Monthly communications by the International Potash Institute, Berne (Switzerland) Subject 12 Industrial crops No. 4/1979 21st suite

O Fertilization of the cotton on some soils of the States of and Barinas, (Fertilicaci6n del algod6n en algunos suelos de los Estados portuguesa y Barinas) 1 , , 2 L. Avilln Rovira L. Vilain2 J. Abreu2 and R. Salas From: Agronomia Trop., Vol. 26, no. 5, 425-439 (1976), Venezuela.

Summary During the years 1969 and 1973, CENIAP, CIARCO and ANCA carried out several experiments in the States of Portuguesa and Barinas with the purpose of. determining the effects of nitrogen, phosphorus and potassium applications on the production of cotton. A randomised block design with fourfold replication of each treatment was employed. The plant varieties tested were Delta Pine Smooth Leaf and Delta Pine 16. Soils were chosen so as to represent three groups, classified according to physical and chemical characteristics: First group: Much weathered soils, acid, of low natural fertility, well drained. Second group: Lightly weathered soils, moderately acid, of medium to high natural fertility, well drained, mainly of medium texture. Third group: Lightly weathered soils of medium to high fertility, having problems of drainage, with fine textures predominating throughout the profile. The major yield increments were due to nitrogen and phosphorus in soils of the first group, with responses to potassium less pronounced. The best treatment was 60-60-40. In the second group of soils significant effects were not recorded except for nitrogen; the best treatment was 60-30-40. In the third group nitrogen and potassium gave the major effects. The best treatments were 60-60-80 and 80-40-40. Differences between forms of nitrogen (ammonium sulphate and urea), tested on the soils of the third group were not apparent. Introduction The cotton is one of the more important enterprises in the agricultural sector of the economy of the States of Barinas and Portuguese, where it occupies 45 per cent of the area in the whole country to this crop. This, in the year 1974 was 64688 ha giving a mean yield of 1076 kg/ha (M.A.C., 1975). With the aim of

Instituto de Investigaciones Agrlcolas Genemales (CENIAP), , Venezuela. * CIARCO, Araure, State of Portuguesa, Venezuela. 1.. establishing a rational fertilizing programme and so improving the low yields observed, experiments have been carried out during some years past by the Ministry of Agriculture and Animal Production through the Centro Nacional de Investi- gaciones Agropecuarias (CENIAP) and the Centro de Investigaciones de la Regi6n Centro Occidental (CIARCO), in collaboration with the Asociaci6n Nacional de Cultivadores de Algod6n (ANCA). This paper reports the results of some of these experiments.

Survey of the literature Westin (1962) in accordance with the 7th Approximation', classified the soils of the Barinas Series as Ochrultic Typudalfs. They are soils of alluvial origin over- lying accumulated material, always distinctly sandy and distinctly clayey, and situated on terraces and alluvial basins at the bases of mountains. Nelson (1967) working with grain sorghum, noted the low fertility of these soils. He reported positive effects from treatments with the fertilizing elements nitrogen and phos- 0 phorus, indicating further that the response to nitrogen depends on the level of available phosphorus. Rodriguez and Aviln (1969), and Aviln and Rodriguez (1972) obtained similar results for the cotton and for , and reported further that the element potassium was a less effective element in raising yields. Nelson (1967) and Avildn and De Rojas (1975) pointed to sulphur as another limiting factor in the productivity of these soils. The soils of the Series were classified by P&ezetal. (1968) in accordance with the 7th Approximation as Ustifluvents. They find their place amongst the soils suited to cotton cultivation (Strebin and Tua, 1966), permitting a vigorous development of the root system (Dumith et al, 1964). In soils of Barinas State with similar physical and chemical characteristics, Rodriguez and AviMn (1969) have reported responses of maize and cotton to nitrogen fertilization, though not as marked as those observed on the Barinas Series.

Materials and methods

The experiments were located in various places, representative of the soil units most commonly cultivated in the cotton growing region. They were conducted in the years 1969 and 1973. A randomized block design with fourfold replication of treatments was used. The experimental plots consisted of three strips 10 m wide with spaces of 1 m between them. For the yield data the central strips were recorded and the figures refer to raw cotton. Density of planting was 60 000 plants per hectare. The variety Delta Pine Smooth Leaf was used in the year 1969, and Delta Pine 16 in 1973. Fertilizer dressings were timed as follows: nitrogen was divided, one third at the time of sowing and the rest 35 days later; phosphorus and potassium were entirely applied at sowing time, in the bottoms of the furrows. The sources used, and the application rates in 1969 and in 1973 were as follows.

Year Element Rates, kg/ha Sources used 1969 N 0,30, 60, and 90 ammonium sulphate (21%) P.O. 0,30, 60, and 90 triple superphosphate (45%) K.0 0,40 and 80 potassium sulphate (50%) 1973 N 0,40, 60 and 80 ammonium sulphate (21%) P20 0,20 and 40 triple superphosphate (45%) K20 0,60 and 80 potassium sulphate (50%)

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In the experiments of 1973, two alternative sources of nitrogen were used with the object of testing for differences in the forms of supply of this element. These sources were urea and ammonium sulphate and they were used in the treatment 60-40-40 (kg/ha of nitrogen, phosphoric acid and potash respectively). The topsoils were sampled to a depth of 0-15 cm at each of the experimental units. The analytical tests applied and their results are given in Table 1. The soils of the Barinas Series represent the first group, the characteristics of which were described by Rowles (1969). The terrains occupied by these soils are high, level terraces adjoining the lower slopes of the mountains, with savannah vege- tation, acid in reaction with low exchange capacity, well drained and with low moisture retentivity and low natural fertility (table 1 ). The soils of the second group typified by the Guanare Series, the features of which were described by Prez Materan at al. (1968) are characterised as slightly developed soils of medium texture in all horizons. Internal drainage is normal, reaction moderately acid, exchange capacity medium to high. They generally occupy the first terrace or flat along a river, the natural vegetation being rain forest. They are soils of moderate natural fertility (table 1 ). The soils of the third group, represented by the Maria Series described by Stagno and Granados (1969) are characterised by having a low exchange capacity and medium acidity, with problems of internal drainage associated with their positions on river flats or very tow-lying banks. The results of soil testing, for individual soils and their recognised groups, were analysed statistically using Tukey's significance test according to the procedure given by Pimentel (1966).

Table 1 Results of laboratory tests on soil samples representative of the experi- ments on cotton fertilization in different localities. Depth of sampling 0-15 cm.

Analysis (4) Locality HDA Apica Est. Exp. Araure Guache Sist. Riego Guanare Texture Ar. (5) F.Ac. F.Ac. Ar. F.Ac. Ar. F. Ar. (1) Phosphorus ppm. 1,5 24 38 13 16 (1) Potassium ppm. 8.5 78 66 139 176 (2) Calcium ppm. 158 462 467 684 741 (2) Nitrate ppm. 19 28 35 74 83 (3) Org. matter % 1.70 4,96 3.90 4,03 4,30 pH, 1:2,5 5,3 5,6 5,00 5.2 5,6 Conductivity 1.5 mmho/cm 25°C 0,05 0,09 0,07 0,07 0,09

(1) Phosphorus and potassium extracted by NaHCO. 0,5 M, pH 8,5. Ratio of soil to extractant solu- tion, 1:20 (2) Calcium and nitrate extracted by CH.COONa 0.125 M. pH 4,2 (3) Organic matter determined by wet combustion according to Walkley and Black (4) Soil Analysis Laboratory I.I.A.G.-CENIAP-MAC. (4) Ar.-sand, F.=silt. Ac.=clay.

Results and discussion

The results obtained for the different groups of soils studied are presented in table 2. The largest response observed in the first group of soils was that due to nitrogen (yield increment 834 kg/ha), followed by phosphorus (555 kg/ha), whilst potassium had a less marked effect on yields (323 kg/ha).

3 ... Soil analysis (table 1) indicates low available phosphorus contents ( <10 ppm.). Moreover, these soils possess a high power of phosphate fixation, given their high degree of weathering (Rowles, 1969) and the effect of this on the response to phosphate fertilization, despite the low requirement of the cotton for this nutrient (Malavolta et al., 1974) must be such as to give larger increments from phosphatic treatments. In the present case however the response to the element may have been obscured by the high coefficient of variation (25%) observed in the experi- ment, or additionally by an inadequate distribution of the rainfall.

Table 2 Effects of N, P and K fertilization on yields of cotton. Field experiments on soils of the first, second and third groups.

Treatment Yields of raw cotton (kg/ha) Mean of 5 experiments No. kg/ha Group I Group 2 Group 3 Yield Increase of N POs KO Experi- Experi- Experi- Experi. Experi- kg/ha yield (per ment mont ment ment mont cent) due to Hda. San Guanare Est. Hda. N,P2O. and Apica NicolAs Araure Guache KO respec- tively 1 0 60 40 764 705 1174 1270 775 938 2 30 60 40 1295" 1011 1343 1620 1235 1301 39 3 60 60 40 1598" 1168 1032 1765 1175 1348 44 4 90 60 40 1439" 1248 1140 1765** 1295 1376 47 5 60 0 40 1003 895 1367 2020* 1090 1275 6 60 30 40 1361 1250 1240 1605 1300 1351 6 3 60 60 40 1598" 1168 1032 1765"" 1175 1348 6 7 60 90 40 1455" 1180 1196 1830*° 1200 1368 7 8 60 60 0 1275' 1012 1190 1550 1170 1239 3 60 60 40 1598 "° 1168 1032 1765"" 1175 1348 9 9 60 60 80 1503"" 1241 1269 2240" 1120 1475 19 10 0 0 0 505 1015 1081 800 800 840

In the second group of soils the mean response to nitrogen fertilization (251 kg/ha) was greatest with the dose 90 kg N per hectare, but this yield only slightly sur- passed that of the dose 30 kg N per hectare which produced an increment of 238 kg/ha. Compared with these effects of nitrogen the recorded yield increases due to phosphorus dressings were low (57 kg/ha). The response to potassium occupied an intermediate position, with 154 kg/ha achieved with the dose 80 kg K20 per hectare. Soil analyses (table 1) show that these soils offer a growing medium of moderate to high natural fertility, which partly accounts for the low response from the appli- cation of fertilizers; for all that, the yields recorded are rather low in general. The coefficients of variation recorded in the experiments were high, namely 26% for San Nicolds and 29% for the Riego Guanare system, similar to that found in the experiments of the previous group, and again capable of being attributed to the need for a favourable distribution of a rainfall, not always entirely adequate, through the season; a need to which our agriculture is generally subject. The yields obtained from the third group of soils are given in tables 2 and 3 which show the results of 1969 and 1973 respectively. In 1969 (table 2) the yield incre- ments above the control figure ranged from 222 to 820 kg/ha, the latter being from the treatment 60-60-80 (kg/ha of N, P2Os and K2O respectively.)

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Response to nitrogen fertilization reached its highest level with the dose of 90 kg N per hectare, this increment being 507 kg/ha when compared with the treatment (0-60-40) without nitrogen. This response was, however, not very greatly different from those given by the doses 30 kg and 60 kg of N per hectare, which were 405 and 447 kg/ha respectively.

For phosphatic fertilization no significant responses were recorded. Treatment with potassium increased the yields to a lesser degree than nitrogen did. The dose giving the largest increment, namely 320 kg of cotton per ha, was 80 kg K2O.

The results of 1973 (table 3) show similar tendencies in the responses to the fertilizing elements. In the case of nitrogen the dose 80 kg N per hectare produced increments as high as 896 kg/ha, and for potassium the dose 60 kg KaO per hectare increased yield by 567 kg/ha. In the case of phosphorus the dose 20 kg P20, per hectare gave yield increments up to 229 kg/ha, but the higher dose (40 kg P20,/ha) induced a decreasing response. Statistical analysis of the data of table 3 indicates that the treatment 80-40-40 shows highly significant yield differences with respect to control. Treatments 60-20-40 and 60-40-60 show significant differences from the same control. Chemical analysis of the soils (table 1) shows that they have high phosphorus contents and this, taking into account the low phosphorus requirements of this crop (Malavolta at a., 1974) explains in part the low response obtained on these soils. The same analyses indicate that potassium contents of the soils are low ( <80 ppm.). The response obtained with potassium fertilization shows the need for a high rate of application, considering also the high potassium requirement of the crop. Tincknel/ et al (1961) report on the effectiveness of potassium in the cultivation of tobacco on these soils.

Table 3 Effects of fertilization with nitrogen, phosphorus and potassium on yields of cotton in field experiments on soils of the third group, State of Portuguesa (Est. Exp. Araure)

Treatments (kg/ha) Yields No. N P.O. K.0 kg/ha 1 0 0 0 1096 2 0 40 40 1192 3 40 40 40 1612 4 60 40 40(S+S) 1534 5 80 40 40 2088"" 6 60 0 40 1408 7 60 20 40 1637" 4 60 40 40(S+S) 1534 8 60 40 0 1130 4 60 40 40 (S+S) - 9 60 40 60 1697' 10 60 40 40 (U+U) 1427 11 60 40 40 (S+U)' 1438 Tukey's test 5% 630 1% 631 Coeff. variation % 18

Ammonium sulphate and urea

5 .. From the results obtained with this group of soils, the conclusion is that the major responses are due to the elements nitrogen and potassium, with phosphorus having little effect on the yields. Ram/rnez (1961) obtained similar results with maize, and also reported an interaction effect between nitrogen and potassium. As regards the source of nitrogen, analysis of the results (table 3) of supplying ammonium sulphate (S) and urea (U) as alternatives, shows no significant differ- ences between the two.

General conclusions The best treatments - in kg/ha of nitrogen, phosphorus (P2Os) and potassium (K20) - were for the soil groups studied:

Soil group Year Best treatment Yield Control Difference kg/ha kg/ha kg/ha First 1969 60-60-40 1598 505 1093 Second 1969 60-30-40 1245 1048 197 Third 1969 60-60-80 1680 800 880 Third 1973 80-40-40 2088 1096 992

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