68 0.70 0.68 SE ±0.029 8 Sendusu 0.54 0.52 0.62 0.53 0.61 0.54 0.69 0.64 Nalumuli 0.60 0.63 0.69 0.63 0.71 0.55 0.58 0.67 SE -10.046 213
APPENDIX V CONTINUED
Phosphorus in dry matter, %
P applied, mg/pot
None MCP TPS DCPR DCPD
Cut Soil 0 3 12 3 12 , 0.75 3 12
1 Sendusu 0.215 0.254 0.316 0.263 0.328 0.242 0.228 0.243 Nalumuli 0.182 0.216 0.292 0.246 0.301 0.208 0.224 0.216 SE ±0.0082 2 Sendusu 0.116 0.138 0.186 0.127 0.197 0.129 0.127 0.125 Nalumuli 0.151 0.174 0.225 0.175 0.245 0.151 0.146 0.144 SE ±0.0063
3 Sendusu 0.133 0.158 0.195 0.151 0.200 0.141 0.151 0.147 Nalumuli 0.154 0.171 0.214 0.168 0.220 0.153 0.170 0.161 SE ±0.0031 4 Sendusu 0.112 0.119 0.145 0.117 0.142 0.115 0.119 0.121 Nalumuli 0.103 0.110 0.146 0.106 0.149 0.104 0.111 0.111 SE ±0.0027
5 Sendusu 0.111 0.117 0.147 0.118 0.145 0.114 0.122 -0.131 Nalumuli 0.117 0.118 0.140 0.114 0.135 0.118 0.117 0.124 SE ±0.0031 6 Sendusu 0.112 0.116 0.149 0.120 0.141 0.112 0.120 0.139 Nalumuli 0.118 0.120 0.154 0.129 0.136 0.125 0.130 0.131 SE ±0.0034 7 Sendusu 0.130 0.142 0.182 0.144 0.165 0.132 0.152 0.170 Nalumuli 0.144 0.150 0.186 0.177 0.166 0.149 0.160 0.172 8 Sendusu 0.134 0.145 0.184 0.156 0.184 0.147 0.147 0.197 Nalumuli 0.155 0.162 0.189 0.158 0.193 1.151 0.172 0.196
Note Cuts 7 and 8: crops of treatment replicates were bulked for analysis so standard errors are not available 214
APPENDIX V CONTINUED
Manganese in dry matter, ppm
P applied, mg/pot
None MCP TPS DCPR DCPD
Cut Soil 0 3 12 3 12 0.75 3 12 .
1 Sendusu 139 137 127 127 126 135 133 126 Nalumuli 116 103 111 116 106 110 110 116 SE ±3.8
2 Sendusu 214 204 187 185 193 198 208 198 Nalumuli 143 140 133 137 136 130 117 124 SE ±7.3
3 Sendusu 227 220 211 213 214 235 209 224 Nalumuli 150 153 155 149 156 148 149 145 SE ±8.8
4 Sendusu 225 228 229 213 227 220 224 234 Nalumuli 153 158 170 159 160 156 151 153 SE ±5.8
5 Sendusu 238 263 319 248 325 254 267 307 Nalumuli 185 184 200 196 198 166 196 181 SE ±12.9
6 Sendusu 312 326 382 309 377 307 327 345 Nalumuli 212 209 215 215 216 190 221 199 SE ±10.6
7 Sendusu 231 292 370 224 311 238 249 366 Nalumuli 215 190 222 182 220 198 210 197
8 Sendusu 272 326 339 282 358 335 342 437 Nalumuli 282 255. 236 260 248 265 247 206
Note Cuts 7 and 8: crops of treatment replicates were bulked for analysis so standard errors are not available
APPENDIX VI
Yields of dry matter, and concentrations,of calcium, phosphorus and manganese in the cotton Cotton Experiment 1 (See Chapter 8)
Dry matter, g/pot
Leaves Stems Roots Total Shed leaves
Soil Sendusu Nalumuli' Sendusu Nalumuli Sendusu Nalumuli Sendusu Nalumuli Sendusu Nalumuli Phosphorus, mg/pot
0 5.73 2.8o 3.96 1.87 2.88 1.53 12.58 6.20 0.795 0.587 2 5.54 3.07 4.62 1.80 2.55 1.33 12.70 6.21 0.915 0.610 4 4.94 3.15 4.00 2.00 2.20 1.46 11.14 6.61 0.930 0.615 6 5.51 3.24 5.01 1.99 2.52 1.71 13.04 6.95 1.047 0.580 SE ±0.295 ±0.309 ±0.210 ±0.721 ±0.1184
Without added iron 5.46 3.22 4.55 2.08 2.65 1.48 12.66 6.78 0.850 0.566 With added iron 5.40 2.91 4.25 1.76 2.43 1.54 12.08 6.21' 0.994 0.630 SE ±0.0209 ±0.219 ±0.148 ±0.510 ±0.0838
2.97 1.97 1.54 12.72 Without added Ca 5.5o 4.62 2.59 6.48 With added Ca 5.36 3.16 4.17 1.87 2.49 1.47 12.02 6.50
SE ±0.0209 ±0.219 ±0.148 ±0.510 APPENDIX VI CONTINUED
Calcium in dry matter, %
Leaves Stems Roots Shed leaves Soil Sendusu Nalumuli Sendusu Nalumuli Sendusu Nalumuli Sendusu Nalumuli Phosphorus applied mg/pot N, li 0 2.52 2.76 0.66 0.71 0.335 0.534 3.34 3.22 cy, 2 2.36 2.84 0.61 0.84 0.320 0.609 3.8o 3.56 4 2.54 2.8o 0.62 0.72 0.328 0.546 3.74 3.88 6 2.49 2.86 0.60 0.83 0.339 0.588 3.8o 3.60 SE ±0.092 ±0.037 ±0.0215 ±0.156
Without added iron 2.43 2.82 0.60 0.76 0.330 0.570 3.73 3.76 With added iron 2.53 2.81 0.64 0.79 0.332 0.569 3.62 3.37 SE ±0.065 -10.026 ±0.0152 ±0.110
Without added calcium 2.23 2.58 0.55 0.66 0.302 0.530 With added calcium 2.73 3.04 0.69 0.90 0.360 0.609 SE ±0.065 ±0.026 ±0.0152 APPENDIX VI CONTINUED
Phosphorus in dry matter, %
Leaves Stems Roots Shed leaves
Soil Sendusu Nalumuli Sendusu Nalumuli Sendusu Nalumuli Sendusu Nalumuli
Phosphorus applied mg/pot
0 0.121 0.191 0.056 0.066 0.102 0.168 0.052 0.052 2 0.137 0.194 0.062 0.078 0.111 0.182 0.062 0.064 4 0.160 0.223 0.062 0.073 0.117 0.185 0.078 0.079 6 0.152 0.217 0.063 0.088 0.118 0.182 0.075 0.069
SE ±0.0102 ±0.0032 ±0.0056 ±0.0061
Without added iron 0.139 0.198 0.059 0.074 0.111 0.178 0.068 0.068 With added iron 0.146 0.214 0.062 0.079 0.112 0.180 0.065 0.064
SE ±0.0072 ±0.0023 ±0.0040 ±0.0043
Without added calcium 0.138 0.206 0.059 0.071• 0.108 0.178 With added calcium 0.146 0.207 0.063 0.081 0.116 0.181 SE ±0.0072 ±0.0023 ±0.0040 APPENDIX VI CONTINUED
Manganese in dry matter,.%
Leaves Stems Roots Shed leaves
Soil Sendusu Nalumuli Sendusu Nalumuli Sendusu Nalumuli Sendusu Nalumuli
Phosphorus applied mg/pot . w 1--, 0 0.047 0.041 0.005 0.005 0.044 0.036 0.047 0.040 co 2 0.044 0.033 0.005 0.004 0.033 0.026 0.062 0.046 4 0.056 0.036 0.007 0.005 0.037 0.030 0.068 0.050 6 0.045 0.040 0.005 0.006 0.038 0.035' 0.067 0.056 SE ±0.0030 ±0.0007 ±0.0039 ±0.0062
Without added iron 0.047 0.037 0.006 0.005 0.037 0.028 0.057 0.053 With added iron 0.050 0.038 0.005 0.005 0.039 0.036 0.065 0.043
SE ±0.0021 ±0.0005 ±0.0028 ±0.0062
Without added calcium 0.049 0.040 0.007 0.006 0.037 0.029 With added calcium 0.048 0.035 0.004 0.004 0.039 0.035 SE ±0.0021 ±0.0005 ±0.0028 219
APPENDIX VIIA
Yields of dry matter, and concentrations of calcium,
phosphorus and manganese in cotton Cotton Experiment 2 (See Chapters 9 & 10)
Dry matter, g/pot
Applied Ca P Mn Leaves Stems Roots Total ppm
6 2 0.5 1.45 0.46 0.69 2.6o 90 2 0.5 1.13 0.30 0.55 1.98 6 3o 0.5 4.39 1.52 1.31 7.23 90 3o 0.5 7.43 2.89 2.35 12.68
6 2 4.5 1.29 0.51 0.65 2.45 90 2 4.5 1.85 0.46 0.81 3.13 6 3o 4.5 4.25 1.56 1.33 7.14 90 3o 4.5 8.83 3.57 2.72 15.12
6 2 8.5 1.21 0.43 0.58 2.22 90 2 8.5 1.87 0.48 0.77 3.12 6 30 8.5 2.64 1.39 0.97 5.00 90 3o 8.5 7.44 3.03 2.34 12.82
6 2 16.5 1.11 0.33 0.48 1.92 90 2 16.5 1.55 0.46 0.73 2.74 6 3o 16.5 2.7o 0.97 0.76 4.43 go 3o 16.5 6.65 2.58 1.85 11.08
SE -±$0.374 ±0.166 ±0.123 ±0.623 APPENDIX VIIA CONTINUED
% of calcium, phosphorus and manganese in dry matter
Manganese Applied Calcium Phosphorus Roots Leaves Stems Roots Ca P Mn Leaves Stems Roots Leaves Stems ppm 0.170 0.024 0.089 2 0.5 0.4 0.25 0.28 0.129 0.086 0.102 6 0.126 0.010 0.064 2 0.5 2.3 0.96 0.68 0.133 0.083 0.100 90 0.513 0.044 0.013 0.040 6 3o 0.5 0.3 0.20 0.41 0.620 0.329 w o.488 0.028 0.028 0.030 w 90 3o 0.5 3.2 0.94 0.82 0.629 0.294 0 0.247 0.063 0.123 2 4.5 0.5 0.25 0.24 0.127 0.108 0.096 6 0.138 0.016 0.073 90 2 4.5 2.3 0.93 0.57 0.127 0.087 0.089 0.518 0.190 0.078 0.104 6 3o 4.5 0.3 0.23 0.42 0.601 0.337 0.540 0.119 0.015 0.131 90 3o 4.5 3.0 0.89 0.83 0.612 0.323 0.103 0.313 0.127 0.205 6 2 8.5 0.5 0.21 0.27 0.124 o.146 0.095 0.194 0.031 0.105 90 2 8.5 2.5 1.04 0.60 0.125 0.100 0.568 0.390 0.162 0.201 6 3o 8.5 0.4 0.19 0.36 0.573 0.560 0.315 0.568 0.227 0.030 0.177 90 30 8.5 3.1 0.83 0.76 0.588 0.185 0.290 2 16.5 0.3 0.16 0.31 0.107 0.146 0.128 0.332 6 0.056 0.102 16.5 2.5 0.95 0.54 0.129 0.088 0.096 0.296 90 2 0.278 0.294 3o 16.5 0.4 0.18 0.36 0.487 0.763 0.568 0.414 6 0.565 0.326 0.049 0.304 90 3o 16.5 3.o 0.81 0.63 0.547 0.279 ±0.0175 ±0.021 SE ±0.09 ±0.035 ±0.068 ±0.0189 ±0.0126 ±0.0299 ±0.0175 221
APPENDIX VIIB
Assays of peroxidase activity and inhibition of IAA oxidation
in cotton leaves
(See Chapter 10)
Inhibition of IAA Peroxidase activity oxidation
Applied pmols purpurogallin Wt of leaf to halve per mg fresh leaf oxygen uptake, Ca P Mn ppm
6 2 0.5 0.312 180 90 2 0.5 0.174 23o 6 3o 0.5 0.067 52o 90 3o 0.5 0.092 1020
6 2 4.5 0.276 8o 90 2 4.5 0.169 230 6 3o 4.5 0.094 400 90 3o 4.5 0.094 460
6 2 8.5 0.266 8o 90 ,2 8.5 0.280 160 6 3o 8.5 0.134 200 90 30 8.5 0.064 58o
6 2 16.5 0.244 130 90 2 16.5 0.157 200 6 3o 16.5 0.180 220 90 30 16.5 0.083 1700
SE ±0.0378 222
APPENDIX VIII
Yields of dry matter and concentrations of calcium, phosphorus
and manganese in cotton Cotton Experiment 3 (See Chapter 11)
Dry matter, g/pot
Applied Ca P Leaves Stems Roots Total Shed leaves mmol 1-1 o o 1.94 0.76 0.70 3.39 0.53 0.1 0 1.50 0.43 0.39 2.32 0.56 0.2 0 1.72 0.60 0.49 2.81 0.55 0.4 0 1.86 0.66 0.59 3.10 0.54 0.8 0 1.67 0.50 0.52 2.68 0.55
0 0.2 4.34 3.02 1.80 9.16 0.52 0.1 0.2 4.99 3.91 1.74 10.64 0.57 0.2 0.2 4.90 3.80 1.83 10.53 0.69 0.4 0.2 4.71 3.45 1.74 9.89 0.52 0.8 0.2 5.79 4.21. 1.92 11.92 0.68
0 0.4 5.26 4.54 2.36 12.17 0.68 0.1 0.4 5.03 4.75 2.21 11.99 0.72 0.2 0.4 5.18 4.46 2.32 11.96 0.76 0.4 0.4 4.82 3.95 2.21 10.98 0.73 0.8 0.4 5.61 4.76 2.26. 12.63 0.83
o 0.8 5.05 4.66 2.16 11.87 0.65 0.1 0.8 4.5o 3.93 1.81 10.24 0.82 0.2 0.8 4.74 4.26 2.01 11.01 0.77 0.4 0.8 5.28 4.66 2.61 12.55 0.77 0.8 0.8 5.15 4.34 2.13 11.62 0.82
SE ±0.287 ±0.310 ±0.156 ±0.684 ±0.062 223
APPENDIX VIII CONTINUED
Calcium in dry matter, %
Applied Ca P Leaves Stems Roots Shed leaves mmol 1-1 o 0 2.13 0.68 0.49 3.37 0.1 0 2.32 0.88 0.56 3.11 0.2 0 2.46 0.87 0.61 3.58 0.4 .0 2.52 0.85 0.50 3.8o 0.8 0 2.74 0.99 0.63 3.90 o 0.2 1.46 0.35 0.43 3.26 0.1 0.2 1.44 0.35 0.40 3.38 0.2 0.2 1.48 0.38 0.42 3.46 0.4 0.2 1.83 0.44 0.44 3.70 0.8 0.2 2.10 0.52 0.52 4.08
O 0.4 1.22 0.27 0.40 3.46 0.1 0.4 1.19 0.29 0.41 3.71 0.2 0.4 1.45 0.34 0.39 3.5o 0.4 0.4 1.76 0.39 0.41 3.85 0.8 0.4 1.99 0.52 0.49 4.21
0 0.8 1.26 0.28 0.37 3.54 0.1 0.8 1.38 0.33 0.43 3.45 0.2 0.8 1.45 0.35 0.47 3.82 0.4 0.8 1.78 0.39 0.45 3.88 0.8 0.8 2.05 0.58 0.51 4.4o
SE ±0.070 ±0.025 ±0.024 ±0.169 224
APPENDIX VIII CONTINUED
Phosphorus in dry matter, %
Applied
Ca P Leaves Stems Roots Shed leaves -1 mmol 1 o o 0.118 0.049 0.090 0.048 0.1 0 0.124 0.056 0.096 0.059 0.2 o 0.116 0.054 0.098 0.054 0.4 0 0.109 0.050 0.090 0.051 0.8 0 0.114 0.064 0.096 0.056
O 0.2 0.296 0.112 0.214 0.058 0.1 0.2 0.288 0.11 1 0.233 0.064 0.2 0.2 0.306 0.125 0.234 0.072 0.4 0.2 0.294 0.118 0.231 0.055 0.8 0.2 0.289 0.108 0.224 0.104
0 0.4 0.432 0.160 0.342 0.098 0.1 0.4 0.448 0.156 0.324 0.108 0.2 0.4 0.432 0.174 0.324 0.137 0.4 0.4 0.478 0.158 0.330 0.132 0.8 0.4 0.454 0.160 0.376 0.106 o 0.8 0.530 0.210 0.516 0.153 0.1 0.8 0.580 0.206 0.539 0.202 0.2 0.8 0.608 0.212 0.568 0.224 o.4 0.8 0.590 0.217 0.560 0.204 0.8 0.8 0.562 0.194 0.533 0.208
SE ±0.0134 ±0.0092 ±0.0225 225
APPENDIX VIII CONTINUED
Manganese in dry matter, %
Applied
Ca P Leaves Stems Roots Shed leaves -1 mmol 1 o 0 0.024 0.006 0.016 0.049 0.1 o 0.019 0.005 0.011 0.034 0.2 0 0.016 0.004 0.015 0.041 0.4 0 0.014 0.003 0.017 0.040 0.8 0 0.015 0.003 0.015 0.038
0 0.2 0.052 0.011 0.024 0.070 0.1 0.2 0.045 0.008 0.020 0.065 0.2 0.2 0.032 0.007 0.020 0.061 0.4 0.2 0.022 0.003 0.016 0.049 0.8 0.2 0.016 0.003 0.012 0.049
0 0.4 0.086 0.016 0.030 0.103 0.1 0.4 0.050 0.011 0.027 0.082 0.2 0.4 0.034 0.009 0.023 0.065 0.4 0.4 0.024 0.004 0.019 0.062 0.8 0.4 0.015 0.003 0.015 0.048
O 0.8 0.103 0.019 0.027 0.116 0.1 0.8 0.076 0.012 0.025 0.094 0.2 0.8 0.059 0.008 0.023 0.106 0.4 0.8 0.036 0.006 0.026 0.075 0.8 0.8 0.015 0.003 0.015 0.048
SE ±0.0036 ±0.0013 ±0.0040 ±0.0054 226
APPENDIX IX
Concentrations of phosphorus in water extracts of soil 35 and 80
days after applying monocalcium phosphate and triple-point
solution to the soil surface
(See Chapter 5)
Phosphorus in extract, ppm
35 days 80 days Soil Source of section soil 'soil soil soil soil soil phosphorus depth, mm 1 2 3 1 2 3
MCP 0-5 20.50 10.82 10.79 12.44 11.18 11.90 6-10 9.72 11.60 6.70 4.82 6.20 5.30 11-15 2.65 5.15 2.72 2.02 2.72 2.25 16-20 0.32 1.15 0.42 0.50 1.00 0.32 21-25 0.12 0.60 0.22 0.30 0.32 0.25
TPS 0-5 9.70 3.35 10.79 6.52 4.45 5.75 6-10 10.28 10.42 6.70 5.60 8.05 6.15 11-15 3.58 4.80 2.72 2.50 4.78 2.70 16-20 0.35 0.80 0.42 0.45 1.35 0.72 21-25 0.15 0.32 0.22 0.05 0.35 0.25
227
APPENDIX X
Composition of nutrient solutions applied to cotton
grown in Sendusu soil
Cotton Experiment 3 (See Chapter 11)
Twenty stock solutions were prepared from basic solutions of the salts used to supply the nutrients. The concentrations of the basic solutions, and the volumes mixed to make the stock solutions, are shown
below and in the accompanying table. The mixtures of basic solutions were made to 1 litre with de-ionised water to make the stock solutions; these were diluted 1:20 with de-ionised water for applying to the cotton plants.
Basic solutions
Volume of solution in 1 1 of stock solution Major nutrients
Ca(H PO ) .H 0 6.3 g 1-1 2 4 2 2 See table Ca(NO ) .4H 0 23.6 g 1-1 3 2 2 See table NH H PO 23.0 g 1-1 4 2 4 See table KNO 101.1 g 1-1 30 ml 3 Mg(NO ) .6H 0 128.2 g 1-1 3 2 2 10 ml MgSO4.7H20 49.2 g 1-1 20 ml -1 NH4NO 40.0 g 1 3 See table
Micro-nutrients
H3B0 2.86 g 3 cus0 .5H 0 0.08 g 4 2 per litre 10 ml ZnS0 .7H 0 0.22 g 4 2 Na2Mo04.2H20 0.063g 228
APPENDIX X CONTINUED
Volumes of basic solutions in 1 litre of stock solution
Basic solution Stock solution concentration of Ca(H PO ).H 0 Ca(NO ).4H 0 NH H PO NH NO 2 4 2 2 3 2 2 4 2 4 4 3 Ca P 6.3 g 1-1 23.6 g 1-1 23.0 g 1-1 40.0 g 1-1 mmol 1-1 ml
0 0 o o o 16 2 o 0 20 0 12 4 o 0 40 0 8 8 0 0 80 0 0 16 o o 160 0 0
O 4 o o 20 12 2 4 80 0 o 16 4 4 80 20 0 12 8 4 80 60 o 4 16 4 80 140 0 0
0 8 0 o 40 8 2 8 80 o 20 12 4 8 16o 0 o 16 8 8 160 40 0 8 16 8 160 120 o o
O 16 o o 80 0 2 16 80 0 6o 4 4 16 160 0 40 8 8 16 320 0 0 16 16 16 32o 80 0 0