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TIARKS, Allan Edward, 1945- CAUSES OF INCREASED CORN ROOT ROT INFECTION OF CONTINUOUS CORN ON NO-TILL UOYTVILLE SILTY CLAY LOAM IN NORTHWESTERN OHIO.
The Ohio State University, Ph.D., 1977 Agronomy
Xerox University Microfilms,Ann Arbor, Michigan 48106 CAUSES OF INCREASED CORN ROOT ROT INFECTION OF CONTINUOUS CORN ON NO-TILL HOYTVILLE SILTY CLAY LOAM IN NORTHWESTERN OHIO
DISSERTATION
Presented In Partial Fulfillment of the Requirements for the
Degree Doctor of Philosophy in the Graduate School of the
Ohio State University
by
Allan E. Tlarks, B. Sc., M. Sc.
*****
The Ohio State University
1977
Reading Committee:
D. M. Van Doren en, Co-advisor G. S. Taylor
A. F. Schmitthenner ACKNOWLEDGMENTS
This seudy was conducted on soils collected from tillage research plots established by Dr. D. M. Van Doren and others. I am grateful for the use of these long term plots, as without this resource, the present study would have been impossible.
I also thank Dr. Van Doren and Dr. G. S. Taylor for their guidance throughout the study* I acknowledge the financial support of the
Agronomy Department in form of a teaching assistantshlp under Dr. F. L.
Himes.
I am indebted to Ms. Lou Ann Cooper for typing and proofing the manuscript. Finally, I would like to extend my appreciation to my fellow graduate students for their Interest and ideas which made the work interesting if not fun.
(ii) VITA
August 16, 1945 Born - Council Bluffs, Iowa
1967 B.Sc. Agronomy, Iowa State University, Ames, Iowa
1967-1969 Peace Corps Volunteer Soil Survey in Tanzania, Africa
1969-1970 Research Assistant, University of Nebraska, Lincoln, Nebraska
1970-1971 U.S. Army
1972-1973 Research Assistant, University of Nebraska, Lincoln, Nebraska
1973 M.Sc., Agronomy, University of Nebraska, Lincoln, Nebraska
1973-1976 Teaching Associate, Department of Agronomy, The Ohio State University, Columbus, Ohio
Present Research Soil Scientist, Southern Forest Experiment Station, Pineville, Louisiana
PUBLICATIONS
"Physical and Chemical Properties of Soil Associated with Heavy Applica tions of Kanure from Cattle Feedlots." Soil Science Society of America Proceedings 38:826-830.
(iii) FIELDS OF STUDY
Soil Science: G. S. Taylor, K. R. Everett, E. 0. McLean, F. L. Himes, R. H. Miller, and L. Wilding
Mic roc1ima t o1ogy J. N. Rayner and A. J. Amfield
Mineralology R. T. Tettenhorst and E. G. Ehlers
Botany C. A. Swanson
Agricultural Engineering L. D . Drew
Statistics H. 0. Harvey
Chemistry Q. Van Winkle and M. H. Klapper
Physics J. H. Shaw
(lv) TABLE OF CONTENTS
ACKNOWLEDGEMENTS ii
VITA H i
LIST OF TABLES viii
LIST OF FIGURES xi
INTRODUCTION !
CHAPTER
I. REVIEW OF LITERATURE 3
A. Morphology, Host Range, and Pathogenicity of Pythium 4
B. Environment of No-tillage Culture and Effects on C o m Root Pathogens 5
1. Soil Temperature 5 2. Soil Water 7 3. Soil Temperature-Soil Water Interactions 10 4. Soil Strength and Bulk Density 10 5. Soil Structure 13 6 . Crop Rotation 14 7. Residue Addition 15
C. Summary 16
II. METHODS AND MATERIALS 18
A. General Information 18
B. General Laboratory Methods 20
1. Root Growth in Soil Core Techniques 20 2. Com Root Baiting Technique 23
C. Preliminary Investigations 23
1. Effect of Bulk Density on Com Growth 23 2. Effect of Pythium on Com Growth 24
(v) D. Measurement of Inoculum Potential of Pythium 25
1. Soil Mass Variable 25 2. Effect of Variable Concentrations of Fungicides 25 3. Soil Mass vs Area of Soil Exposed 27 4. Infinite Thickness 27 5. Surface Area Varied 28
E. Field Core Experiments 29
1. Autumn Core Experiment 30 2. Bulk Density of Clods 31 3. Spring Core Experiment 31 4. Samples of Com Plants From the Field 32
F. Evaluation of Specific Hypotheses 33
1. Bulk Density by Pythium Interaction 33 2. Effect of Organic Amendments 35 3. Pythium Buildup Along Cracks 39
III. RESULTS AND DISCUSSION 44
A. Preliminary Investigations 44
1. Effect of Bulk Density on Com Growth 44 2. Effect of CRP Infection on Corn Growth 46 3. Discussion of Preliminary Investigations 49
B. Measurement of Inoculum Potential of Pythium in Soil 50
1. Soil Mass Variable 50 2. Effect of Variable Concentrations of Fungicides 50 3. Soil Mass vs Area of Soil Exposed 54 4. Infinite Thickness 54 5. Surface Area Varied 54 6 . Discussion of Inoculum Potential Measurement 60
C. Field Core Experiments 67
1. Autumn Core Experiment 67 2. Bulk Density of Clods 73 3. Spring Core Experiment 73 4. Samples of C o m Plants From the Field 78 5. Discussion of Field Sampling 81
(vi) D« Evaluation of Specific Hypotheses 84
1. Bulk Density by Pythium Interaction 84 2. Effect of Organic Amendments 87 3. Pythium Buildup Along Cracks 98
IV. SUMMARY AND CONCLUSIONS 110
V. APPENDICES 113
VI. LITERATURE CITED 124
(vii) LIST OF TABLES
Selected properties of Hoytvllle and Wooster soils. 19
Variables used In the series of experiments to evaluate methods of measuring Inoculum potential of Pythium in soil. 26
Summary of variables used to test the three hypotheses proposed as causing increased infection of c o m roots in no-tillage c o m plots. 34
Regression equations and regression coefficients between bulk density (X) and nine parameters of c o m (Y) . 45
Root weight and root length of c o m plants grown in flasks over Hoytvllle soil from continuous sod and continuous c o m plots. 48
Root lengths of five c o m plants grown above three different amounts of Pythium infested soil. 51
Effect of four concentrations of the fungicide, pyroxychlor, on root growth of c o m plants exposed to Pythium infested soil. 52
Length of c o m root from five plants grown in flasks containing different numbers of vials filled to different depths of soil. Each value is the average of six replications. 55
Root lengths of c o m plants grown in cores of Hoytvllle and Wooster soils that had been subjected to three field and three laboratory treatments. 69
Total phosphorus content of c o m plant tops grown in Hoytvllle and Wooster soil cores. 71
Phosphorus uptake per unit length of root of c o m plants grown in Hoytvllle and Wooster soil cores. 72
Bulk densities of Hoytvllle soil as determined from core and clod samples and of Wooster soil as determined by core samples. The samples were taken in October, 1975. 74
(viii) Root length of c o m plants grown on Hoytvllle soil cores representing four field treatments and subjected to four laboratory treatments. The cores were collected In April, 1976. Each value Is the average of three replications. 76
Phosphorus content of corn tops and uptake per unit of root length from Hoytvllle soil cores representing two field treatments and two laboratory treatments. Each value is the average of 12 determinations. 77
Dry plant weights and total potassium and phosphorus contents of corn plants collected from the field plots at Hoytville 57 days after planting. Each value is the average of nine determinations. 79
Effect of bulk density of Hoytville soil and pyroxychlor fungicide treatment on root length and uptake of potassium and phosphorus per unit length of root. Each value is the average of 12 determinations. 85
Root lengths of c o m plants grown in flasks containing Infested soil, treated with various amounts of c o m stalk residue, or steamed soil incubated for different times. 89
Root lengths of c o m plants grown in Hoytville soil cores treated with and without an organic amendment, pyroxychlor, and ammonium nitrate. Each value is the average of four replications. 92
Water contents, carbon dioxide evolution, and root length growth in soil collected from tillage- rotation plots at Hoytville in April, 1976. Each value is the average of six replications. 94
Growth of c o m plants in blocks of soil constructed from 5 cm cubes of Pythium Infested soil. The surfaces of the cubes of the infested treatments were inoculated with a Pythium infested medium. Each value is the average of four replications. 101
Effect of bulk density of Hoytville soil on the early growth of c o m and nutrient uptake. 114
Parameters of c o m growth and nutrient uptake in Hoytville soil cores representing three field treatments and three laboratory treatments. Each value is the average of three replicates. 115
(ix) Parameters of c o m growth and nutrient uptake in Wooster soil cores representing three field treatments and three laboratory treatments. Each value is the average of three replicates. 116
C o m plant growth in Hoytville soil cores representing four field treatments and four laboratory treatments. The soil was sampled in April, 1976. Each value is the average of three replicates. 117
Nutrient uptake by corn from Hoytville soil cores representing four field treatments and four laboratory treatments. The soil was sampled in April, 1976. Each value is the average of three replicates. 118
Corn growth in Hoytville soil cores, with and without pyroxychlor treatment, packed at two bulk densities and equilibrated at two water potentials. Each value is the average of three replications. 119
Nutrient uptake by c o m grown in Hoytvllle soil, with and without pyroxychlor treatment, packed at two bulk densities and equilibrated at two water potentials. Each value is the average of three replicates. 120
C o m growth and nutrient uptake from soil cores treated with and without pyroxychlor and c o m stalk residue. Nitrogen was not added. Each value is the average of four replications. 121
Cora growth and nutrient uptake from soil cores treated with and without pyroxychlor and c o m stalk residue. Nitrogen was added to all cores at 330 ug/g. Each value is the average of four replications. 122
Corn growth and nutrient uptake from soil cores made with four different internal crack areas. Each value is the average of eight determinations. 123
(x) LIST OF FIGURES
Cross section of apparatus used for soil core experiment.
The effect of hulk density of Hoytville soil packed into cores on root length of 14-day-old c o m plants.
Root lengths of c o m plants grown over Pythium infested soil treated with six concentrations of the fungicide GA-1-62. Each data point is an average of three replications.
Root lengths of 10-day-old c o m plants grown in flasks containing vials with various depths of Pythium infested soil. Each experimental value is the average of four replicates.
Root lengths of 10-day-old c o m plants exposed to four different areas of Pythium infested soil, with distilled water as the rooting medium. Each data point is the average of six replications.
Root lengths of 10-day-old c o m plants exposed to four areas of Pythium infested soil, without treated soil to supply nutrients # and with soil treated with a fungicide to supply equal amounts of nutrientsO. The rooting medium is distilled water. Each experimental value is the average of three replications.
Root lengths of 10-day-old c o m plants exposed to four different areas of Pythium infested soil, with 1/2 Hoagland's solution as the rooting medium. Each data point is the average of six replications.
The effect of bulk density on the root length as predicted from a growth chamber experiment— with 95 percent confidence intervals— plus actual dry plant weights from field plots treated with two rotation and two tillage variables. 9. Effect of added c o m stalk residue and time of Incubation on root length, expressed as percent of roots grown in steam treated soil. 90
10. Relationship between rate of CO2 evolution of soil and length of c o m roots grown in flasks containing the same soil. 95
11. Effect of internal crack area of soil cores on root length of c o m plants grown in the soil. Each value is the average of eight determinations. 100
12. C o m growth in soil blocks constructed from 5 cm cubes of Pythium infested soil. The edges of the soil cubes of the Infected treatment were inoculated with a Pythium infested medium. Note the difference in the top growth of the c o m plants. 103
13. C o m growth in soil blocks cons true ted from 5 cm cubes of Pythium infested soil. The edges of the Infected treatment were inoculated with a Pythium infested medium. Note the difference in the number of m o t s growing out of the blocks and that large roots follow cracks. 104
(xii) INTRODUCTION
Growing corn without tillage was not feasible until the introduc tion of herbicides in the early 1960’s. Since then much research has been done to determine the adaptability of the practice to different
soils, climates, and crop rotations. Results with no-tlllage vary,
improving, decreasing or not affecting yield compared to plowing.
The yield depression usually occurs on poorly drained soils. The
Hoytvllle soils in Ohio are an example. Van Doren et^ al_. , (1976) found
corn yields were reduced by about 15 percent when grown under no-till
age continuous corn culture compared to a no-tillage corn-soybean
rotation or continuous corn where the soil was plowed.
A root pathogen Pythium graminicola has been cited as a possible
cause of the yield reduction (Van Doren et al., 1976). Rao (1976)
found that infection of corn roots was indeed higher on plants grown
under no-tillage continuous corn culture. Pythium granlnlcola was most
likely the cause of the increased infection, although other pathogens
were also identified. However, he was not able to identify the reasons
for the Increased root infection.
The present study was undertaken to elucidate the causes of the
Increased root rot infections on corn. Two approaches were used in
this study. The first was a bioassay technique developed by Rao (1976)
and Improved in this study. The objective of the bioassay was to find
the inoculum potential of Pythium under various conditions.
1 2
The bioassay or corn root baiting method Is nearly specific for
Pythium as water serves as the rooting medium. Any pathogen causing infection probably has zoospores and must tolerate nearly anaerobic con ditions. Pythium is one of only a few pathogens that infects c o m roots and meets these criteria. In the second approach, c o m plants were grown in soil so that root growth could be studied when soil or environ mental parameters were varied. Several CRP besides Pythium graminicola could cause plant growth reduction in the soil core studies because the roots were growing in the soil and because anaerobic conditions never developed.
Three hypotheses were advanced using these two techniques. They were:
(1) Soil bulk, density is higher in no-tilled plots. Cora roots growing in the higher density soil are weakened and have a higher suscep tibility to corn root pathogens.
(2) Residue turned under by plowing is an energy source for compet itors of corn root pathogens, which should reduce the inoculum potential compared to the no-tlllage practice where residue remains on the surface.
(3) Permanent cracks develop in the soil that is not plowed each year. Corn roots follow these planes of weakness and some root rot infection occurs. The following years, the roots grow into the same cracks where the corn root pathogen potential is higher than in the soil matrix. I. REVIEW OF LITERATURE
Research on no-tillage or similar cultural practices has been popular in the last 20 years. Enough literature has accumulated that several excellent reviews have been made (Hlllel, 1972; Soil Conservation
Societya 1973). Because of the availability of these reviews and be cause many of the studies are not pertinent to the problem at hand, this review deals only with c o m root rot and the pathogens involved, and properties of no-tillage culture which affect the severity of corn root rot.
Many pathogens are capable of infecting corn roots (Ho, 1944; Rao,
1976). Of this spectrum of organisms, Pythium graminicola is a common primary Invader of corn roots. While it is not possible to establish that Pythium graminicola is the main corn root pathogen involved in no-tillage c o m fields on the basis of the literature, Pythium gramini cola Is the most probable organism Involved. Pythium graminicola has been plated from c o m roots in several studies (Ho, 1944; Hampton and
Buchholtz, 1959; Rao, 1976). In the above studies, techniques were used that selectively Isolate Pythium from the soil such as the corn root baiting or the crested wheatgrass methods.
While isolation and plating procedures were not used in the present study, emphasis was placed on Pythium graminicola. If related organ isms such as other species of Pythium are involved, the general morpho logical and ecological conditions will still be relevant.
3 4
A. Morphology, Host Range, and Pathogenicity of Pythium
The brief morphological description given here is not for identi fication purposes but to relate to the ecology and dispersion of the fungus in the soil. Middleton (1943) and Waterhouse (1968) describe
Pythium graminicola as having granular and hyalin or colorless mycelium
3-7 urn in diameter. Asexual reproduction is by motile zoospores which are 8-11 urn in diameter and are motivated by lateral bicillate. The zoospores are produced in sporangia which vary in size but each spor angium contains 15-48 zoospores. The sexual reproduction bodies, oogonia, are 16-36 pm in diameter.
According to Ho (1944) Pythium graminicola was first reported on corn roots in 1921 although species identification was not made at the time. Several scientists reported Pythium root rot on sugar cane, barley, and wheat, but most of the early work is confusing because the species were not fully described. Middleton (1943) lists the following temperate crops as being susceptible to P^ graminicola: oats, barley, millet, rye, sorghum, wheat, and corn.
Pythium graminicola initiates lesions on young roots near the tips and later spreads to form streaks or girdles on roots (Rao, 1976).
Hampton and Buchholtz (1959) isolated P^_ graminicola predominately from young, nearly white, roots. Rao (1976) and Hampton and Buchholtz (1959) found P_^ graminicola to be an initiator of root rot with secondary path ogens such as Fusarlum spp. cultured only from advanced lesions. These investigators could not culture Pythium from c o m roots in July or August or about 50-70 days after planting. Dry soil conditions and high temp eratures were given as the reason. However, Foth (1962) reported corn root growth in the 0-15 cm depth of soil was minimal between 54 and 67 5 days after planting. Since Pythium evidently attacks only young roots, it is not surprising that the pathologists could not culture the fungus from roots during this time period.
Pythium graminicola is difficult to Isolate directly from soil because of low populations compared to spore forming fungi and is over grown on culture plates. Rao (1976) attempted direct Isolation by several methods but had no success. Several root baiting techniques which depend on the ability of zoospores to infect uninjured roots, have proven to be successful. Knaphus and Buchholtz (1958) used crested wheatgerm, which is highly susceptible to Pythium infection, to evalu ate the inoculum potential in soil. More recently, Rao (1976) developed a c o m root baiting technique to isolate P_;_ graminicola. Rao also made an attempt of using his technique to quantify the inoculum potential of the soil. However, he found no differences in the infection rates as measured by counting lesions on roots, when he varied the weight of soil in the flasks.
B. Environment of No-tillage Culture and Effects on C o m Root Pathogens
Compared to plowing, no-tillage culture changes several environ mental factors which affect the ability of fungi to infect c o m roots.
The factors soil temperature, soil water content, soil temperature-soil water interactions, soil strength, and bulk density, soil structure, crop rotation and residue additions to the soil are discussed here.
1. Soil Temperature
In a study in Iowa, Minnesota, Ohio, and South Carolina, Van
Wijk e£ al. (1959) found a three ton per acre oat straw mulch reduced soil temperatures at the 10 cm depth by about 1 C. In Virginia, a similar mulch reduced maximum soil temperatures at the 10 cm depth by 6
1-3 C In May and 3-4 C in June (Moody et al., 1963). Allmaras and
Nelson (1971) measured a 2 C reduction in average soil temperature in
Minnesota under a 4.5 metric ton/ha. straw mulch compared to bare soil.
Leach (1947) reported that If the growth rate of a pathogen is compared to the growth rate of the host at the same temperature, the probability of infection occurring at that temperature can be inferred.
By using separately reported temperature-growth rate relationships for
Pythium and corn, such an analysis will be made. Middleton (1943) measured the rate of growth of Pythium graminicola on corn meal agar at temperatures ranging from 1-46 C in three degree Increments. He found the maximum growth at about 28 C. The temperature-growth rate curve of the fungus contains no abrupt peaks but rather a gradual change in growth as temperature was varied. Walker (1969) varied soil temperature in 1 C increments and measured corn growth. The maximum corn root growth was at 26 C. The shape of the temperature-growth rate curves for
P. graminicola and corn roots is similar except in the 16-19 C range where the relative growth rate of the fungus is faster. At 25 C the c o m root growth was at a maximum compared to the fungus. This data must be interpreted with caution because the organisms were grown under different environmental conditions by different scientists. However, the results of the analysis do agree with corn root Infection data where corn is grown in Pythium infected soil. Ho (1944) measured a 10-20 percent decrease in root length of c o m grown in Pythium infested soil compared to steamed soil, when soil temperatures were at a constant
25 C. When the night temperatures were allowed to drop to 9 C, the
Pythium caused a 40-50 percent decrease in root length. In nearly saturated Hoytville soil, the percent of c o m roots infected by Pythium 7 was higher at 15 C than at 25 C (Rao, 1976). However, the most severe
Infection was at 30 C.
While leaving a mulch on the soil surface does decrease soil tem peratures by 1—3 C, the difference is not enough to substantially increase the potential Infection by Pythium or other CRP. Van Doren et al., (1976) noted that while rotations and continuous corn cultures with no-tillage on Hoytville soil have about the same amount of mulch and therefore nearly the same temperature regime, yield reductions were greater in the continuous c o m plots. On the basis of the interaction, the influence of field culture practices on soil temperature was deter mined to be insignificant to affect CRP substantially. Temperature was not used as a variable in the present study.
2. Soil Water
Jones et al. , (1969) measured soil water contents through the
growing season in soil that had been plowed or not tilled. They found no difference in soil water contents in the spring but about 15-20 per
cent more water in the untilled soil in midsummer and fall. In Kentucky
the 0-8 cm layer of soil contained more water when left untilled com
pared to plowing (Blevins et al., 1971). However, the soil water con
tents in the 8-15 cm layers were about the same. Allmaras and Nelson
(1971) used tensiometers to measure soil water potentials in plowed soil without mulch, plowed soil with mulch, and no-tillage without mulch.
The matrlc potential at the 15 cm depth was unaffected by tillage when mulch was not applied. Applications of 4.5 matric tons/ha. of straw
mulch reduced evaporation but the effect on matric potential was small.
The two plowed treatments had about the same bulk density so that the
air-filled porosity would be the same for the two soils if the matric 8 potentials were nearly the same. Saturation or high water content In a soil can increase the amount of infection by CRP in three possible ways: (1) The water supplies a continuous medium for zoospores to
travel through. (2) The rate of diffusion of root exudates Increases as soil water content increases. The root exudates may be energy sources which trigger germination of oospores or chemicals that attract zoospores.
(3) High soil water content reduces soil aeration which affects the physiology of roots causing increased production of roots exudate.
None of the above points were studied per se in the present research.
Host of the literature to be discussed were laboratory studies and can not be directly related to the field. However, literature will be dis cussed which gives a perspective of possible relationships which occur
in continuous corn no-tillage field conditions.
Pythium graminicola has motile zoospores so active movement through
continuous water films can be Important in the dispersion of the fungus.
The nature of locomotion of zoospores and compounds causing chemotaxis
or attraction of zoospores has been studied using several fungal species
that are similar to Pythium graminicola. Zoospores of Phytophthora
cinnamon! follow a helical path when traveling through Isotropic solu
tions (Allen and Newhook, 1973). The amplitude of the helix was found
to be 43 um and the speed 116 um/sec. If tubes containing low concen
trations of ethanol were placed into the zoospore suspension, the zoo
spores concentrated near the tips of the tubes. The authors concluded
that water filled pores of at least 190 pm diameter are necessary for
substantial active movement of zoospores. The ethanol gradient required
for zoospores to move out of 190 um diameter capillaries was 1 mm 9 per 16 urn. Royle and Hickman (1964a) found zoospores of Pythium aphanldermatum travel In the characteristic helical or corkscrew motion until root exudates were placed in the suspension. Then the zoospores halted, milled around, and converged to the source of the exudate travel ing in the normal pattern. If the exudate concentration gradient was small, the zoospores moved in loops and jerks but generally towards the exudate source. In a second paper, Royle and Hickman (1964b) used several animo acids, sugars, and combinations thereof in an attempt to find the chemicals in root exudates causing chemotaxis. Many of the materials used did attract the zoospores but none were as effective as the root exudates. Ethanol was not studied in this series of papers.
Hickman (1970) reported that zoospores accumulate around the region of maturation of roots, root hairs, and along cuts in the roots. The region of accumulation varies with the type of root indicating that the site of root exudate varies with plant species. Some evidence shows that several compounds may be involved. One group attracts the zoospores, a second group traps the zoospores, and a third group of chemicals trigger germination. Agnihotri and Vaartaja (1967) found root exudates enhance pathogenesis by supplying nutrients for the growth of fungi such as Pythium.
The amount and type of root exudates and site of release vary with the environmental conditions of the roots. Kerr (1964) found that pea seeds lost more weight and therefore more sugars as soil water con tents increased. The loss of dry weight increased linearly as water contents increased in sandy loam and loam soils. The percent of pea roots infected correlated with the loss of dry weight. Bolton and
Erickson (1970) measured ethanol production in tomato plants that were 10 growing in flooded soils. The rate of ethanol exudation from the roots was increased 100 fold after 12 hours of flooding.
While the cultural practice of no-tillage does affect the soil water content somewhat, the amount of change is probably too small to be of any significance to soil organisms, especially in poorly drained soils.
3. Soil Temperature-Soil Water Interactions
The activity of Pythium in soil as measured by root infection may be highest when soil temperature and soil water contents interact to give optimum environmental conditions for the growth and dispersion of the fungus or to give conditions where root exudate production is high. McLaughlin (1947) isolated Pythium from soils at weekly intervals through a growing season. A good correlation existed between Pythium levels and soil temperature and soil water measurements. A combination of high soil temperatures and low water contents reduced Pythium activ ity to very low levels. Hampton and Buchholtz (1959) Isolated Pythium graminicola from corn roots in May, June, and September but found few
Pythium infected roots in July and August when temperatures were high and precipitation low. However, these results may be caused by decreas ed feeder root activity in the surface soil as previously mentioned.
Rao (1976) found the greatest amount of corn root infection occurred at
23 C in dry soil but in wet soil the smallest infection was at 23 C com pared to 15 and 30 C.
4. Soil Strength and Bulk Density
Plowing soil causes several changes in the physical condition or the tilth of soil. Mechanically the plow cuts loose, granulates, and inverts the furrow slice. Benefits of plowing are loosening of the soil and covering the surface residue (Baver, 1956). In the field, the 11 loosening effect can be quantified by measuring soil strength, porosity, and bulk density. While several investigators have attempted to measure the effects of tillage on these parameters, the results have been dis appointing because of the variability both between and within the sample sites. Page et al., (1946)found spring plowing Miami soil increased soil aeration porosity from 14 to 26 percent and a corresponding decrease in soil strength. The measurements were made in the fall. In an Indiana study the aeration porosity in the fall, was six percent in untilled soil and 24 percent in plowed soil just after the tillage operation. By spring, both treatments had six percent aeration porosity (Kohnke and
Barber, 1968). Penetrometer measurements in Virginia showed little dif ference in strength between plowed and unplowed plots in the spring.
By midsummer, when water contents were lower, umplowed soil resisted penetration 1.2 times as much as plowed soil (Jones et al., 1969). Shear and Moschler (1969) measured bulk densities one year after tillage and found plowing decreased the bulk density from 1.48 to 1.43 g/cm . How ever, the differences were not statistically significant because of high variability.
Increasing the bulk density changes several soil parameters which affect root growth. The reduced pore size changes the soil water poten tial at a given water content. Also, gas exchange is reduced as pore size and volume is decreased. The smaller pores themselves and the
Increased soil strength of compacted soils may restrict root growth.
Wiersum (1957) found that roots will not penetrate pores that are smaller than the roots. Russel and Goss (1974) using glass beads in flexible diaphrams found applied pressure as low as 0.5 bar substantially reduced root growth. As the soil strength increased, root diameter was increased. 12
Because of the confounding of several variables, studies using soil
to elucidate the effect of bulk density on root growth are difficult to
evaluate. Meredith and Patrick (1961) found root weights decreased linearly with increased compaction. A critical bulk density where root
growth dramatically declined was not apparent. The roots of Sudan grass were reduced by a factor of ten as the soil bulk density was Increased
from 1.4 to 2.0 g/cm^ (Zimmerman and Kardos, 1961). In a field study oxygen contents, temperatures, and water contents did not correlate with corn yields but the yields were significantly reduced by higher bulk densities (Phillips and Kirkham, 1962).
Since the hyphae of Pythium is small compared to pore diameters of soil, growth of the pathogen is not directly affected by bulk density.
However, Griffin (1963) found that fruiting bodies of Pythium ultlmum were not formed in porous medium if the voids were 15 ym in diameter or smaller. The oogonia of P^ ultimum are usually about 20 ym in diameter.
The number of soybean roots infected by Phytophthora increased from 2.5 percent to 35-45 percent when the soil bulk density was increased from
1.20 to 1.49 g/cm^ (Fulton et al., 1961). The high incidence of infec tion did not occur if only the top half of the 0-8 inch layer of soil was compacted. Increasing the water content of the soil did not signifi cantly affect the amount of infection. Kerr (1964) pointed out that greater bulk densities would increase the rate of root exudate movement through the soil. In a study of Pythium ultimum on pea roots, percentage infection increased from 0 percent at a bulk density of 1.0 g/cm^ to 63 percent at a density of 1.5 g/cm^. Pythium ultimum does not form mobile zoospores so movement of the fungus was probably by hyphae extension. 13
5. Soli Structure
Plowing breaks soil into smaller aggregates creating new sur faces and cracks. If the soil is not tilled for several seasons, new peds will form. In a lab experiment, where soil was wetted and dried or frozen and thawed for several cycles, the crack pattern formed after two cycles remained after five cycles. In other words, the rewet ted soil did not seal the previous cracks and fracture elsewhere(Czeratzki and Frese, 1958). Blake, et al. (1973) found that surface applied water moved more rapidly through cracks than through soil peds. Although measurements were not made the data indicated that the cracks were saturated before the ped interiors.
Soil cracks can play an important role in root growth. Roots may be stretched, broken, compressed, or flattened In cracking soils (White,
1975). Hubbard (1950) reported that certain species of native grasses could not survive in high clay content soils because of the tearing action of the cracking soil. Roots may follow cracks and the resulting root pattern reflects the geometry of the soil rather than the natural rooting pattern of the plant (Sutton, 1969). The crack pattern of high clay content soils may also affect nutrient uptake, especially the immobile ions such as phosphorus. Wiersum (1962) found the phosphorus content of plants increased as the soil aggregate size decreased. A structurally
Induced phosphorus deficiency could be important in studies involving
Pythium since infection has been found to be more severe in phosphorus deficient soils (Vanterpool, 1940).
Unfortunately, the effect of structural units in soil on microbial populations has not been a major objective of research. Dobbs and Hinson
(1960) cross-sectioned clods and measured fungal populations across the 14 break. Surprisingly they found the greatest numbers in the middle of the clod. However, the method they used did not allow enumeration of
Pythium and related fungi. In a study of Pinus radiata growing on high clay content soils, roots infected with mycorrhiza were found only in cracks between structural units (Atkinson, 1959). The pattern ofa prior rooting system can also influence the location of a fungus within the soil. Trujillo and Snyder (1963) found Fusarium oxysporium did not spread through the soil but remained concentrated around roots it had infected.
Plowing increases soil aeration, especially immediately after the tillage operation. As aeration increases, the production of ethylene by bacteria decreases and allows for larger populations of pathogenic fungi
(Smith, 1976). Because of relatively high ethylene production in dense peds, root Infection is usually the lowest in natural ecosystems, moder ate in no-tillage culture, and highest when the soil is tilled annually.
However, phycomycetes, which Include Pythium, show a tolerance to ethy lene while their competitors do not (Smith, 1976). Therefore, Pythium may have an advantage in unplowed soil. If this is true, the inoculum potential would be the highest in the interior of the peds and substan tially lower In the soil cracks where the oxygen content is higher and the ethylene concentration is lower.
While the relationships between soil structure, root growth, and plant pathogens have not been studied specifically, there is evidence that these factors can be important.
6 . Crop Rotation
In most research of no-tillage that included rotations, the amount of fertilizer added was adjusted to allow for differences in nutrient 15
uptake by the different crops. Therefore, the main effect of crop
rotation, assuming equal runoff or that water was not limiting as is
the case at Hoytville, is in the microorganisms in the soil. Control
of plant pathogens by crop rotation is a standard practice that depends
on the pathogen being host specific.
A good rotation requires the crops to be taxonomieslly remote from
one another (Garrett, 1970). Knaphus and Buchholtz (1958), using crested
wheatgrass to evaluate Pythium graminicola population levels, found
alfalfa and bluegrass reduced the population of the pathogen by 33-55
percent of the original level. Williams and Schmitthenner (1963) report
ed soybeans to be the best for reducing potential infection of the next
year's corn crop. The worst condition existed if co m followed c o m for
more than one year. Rao (1976) reported that 45 percent of the c o m
roots in continuous corn plots were infected by Pythium in July while in
corn following soybean plots the infection was 21 percent. Therefore,
the rotation of corn and soybeans does significantly reduce the popula
tion levels and actual root infection by Pythium.
7. Residue Addition
The addition of crop residues to the furrow slice by plowing
rather than leaving the material to decompose on the surface, can be a
biological method of controlling plant disease. Addition of the residue
supplies an energy source to the wide range of organisms present other
than the pathogen. The other organisms limit growth or reproduction of
the pathogen by competition for soil nutrients and oxygen. Also, the
population of predators and parasites of the pathogen can be increased by addition of organic residues. Okpala (1975) reduced the damping-off of lettuce, caused by Corticium praticola by adding grass meal. Incidence 16
of damping-off and survival time of the pathogen was increased by adding
corn meal, however. The author concluded that residues of high C/N
ratios would not increase pathogen levels but residues with low C/N
usually do.
If added organic residues are to decrease the population of a plant
pathogen by increasing the population of parasites, the occurrence of
the parasites must be shown. Drechsler (1963) indentified a parasite of
several species of Pythium. Dactylella stenomeces was Isolated from
leaf mold and was found to be both an active and passive parasite of
mature Pythium oospores- The parasite seems to reduce the Pythium popu
lation the most in wet soils.
If parasites of Pythium graminicola can be stimulated by the addition
of organic residues, plowing cornstalks under would be a factor in reduc
ing the root infection of the c o m by Pythium.
C. Summary
The literature reviewed here indicates that baiting techniques to
isolate Pythium graminicola are available but the techniques are not useful to evaluate the inoculum potential of the soil.
No-tillage culture does not change the temperature and water status of the soil compared to plowing but the changes are probably too small
to influence the amount of c o m roots Infected by pathogens such as
Pythium.
The percent of roots infected has been shown to be higher in some cases where the soil bulk density is high and the pathogen is present.
Biological control of root infecting pathogens by additions of organic residues has had mixed success, with the chance of positive results increased if the residue has a high C/N ratio. 17
Soil structure with cracks around dense peds, as exists in soils similar to Hoytville, does influence root growth, nutrient uptake and microbial population densities. II. METHODS AMD MATERIALS
A. General Information
All soil used In this study was collected from plots representing selected treatments of a long term tillage and rotation experiment.
Details of the experimental variables and soil characteristics have been reported by Van Doren et al. (1976). Most of the present study deals with Hoytville soil collected from the field plots at the
Northwestern branch of Ohio Agricultural Research and Development Center near Hoytville, Ohio. For a comparison of results, samples for one experiment were also collected from similarly treated plots on Wooster soil at the main OARDC Station at Wooster.
Physical and chemical properties of Hoytville and Wooster soils, as reported by Van Doren et^al. (1976) are summarized in Table 1.
Additional Information about Hoytville soils taken from Blevins and
Wilding (1968) is also included in Table 1. Hoytville silty clay loam is classified as a Mollic Ochraqualf. Hoytville soils were formed on glacial till reworked by lake action. The experimental area is flat, very poorly drained and tiled. The A horizon is saturated much of the early spring of climatically normal years. As the soil dries in July and August, cracks form and become 1-2 cm wide and probably about 60-90 cm deep.
The Wooster soil is a silt loam, classified as a Typic Fragiudalf, on a 2-5 percent slope. The site is well-drained and subject to erosion.
The soil structure in the A horizon is very weak and tends toward massive 18 19
Table 1. Selected properties of Hoytville and Wooster Soils.*
Soil Soil Property Hoytville Wooster
Percent (%) Sand 21.0 25.0
Percent (%) Silt 39.0 60.0
Percent (%) Clay 40.0 15.0
Percent (%) Organic Matter 4.0 2.4 pH, B Horizon 7.0 5.0
Minimum Saturated, Hydraulic Conductivity, cm/hour 0.1 0.6
Bulk Density of B22 Horizon, g/crn^ 1.54 ---
Montmorillonite in A Horizon, Percent (2) of Clay 15.0 ---
Illite in A Horizon, Percent (%) of Clay 35.0 ---
* Data from Van Doren e£ al. (1976) and Blevins and Wilding (1969). 20 in plowed plots. As the shrink-swell potential of the Wooster is low, visible cracks are not formed, even in the dry months.
Four field treatments, combinations of two tillage practices and two rotations, were included in this study out of a possible nine. The tillage practices were plowing followed by two secondary operations, and direct seeding with a no-tillage planter equipped with fluted coulters.
Plowing was done in the fall on the Hoytville and in the spring on the Wooster. The rotations were continuous c o m and com-soybeans. At the time of this study, the treatments had been applied for 15 and 16 years for the Hoytville and Wooster sites respectively. Fertilizer, insecticide, and herbicide applications were made according to Ohio
State University recommendations and are reported in Van Doren et al.
(1976).
Except for the field core experiments and field inoculum potential studies, all of the soil used in the present study was collected from the no-tillage continuous c o m plots at Hoytville. The soil from the three replicates was mixed, dried to about 15-20 percent water content and passed through a 2 mm sieve. The soil was never allowed to dry to water contents less than 15 percent. The method of handling the soil for the field core and inoculum studies will be described later in this section.
B. General Laboratory Methods
1. Root Growth in Soil Core Techniques
The soil core technique was designed to study root growth in soil in a manner that would allow good control of the physical properties of the soil. The inside dimensions of the soil cores used were 7.6 cm in diameter x 7.6 cm long. As will be described later for each specific experiment, sieved soil was packed to a known bulk density or undisturbed field cores were used with or without other laboratory preparation. The cores were placed on a disc of blotting paper in a plastic petri dish.
A hole had been drilled in the center of the dish and a small tygon tube attached by glue such that the inside surface of the dish was flat (Fig.
1.) The soil core-petri dish assembly was put into a two-liter plastic pot. Enough washed quartz sand was placed in the bottom of the pot so the top of the soil core was 2 cm below the top of the pot* Sand was placed around and on top of the soil core. The tygon tube was attached to a manifold where the water potential could be controlled from 0 to
60 cm of water. One corn seedling (PAG SX-424) was placed in the middle of the sand layer on top of the soil core and centered on the soil core.
The sand was saturated with distilled water when the corn was planted and then allowed to drain. The plants were grown in a growth chamber for 14 days. At harvest, the top was cut off at the top root node, weighed, dried at 70 C, and weighed again. The sand was brushed from around the soil core and the roots clipped off at the sand-soil inter face. The soil core was weighed to obtain the soil water content at harvest. The soil was then washed from the roots by slowly dripping water on them for 1-2 days. To prevent loss of fine roots, the soil was not shaken or pulled apart. The roots were then dried and weighed. The length of the roots was estimated by the line intersection method (Newman,
1966). The dried tops were ground to pass a 0.8 mm sieve and digested with a nitric-perchloric acid method (Isaac and Kerber, 1971). Potassium was determined with a Beckman flame photometer. Phosphorus was measured using the ascorbic acid reduced phosphomolybdate method as modified by
John (1970). 22
CORN SEEDLING
\o Q O O o O O 0 o o o oo c o 0 0 0 OO0 * O O o Q { 2 o °o o o oo o n°J* oO oO O O f Oo o SAND/// 000 0 o ?o ir„ o o 0 o o o oo 0 % 0 «O 1 0 0° ■ :* **- , OO O 0 0 I o o o o ° O d % \ OOoO ■# 1 PLASTIC oo«« *** * */ ' * • • ...... I ° * *o 00 S 01 L POT I ° 0o° :* » o o o 1 o o ° * \ - \ O OO •1 >. * -r* 6 O O O j 1 o Qq O **■ I •V*- . "■ I oo 0 . > PETR! \° o 0 . -****- w* *•,•»V:‘ ^ / / /b l o t t in g DISH s^L°n<» PAPER
£ SAN D
TUBING TO SUCTION
Figure 1.— Cross section of apparatus used for soil core experiment. 23
2, C o m Root Baiting Technique
Pythium graminicola cannot be plated directly from soil so a
baiting technique developed by Rao (1976) was used to determine the
Pythium inoculum potential in soil. The test soil was placed in a 500
ml flask. The flask was filled with distilled water and let stand for
1-2 hours to allow the water to clear. Floating residue was removed.
A nylon netting was fastened over the mouth of the flask and secured with
a rubber band. Five c o m seedlings (Pioneer 3780), which had been germi
nated for five days on blotting paper, were placed on each flask so the
roots penetrated through the netting into the water. The radicles were
about 5 cm long at the time of transplanting. Each flask was wrapped in brown paper to prevent light Influencing root growth and to retard algae
growth. Clear plastic bags were placed over the c o m plants to retard evapotranspiration. The flasks were placed in a growth chamber for ten
days. Water was added to keep the level within 0.5 cm of the top of the
flask. When possible, temperatures were kept in the 16-19 C range where maximum infection would occur according to Leach (1947) analysis. After
the growing period, the radicle root system was removed and dried. Root
length was determined by the line Intersection method (Newman, 1966).
C. Preliminary Investigations
Two preliminary experiments were run to find the growth parameters
that would be the most sensitive to changes in the physical properties of the soil, mainly bulk density, and to differences in the Pythium
Inoculum potential of the soil.
1. Effect of Bulk Density on C o m Growth
Hoytville soil collected from the no-tillage continuous corn plots was dried to about 17 percent water content and passed through a 24
2 mm sieve. The soil was packed into soil cores at six bulk densities which were 0.95, 1.10, 1.25, 1.40, 1.55, and 1.70 g/cm^. The soil
required for each core was divided into six equal increments and each
increment was pressed into the cylinder with a hydraulic press. To
insure uniform packing, about 1/4 of the previous increment was scarified before the next increment was added. After packing, the sanples were
saturated and equilibrated at -60 cm of water on a tension table. The
soil samples were placed into the pots in the growth chamber as previously
described. A -0.06 bar water potential was maintained on the soil during
the growing period. Temperatures were 18 C for a 12-hour dark period and and 27 C during the light period. The treatments were replicated three times. At the end of the experiment, root length and other parameters of c o m growth were measured as described above.
2. Effect of Pythium on C o m Growth
To evaluate the effect of Pythium infection of c o m root growth, a flask experiment was established using soil known to be infested with
Pythium collected from the no-tillage continuous corn plots and soil collected from a grassed area near the plots which was thought to be
Pythium free. Samples of both soils were steamed at 55 C for one hour
to kill any Pythium present. Enough soil to cover the bottom of the
flasks with a layer 1 cm thick, 150 grams, was placed in each flask. The
flasks were then filled with distilled water and c o m grown for ten days as previously described. Temperatures in the growth chamber were 10 C and 27 C for the dark and light periods respectively. After ten days,
the radicle root systerr of each plant was removed and the dry root weights and lengths werti measured. As mentioned in the introduction, this method
is selective for Pythium. 25
D. Measurement of Inoculum Potential of Pythium In Soil
As previously noted, Pythium cannot be directly plated from a soil*
so a baiting technique must be used to evaluate the amount of Pythium In
a soil sample. Rao (1976) used c o m seedling roots immersed in water to
measure the potential amount of infection. However, his data did not
show any differences in number of lesions on the roots when the amount
of soil in the flasks was varied. The following set of experiments
summarized in Table 2 were established to attempt to improve on the
method.
1. Soil Mass Variable
The dry weight equivalent of moist soil placed in the flasks was
50, 100, and 150 grams. The standard flask procedure was then followed.
Growth chamber temperatures were 16 C and 19 C for the dark and light periods respectively. Three replicates were used.
2. Effect of Variable Concentrations of Fungicides
a) Pyroxychlor. The experimental fungicide pyroxychlor (Dow
Chemical Company, Reference Number 1-5395-48) was obtained as a liquid with 60.8 mg of active ingredient per ml of material. The material was
diluted and added to the soil to make final concentrations of 0, 1, 2,
and 4 ug of active material per gram of soil. The amount of soil used
per flask was 150 grams oven dry basis. The treatments were replicated
three times. Because of a malfunctioning growth chamber, the temperatures were not precisely controlled but were about 18 and 27 C for the dark
and light periods respectively.
b) GA-1-62. The experimental fungicide, GA-1-62 (supplied by
CIBA-Geigy), was obtained as a 10 percent active ingredient dust. The material was added to the soil in concentrations of 0, 15.9, 31.8, 62.5, Table 2. Variables used In the series of experiments to evaluate methods of measuring Inoculum potential
of Pythium In soil.
Chamber Variables Temperature Growth Variable Exposed Soil Number of Name of Experiment Dark Light Medium Fungicide Soil Area Thickness Replicates
Pyroxychlor 18 27 Distilled Pyroxychlor Constant Constant water 0-4 yg/g 70 cm2 2 cm
GA-1-62 16 19 One-half GA-1-62 Constant Constant Hoagland 0-250 yg/g 15 cm2 3 cm
Mass vs Area 16 19 One-half None 5 or 10 2 or 4 2 2 Hoagland cnr cm*
Infinite Thickness 16 19 One-half None Constant 0-4 cm Hoagland 10 cm^
Surface Area (Water) 16 19 Distilled None 0OJ 15 Constant water cm 3 cm
Surface Area (Water 18 27 Distilled GA-1-62 0 or 15 Constant and Fungicide) water 0-175 yg/g cm2 3 cm
Surface Area (Nutrient 16 19 One-haIf None 0 or 15 Constant Solution) Hoagland cm5 3 cm
O' 27
125, and 250 pg of active material per gram of soil. The soil vas not allowed to spread In the bottom of the flask as before but was packed
Into glass vials to control the surface area of the soil exposed to the roots. The vials were 7 cm high and 2.5 cm in diameter. Twenty grams of soil were placed into each vial and packed to a bulk density of 1.0 g/cm^ with a rubber stopper leaving a smooth surface. The depth of soil in each vial was about 3 cm. Three vials were placed into each flask.
Rather than distilled water, 1/2 strength Hoagland*s nutrient solution was used as the root growth medium. C o m plants were placed in the flasks in the normal manner and grown for ten days at 16 and 19 C dark and light cycles respectively.
3. Soil Mass vs Area of Soil Exposed
The glass vials used to contain the soil in the last experiment offered an opportunity to vary independently the weight of soil and the area of soil exposed to the roots. Ten or twenty grams of soil were weighed into each vial and packed to a bulk density of 1.0 g/cm^ to give soil depths of 2 or 4 cm respectively. The number of vials set into each flask was one or two. The experimental design was a 2 x 2 factorial with soil depth and number of vials as the two variables giving soil weights of 10, 20, or 40 g/flasks. Each treatment was replicated six times. The standard flask procedure, using 1/2 strength
Hoagland’s solution rather than distilled water as the rooting medium, was followed. Growth chamber temperatures were 16 C during the dark period and 19 C during the light period.
4. Infinite Thickness
In order to find the maximum thickness of soil that can be between c o m roots and Pythium and still have Infection, vials were packed to different thickness of soil. Enough soil was weighed into the vials to 28 give soil depths of 0, 0.25, 0.50, 1.0, 2.0, and 4.0 cm. The soil was packed to a uniform density of 1.0 g/cm^. Two vials were placed into each flask so the exposed surface area was held constant at 10.1 cm^.
The rooting medium was 1/2 strength Hoagland*s solution. The growth chamber temperatures were 16 and 19 C for the dark and light periods respectively. Each soil depth was replicated four times. Otherwise, the standard flask procedure was used.
A regression program (Dixon, 1974) was used to fit the data to an exponential equation of the form Y = A - Be“^X where Y is the root length in meters, A is the root length at infinite soil depth, B is the maximum reduction in root length caused by the Pythium infection, C is a fitted constant, and X is the soil depth in cm. The equation is similar to the self-absorption equation used in radioisotope studies.
5. Surface Area Varied
a) Hater. The previous experiments Indicated that the amount of root damage caused by Pythium is related to the surface area of soil exposed to the roots, provided that infinitely thick samples are used.
To clarify the type of relationship between area of soil exposed and root length, a flask experiment was designed with the number of vials varied from 0 to 3 to give areas of soil exposed of 0, 5.1, 10.1, and
15.2 cm^. The soil was packed into the vials to a bulk density of 1.0 g/cm^ and a thickness of 3 cm. Distilled water was used as the rooting medium. Standard flask experiment methods were used with each treatment replicated six times. Temperatures in the growth chamber were 16 and
19 C.
b) Water-fungicide treated soil. As the c o m roots were af fected more by nutrient deficiency than Pythium infection in the previous experiment, a method was designed to separate the Increased growth 29
caused by more soil nutrients from the decrease caused by the Pythium
infection. One set of flasks was set up as In the previous experiment with 0, 1, 2, and 3 vials of soil per flask. A second set was assembled
in the same fashion except vials of soil treated with 175 ug of GA-1-62
fungicide per gram of soil were placed in the second set so that all
these flasks contained three vials of soil. The number of untreated
vials in the second set varied from 0 to 3 and the number of treated 3 vials from 3 to 0. The soil was packed to a bulk density of 1.0 g/cm and a thickness of 3 cm. Pregerminated c o m was placed on the flasks and grown in the standard manner. Distilled water was used as the
rooting medium. Each treatment was replicated three times. Growth
chamber temperatures were 18 C for the dark period and 27 C for the
light period.
c) Nutrient solution. In order to mask the nutrient supply of
the soil, so root damage would be caused only by Pythium, an experiment was set up as in the surface area varied (water) experiment except rather
than distilled water, nutrient solution was used as the rooting medium.
The soil was packed in the vials at a bulk density of 1.0 g/cm^ and a depth of 3 cm. The number of vials per flask was varied from 0 to 3 to
give areas of soil exposed of 0, 5.1, 10.1, and 15.2 cm^. Each treat ment was replicated six times. Temperatures in the growth chamber were
16 and 19 C.
E. Field Core Experiments
In order to evaluate the physical properties of soil that could be causing an increased amount of Pythium infection in the no-tlllage plots, undisturbed soil core samples were taken. Bulk density and water content at -0.06 bar potential measurements were made. Various laboratory 30 treatments were applied to the soil samples and then corn olants were grown into the soil cores. The experiments were conducted using Hoytville and Wooster soils collected in the fall of 1975 and again on Hoytville soil samples collected in the spring of 1976. In conjunction with these experiments, bulk densities were measured with the clod method and six- week—old corn plants were harvested from the field plots and analyzed.
1. Autumn Core Experiment
Undisturbed soil cores, 7.6 x 7.6 cm, were collected using an
Uhland type sampler in October 1975. Samples were taken from the Hoyt ville and Wooster sites. The cores were taken from plots representing the no-tillage continuous corn, no-tillage corn-soybean rotation and plowed continuous c o m treatments. The rotation plots were in corn at the time of sampling. Sampling was completed before any autumn tillage operations were performed. Each of the three replicates in the field were sampled, and four core samples were taken from each plot for a total of 36 samples of each soil series. The samples were taken at about the
2-9 cm depth.
All the cores were saturated and equilibrated at -0.06 bar water potential. The cores were divided into four sets, with each set contain ing one core from each field plot. One set was oven dried to obtain a bulk density value. The second set of cores was dried to about 15 percent water content, passed through a 2 mm sieve, and repacked into a 7.6 x
7.6 cm core. An air-steam mixture at 55 C was forced through the third set of cores for one hour. The fourth set of cores was not treated after equilibration at -0.06 bar water potential.
The cores, excluding the oven dried set, were resaturated and equilibrated at -0.06 bar water potential and placed in the pots in the growth chamber (Figure 1). C o m was grown in the pots for 14 days with 31 temperatures of 18 C during the dark period and 27 C for the light period.
Soil water potential was maintained at -0.06 bar during the growing period. The plants were harvested and analyzed as previously described.
The data were analyzed as a split plot design with field treat ments as the main effects and laboratory treatments as the sub-units.
2. Bulk Density of Clods
Clod samples for bulk density measurements were taken from the plots at Hoytville in October 1975, using a spade to expose the clod3.
Three clods were carefully selected from each plot. Each clod was dipped in Saran once in the field and placed in a pint ice cream carton for storage. In the laboratory, the standard soil characterization procedure was followed. Briefly, the clods were weighed, dipped in
Saran two more times, rewelghed in air and water. A small corner was cut off each clod and the clods were saturated for two days. The clods were then equilibrated at -0.06 bar water potential, weighed, equilibrated at -0.33 bar water potential, and reweighed. The clods were then oven dried, dipped in Saran and reweighed in air and water. Field and oven dry bulk densities were calculated.
3. Spring Core Experiment
Undisturbed soil cores, 7.6 x 7.6 cm, were collected from the plots at Hoytville on April 16, 1976. The field treatments sampled were continuous corn, plowed and untilled, and corn-soybean rotation, plowed and untilled. The rotation plots were being planted to corn at sampling time. Four samples were taken from each of the three replications of each of the four treatments for a total of 48 cores. The samples were split into four groups, each group containing a sample from each field plot. 32
One set of cores was dried to about 5 percent water content,
passed through a 2 mm sieve, and repacked into a 7.6 x 7.6 cm cylinder.
A second set was also dried and sieved. Prior to repacking, the second
set was sprayed with 25 ml of pyroxychlor solution to obtain a final
concentration of 2.3 pg/g of soil. The third set was not sieved, but
25 ml of the pyroxychlor solution were pouredon the top of the cores
and allowed to diffuse through the soil. The fourth set was not treated
in the laboratory.
The cores were placed into pots in the growth chamber (Figure 1).
Corn was grown in the pots for 14 days as previously described. Growth
chamber temperatures were 16 and 19 C for the dark and light period
respectively. The corn was harvested and growth measured as described
above. Before the soil was washed from the roots, a small sample of
about 10 grams was removed from each core for water content determinations.
Using the water content of the sub-sample and the wet weight of the
entire core, oven dry weight and bulk density of each core were calculated.
The experimental design was a split plot with field treatments
as the main effects and laboratory treatments as the sub-units. The
main effects were analyzed as a 2 x 2 factorial of tillage and rotation.
The sub-units were also analyzed as a 2 x 2 factorial of laboratory
tillage and fungicide treatments.
4. Samples of Corn Plants From the Field
Where the primary concern of application of research data is to
field results, laboratory work must be confirmed by field growth. To meet this requirement, corn plant samples were collected from the field on June 12, 1976, or 57 days after planting. The plants were about 40 cm high at sampling. Three plants were collected from each of the three
replicates of the continuous corn, plowed and untilled, and com-soybean 33 rotation, plowed and untilled plots. One plant was selected from 1.5 meter intervals which were 6.1 meters apart along a row. The plants were selected randomly from the interval by choosing a plant and harvest ing the fifth plant to the right of the chosen plant. The plants were dried at 70 C, weighed, and analyzed for potassium and phosphorus.
F. Evaluation of Specific Hypotheses
Experiments were developed to test each of the three proposed mechan
isms for causing the decrease in corn yield of no-tillage continuous
c o m compared to plowing or no-tillage com-soybean rotations. Testing
the first hypothesis, that high bulk densities and the presence of Pythium
together reduce corn growth more than either alone, required one experi ment. Three experiments were used to evaluate the possibility that c o m
stalk residue turned under by plowing limited the buildup of Pythium
inoculum in the soil. Three experiments were also designed to test the hypothesis that Pythium concentrates in the structural cracks of the
Hoytville soil, where chances of infecting roots are high. The experi ments are sumnarized in Table 3.
1. Bulk Density by Phythium Interaction
In order to evaluate the effect of bulk density and water potential on the amount of Pythium infection on roots, a 3 x 2 factorial experiment was designed with bulk density, water potential and presence of Pythium as the variables. Bulk densities of 1.20 and 1.50 g/cm^ were obtained by packing the soil into 7.6 x 7.6 cm cylinders as previously described.
Water potentials of 0 and -0.06 bar were maintained on the soil in the growth chamber, using the petri dish tension tables in the pots (Figure 1) .
The soil used was collected from the no-tillage continuous corn plots at
Hoytville and was infested with Pythium. Control of the Pythium was obtained by mixing 2.3 ug/g of pyroxychlor with the soil before packing Table 3. Summary of variables used to test the three hypotheses proposed as causing increased infection of c o m roots in no-tillage corn plots.
Chamber Type of Time of Bulk Hypothesis Name of Experiment Temperatures Experiment C o m Growth Fungicide Density Dark Light Days g/cm-* C Bulk Density by Pyroxychlor Pythium Interaction --- 16 19 Core 14 0 or 2 ug/g 1.20-1.50
Effect of Organic Amendments Flask Experiment 16 19 Flask 10 Steam (55 C) --
Organic Residue IS 27 Core 14 Pyroxychlor 1.20 0 or 2 ug/g Carbon Dioxide Evolution 16 19 Flask 10 None --
Pythium Buildup Along Cracks Inoculum Potential Along Cracks 16 19 Flask 10 None ...
Root Growth Along Cracks 18 27 Core 14 Pyroxychlor 1.50 2 Ug/g Root Growth Along Pythium Infected Cracks 16 19 Block 21 None 1.51 35
the cores. The pyroxychlor was diluted with 25 ml of distilled water
and sprayed on the soil in a wide mouth Jar to distribute the fungicide
uniformly and prevent loss of the material. The untreated samples were
sprayed with 25 ml of water as well so the soils were at the same water
content when packed. The water content at packing was 25 percent. The
cores were stored In pint cartons at 4 C for two weeks before using.
The cores were placed in the pots In the growth chamber and c o m
was planted. The temperatures in the growth chamber were 16 and 19 C for
the dark and light periods respectively. The c o m was harvested 14 days
after emergence. The plant growth and nutrient uptake parameters were
measured as previously described.
2. Effect of Organic Amendments
Three experiments were designed to test the hypothesis that c o m
stalk residue turned under by plowing limited the population of Pythium or at least reduced Pythium infection. In the first experiment, organic
residue was added to the soil in flasks and the standard c o m root bait
ing technique was used to evaluate the inoculum potential. In the second
experiment, organic residue was mixed with soil and packed into cores.
C o m was grown into the cores using the standard core methods. In the
third attempt to evaluate the effect of organic residues in Pythium 2 infection, CO evolution rates of soil were compared to the root length of c o m plants grown in the flask containing the soil.
1) Flask experiment.
Soil from the no-tillage continuous c o m plots at Hoytville was dried to about 20 percent water content and passed through a 2 mm sieve. Soil was weighed into 150 gram subsamples and residue was mixed with the soil at the rates of 0, 0.5, 1.0, and 1.5 percent by weight. 36
The residue consisted of entire mature c o m stalks, excluding ears, ground in a Wiley mill and dried at 70 C. Nitrogen as NH^NO^ was added to bring the C:N ratio of the soil-residue mixture to approximately 10:1.
The amounts of added nitrogen were 0, 0.15, 0.30, and 0.45 g NH^NO^ per flask for the respective residue treatments. Water was added to bring the soil to about field capacity. The soil was allowed to incubate at room temperatures for 0, 2, and 4 weeks. Soil that had been treated with a 55 C steam treatment was included in each time of incubation set of samples. Three replications of each treatment was used.
After incubation, the soil was put into 500 ml flasks and c o m grown in the flasks using the procedures previously described.
Vials were not used to contain the soil, but the soil was simply spread out in the bottom of the flasks. The surface area exposed was about 70 cm and the soil depth was about 2 cm. However, neither were precisely controlled. Temperatures in the growth chamber were 16 and 19 C for the dark and light periods respectively. After ten days, the root length of the radicle root system was determined- The root length as a percent of the root length of the plants grown over the steamed soil at the same time of Incubation was calculated.
2) Organic Residue Additions To Soil Cores
a) Without nitrogen. The data from the experiment where organic residue was mixed with soil in the flasks indicates that organic residue may control Pythium infection, at least in the specialized flask environment. A soil core experiment was designed to find if biological control would work when the corn roots were actually in the soil.
Hoytville soil, collected from the no-tillage continuous c o m plots, was dried to about 15 percent water content and passed throuj^i a 2 nun sieve. The fungicide, pyroxychlor, was added at the rate of 2.3
yg/g of soil for the control soils. Organic residue, consisting of the
ground mature c o m stalks described in the last experiment was added to
the soil at 0 and 1 percent by weight. The pyroxychlor was sprayed onto
the soil before packing the cores. The pyroxychlor was diluted with 25 ml of distilled water. Soil for untreated cores was sprayed with 25 ml of distilled water. The organic residue was added to and mixed with the soil before the cores were packed. The soil was packed into 7.6 x 7.6
cm cores at 1.20 g/cm^ bulk density. The cores were placed into the pots
in the growth chamber 24 hours after packing and c o m planted immediately.
The experimental design was a 2 x 2 factorial with factors being fungi cide and added residue, each at two levels. C o m plants were grown for the standard 14 days after emergence as described In the general information section. The water content of the soil cores was maintained at near saturation. Because of a malfunctioning growth chamber, the temperatures were not controlled but were about 18 and 27 C for the dark and light periods respectively. At harvest, the plants and roots were removed and the standard data collected as previously described.
b) With nitrogen. In the previous experiment using soil cores to test the effect of added organic residue on Pythium root infection, a deficiency of nitrogen decreased root growth substantially. To alle viate the problem the same experiment was repeated except nitrogen at the concentration of 330 yg of nitrogen per gram of soil was added in the ammonium nitrate form. The nitrogen was dissolved In the 25 ml of water sprayed on as a carrier of fungicide. All cores received the ni trogen treatment. Water content of the cores, temperatures and other environmental factors during the corn growth period were maintained as close as possible to the conditions of the last experiment. 38
3) Carbon Dioxide Evolution
In order to further evaluate the possibility of c o m stalk residue, which is buried by plowing, affecting the inoculum potential of
Pythlum in the soil, field plots at Hoytvllle were sampled April 16,
1976. The field treatments were continuous com, plowed and no-tillage and com-soybean rotation, plowed and no-tillage. The rotation plots were in c o m at sampling time. Each plot was sampled in two locations.
Laboratory measurements were replicated six times. Carbon dioxide evo lution from the soil was measured and the inoculum potential of the soil was measured on the same soil.
The plowed plots were sampled at the 3-10 cm depth in random locations in each plot. Two samples were collected from each of the no- tillage plots, one sample from the soil along the cracks and the other from the ped interiors. The soil was loosened with a spade and clods at about the 5 cm depth were removed. Soil material was scraped off the clod surfaces and used as the "crack” samples. The remaining part of the clod was considered to be ped interior. The crack material was about
0 .5-1.0 cm thick and was easily identified as the ped interiors were much denser. The soil samples were stored at 4 C for about four weeks until analyses were started.
The soil was passed through 4.38 mm sieve and a subsample was taken for water content determination. For carbon dioxide evolution measurements, 150 grains of soil on a dry weight basis were placed into
500 ml flasks. A 35 ml glass vial containing 5 ml of 0.3 N sodium hy droxide was placed on top of the soil and the flask sealed. Vials were changed every 48 hours, three times, for a total period of 144 hours.
The evolution study was done in a growth chamber at 16 and 19 C. The flasks were covered with paper bags to prevent algae growth. 39
After the carbon dioxide evolution study was completed, the flasks were filled with distilled water. C o m seedlings were placed on the top of the flasks with the roots penetrating into the water. The normal corn root baiting technique was followed. The c o m was grown for ten days at 16 and 19 C. The radicle root systems were removed and measured.
3. Pythium Build Up Along Cracks
In the last experiment, the root growth of c o m plants exposed to soil material from along the cracks area of plots in no-tillage continuous c o m was less than the root growth over soil from any other sample site. The material along the cracks may have a higher inoculum potential of Pythium and if roots follow the planes of weakness along the cracks, greater root infection would take place. A series of experi ments were developed to: (1) measure the inoculum potential of the soil material from along the cracks and from the ped interiors, (2) find if roots do follow cracks at the bulk densities found in the field, (3) find if root growth is actually reduced by a higher inoculum potential along the cracks.
a) Inoculum potential along cracks. Samples of soil were col lected from the no-tillage continuous c o m plots at Hoytville on July 3,
1976. The soil material along the cracks was collected by scraping
0 .5-1.0 cm of loose soil from the surface of clods exposed with a spade.
After the entire surface of a clod was scraped off, the remainder of the clod was retained as soil from the ped interior. Sampling depth was about 5-10 cm. The field plots were sampled in four locations so 12 replicates of each of the two microsites were tested for inoculum poten tial. 40
The c o m root baiting technique as refined in the present
study was used to measure the inoculum potential. As refined procedure
has not been described as a unit before, the complete procedure follows.
Soil at the field moisture content of about 25 percent was passed through
2 2 mm sieve. Twenty grams of the crushed soil uere packed into 7 cm high
x 2,5 cm diameter glass vials at a bulk density of 1.0 g/cm . The soil
was packed to a depth of 3 cm with a rubber stopper to obtain a smooth
flat surface. Two of the vials were placed in a 500 ml flask, so that 2 the area of soil exposed was 10.1 cm . The flasks were filled with 1/2
strength Hoagland's solution, wrapped in brown paper and the mouths of
the flasks covered with a piece of nylon netting. Five pregerminated
c o m seedlings were placed on the netting with the roots emersed in the nutrient solution. The radicles were about 5 cm long at the time of
transplanting. The corn plants were covered with a clear plastic bag to
retard evapotranspiration. The flasks were placed in a growth chamber
for ten days. Temperatures were 16 C for the 12 hour dark period and
19 C for the light period. After the growth period, the c o m plants were removed and the radicle root system cut off and dried. Root length was determined by the Newman (1966) line intersection method.
b) Root growth along cracks. If the higher inoculum poten tial along the cracks are to cause a greater infection of the c o m roots, roots must preferentially follow cracks at the bulk densities found in the field.
A soil core experiment was designed to find the effect of different amounts of crack surface on root growth. The soil cores were made from Hoytville soil collected from the no-tillage continuous c o m plots. The soil was dried to about 15 percent water content and 41 passed through a 2 mm sieve. Before packing, the soil was treated with
2.3 pg/g of pyroxychlor to kill any Pythium present. The cores were packed to a bulk density of 1.50 g/cm in the normal manner except hori zontal cracks were made in the cores while packing. The cracks were formed by placing paper disks between the increments of soil added during packing. After the entire core was completed, the core was easily sepa rated and the paper removed. The cores was then reassembled to the standard 7.6 cm height. The number of cracks were varied from 0 to 5 to 2 give internal crack areas of 0, 45.6, 91.2, and 228.0 cm . Geometrical configurations were not the same for the different number of cracks.
Location of the cracks were 3.8 cm from the top of the core when one crack was used, 2.53 and 5.07 cm from the top for two cracks and in the cores with five cracks, the cracks were spaced every 2.54 cm.
The core was placed in the pots in the growth chamber
(Fig. 1). Water potential of the soil was maintained at -0.06 bar during the c o m growth period. The c o m plants were grown for 14 days after emergence. Growth chamber temperatures were not controlled because of mechanical malfunction but were about 18 and 27 C. At harvest, the tops and roots of the c o m were removed and the standard parameters were measured as previously described.
c) Root growth along Pythium infected cracks. The following experiment was designed to test the hypothesis that root growth in soil con taining cracks with a high Pythium inoculum potential would be less than in soil with cracks with low inoculum potential.
Cubes of soil, approximately 5 cm per side, were comp- pressed in a form designed to fit a hydraulic press. The form was a metal box with square base 5 cm per side and 15 cm high. The box was 42
made from two pieces of angle iron bolted together, so the soil cube
could be removed by separating the frame. A metal plunger with a square
base fitting the form was used to pack the soil. The soil used to make
the cubes for the top 20 cm of the blocks was collected from the no
tillage continuous corn plots at Hoytville.
Soil collected from the B horizon of a nearby site was
used to make the cubes for the bottom layer of the blocks. The soil was
dried to about 15 percent water content and passed through a 2 mm sieve.
The soil from the A horizon was infested with Pythium. Enough soil was
weighed and placed into the form to make a final volume of 125 cm^ with
a bulk density of 1.51 g/cm^. Dry clods from the field had a bulk den
sity of 1.63 g/cm^ and a preliminary study showed that cubes packed to a
bulk density of 1.51 g/cm^ shrank on drying to a density of 1.63 g/cm^.
The cubes were assembled to make blocks of soil 25 cm
high and 15 cm on each side. The bottom row of cubes was made from the
B horizon material. The B horizon material was used as a buffer volume
between the Pythium infected soil and the base of the blocks. The center
cube of the top layer of each block, where the c o m was planted, was
packed to a bulk density of about 1.0 g/cm-*.
Before assembling the blocks, the cubes used to make the
infested blocks were rolled in a Pythium infested medium. The medium was made of two parts sand, one part c o m meal, and one part of mashed
roots known to be Pythium infected. A thin layer of the medium was spread out on a lab bench and the cubes rolled through the medium. The medium was kept saturated with distilled water so the medium would adhere to
the sides of the soil cubes. 42 made from two pieces of angle iron bolted together, so the soil cube could be removed by separating the frame. A metal plunger with a square base fitting the form was used to pack the soil. The soil used to make
the cubes for the top 20 cm of the blocks was collected from the no- tillage continuous c o m plots at Hoytville.
Soil collected from the B horizon of a nearby site was used to make the cubes for the bottom layer of the blocks. The soil was dried to about 15 percent water content and passed through a 2 mm sieve.
The soil from the A horizon was Infested with Pythium. Enough soil was weighed and placed into the form to make a final volume of 125 cm^ with a bulk density of 1.51 g/cm^. Dry clods from the field had a bulk den sity of 1.63 g/cm^ and a preliminary study showed that cubes packed to a bulk density of 1.51 g/cm^ shrank on drying to a density of 1.63 g/cm^.
The cubes were assembled to make blocks of soil 25 cm high and 15 cm on each side. The bottom row of cubes was made from the
B horizon material. The B horizon material was used as a buffer volume between the Pythium Infected soil and the base of the blocks. The center cube of the top layer of each block, where the corn was planted, was packed to a bulk density of about 1.0 g/cm-*.
Before assembling the blocks, the cubes used to make the infested blocks were rolled in a Pythium infested medium. The medium was made of two parts sand, one part c o m meal, and one part of mashed roots known to be Pythium Infected. A thin layer of the medium was spread out on a lab bench and the cubes rolled through the medium. The medium was kept saturated with distilled water so the medium would adhere to the sides of the soil cubes. 43
After the blocks were assembled, they were wrapped In plastic and brown paper. The blocks were placed in the growth chamber and the c o m planted. Temperatures in the growth chamber were 16 C during the 12 hour dark period and 19 C during the light period. The blocks were saturated for 24 hours, 6 and 13 days after the c o m emerged by putting the entire block into a tub of water. Separate tubs were used for the infected blocks. Each of the two treatments were replicated four times.
The c o m was harvested 21 days after emergence. The top was cut off, weighed, dried at 70 C and weighed. The roots growing along the exterior of the blocks were cut off and measured. The B horizon material was separated from the remainder of the block. The A and B horizon soil material was then washed off of the roots and the roots were dried and measured for weight and length. III. RESULTS AND DISCUSSION
A. Preliminary Investigations
A preliminary soil core experiment where bulk density was the vari
able and a flask experiment was performed to find which parameter of corn
growth was the most sensitive and reliable to changes in physical and
pathological properties of the soil.
1. Effect of Bulk Density on Corn Growth
Soil cores were packed to six bulk densities and c o m grown on
these cores for 14 days following the given procedures. Root length and weight, top height, fresh and dry top weight, and potassium and phosphorus content in the tops were determined and are shown in Appendix A.
A backward elimination procedure (Service, 1972) was used to pick the best polynomial model between bulk density and each of the nine para meters of c o m growth. The significance level for a variable staying in the model was 20 percent. The best fit model and the R for each of the parameters are given in Table 4. Of the nine, changes in root length were the most closely related to changes in bulk density. The cubic equation 2 has a R of 0.82. The next best regression coefficient was 0.57 for total phosphorus uptake. Root weight decreased curvilinearly with increasing bulk density but the R of the equation was only 0.23.
Root length was highly correlated (r = 0.8) with top height, fresh and dry top weights, and total uptake of potassium and phosphorus.
The correlation between root length and dry root weight was statistically significant but the correlation coefficient of 0.67 is less than that of
44 A5
Table A. Regression equations and regression coefficients between bulk density (X) and nine parameters of corn growth (Y).
Variable Equation R2
Root Length Y - 58.7 - A2.5X2 + 1A.3X3 0.82
Plant Height Y - 37.5 - 2.3X3 0.51
Fresh Top Weight Y - 7.A5 - 3.3AX 0.51
Dry Top Weight Y - 772 - 331X 0.A2
Dry Root Weight Y “ 23 - 55X2 0.23
Potassium Concentration Not significant —
Phosphorus Concentration Not significant —
Potassium Uptake Y - 19.7 - 2.5AX3 0.A6
Phosphorus Uptake Y = 1.65 - 0.73X 0.57 46 the other five parameters. Root length was not related to the concentra tion of potassium or phosphorus in the tops.
The relationship between root length and soil bulk density is shown in Figure 2. The root length decreased rapidly between bulk densi ties of 0.95 and 1.55 g/cm^, and then declined slowly as the bulk density was increased to 1.70 g/cm . Root diameters as observed but not measured under a biocular microscope increased dramatically between bulk densities of 1.25 and 1.40 g/cm^.
The concentration of potassium and phosphorus in the plant tops was not affected by the bulk density of the soil. However, the total amount of both nutrients in the plant tops decreased as bulk densities
Increased.
Water content of the soil at harvest was about the same as when the c o m was planted or at the potential of -0.06 bars. As the cores swelled when wetted, actual aeration porosities could not be calculated.
2. Effect of CRP Infection on C o m Growth
The dry root weights and root lengths as determined by the line intersection method are given in Table 5. The dry root weights did not vary with soil or steam treatment. However, the root length of plants grown in presence of unsteamed soil from continuous corn plots was 4.7 m compared to 8.4 m for the steamed treatments. Pathogens which can be killed by steaming, presumably Pythium, as the selective baiting technique was used, reduced the root length by about 50 percent. The steam treat ment killed all of the CRP as the roots grown over the treated soil did not have any lesions. The sod soil did not exhibit any evidence of CRP as both the steamed and untreated soil allowed the same root growth of about 8-10 meters. plants. 14-day-old oflength root on cores into packed soil Hoytville of density of bulk effect 2.--The Figure
ROOT LENGTH,m/core 0 3 20 0.95 - 1.10 BULK DENSITY, g/fcm3 DENSITY, BULK 1.25 1.40 - 0.82- 14,4X + 42,5X 1.55
48 Table 5. Root weight and root length of corn plants grown in flasks
over Hoytville soil from continuous sod and continuous corn plots.
Previous Crop Steam Treatment Dry Root Weight Root Leneth mg m
Sod None - /94.8a 10 .0a
Sod 55 C 72.2a 8.4a
Corn 55 C 80.9a 8.4a
C o m None 70.1a 4.7b
1/ Values followed by the same letter in each column are not different
at the 5 percent probability level. 49
Observation of the roots showed the main reduction of root length was in the secondary roots. The main roots were somewhat shortened and
curled but the growth of many of the secondary roots had evidently been
stopped in the budding stage. The roots grown above steamed corn field
soil or above sod soil were clean and white with large numbers of secon
dary roots each about 5 cm long. Primary root length was also reduced by
CRP but the effect was small compared to secondary roots. Few tertiary
roots were present in any of the treatments.
3. Discussion of Preliminary Investigations
The two preliminary experiments show that root length is a reliable parameter to evaluate the effects of the soil physical conditions and CRP
infection in the root. Root weight was not a good measurement of the effects of these two soil variables because of changes in the morphology of the root. Increasing the bulk density caused the roots to be shorter and thicker as was also shown by Russel and Goss (1974) . The increase
in bulk density caused a quadradic reduction in root weight rather than linear as shown by Meredith and Patrick (1961), but the root weights gave a lower correlation with the other parameters of corn growth than did root length.
Soil particles adhering to roots in the bulk density experiment would increase the variability of root weight and would partially explain the lack of correlation between root length and root weight. However, the roots from the flask experiment were grown in water, eliminating the possibility of foreign material affecting the root weights. The Pythium infection also caused a change in the root morphology by stopping the growth of secondary roots. For these reasons, root length was used as the major parameter of c o m growth in the remainder of the study although data on the other parameters was still collected. 50
B. Measurement of Inoculum Potential of Pythium in Soil
In the preliminary study of the effect of Pythium infection on corn roots, root length was found to be a good indicator of the presence of
Pythium. As Rao (1976) could not quantify the amount of Pythium inoculum in a soil using lesion counts or size of lesions, root length was inves tigated as an alternate parameter of measurement.
1. Soil Mass Variable
The root lengths and weights of corn plants grown In flasks con taining 50, 100, and 150 grams of soil dry weight equivalent are shown in
Table 6 . The weight of soil had no consistent effect on the lengths of the roots. The weights of the roots increased somewhat as the amount of soil in the flasks Increased although the differences were not statisti cally significant.
2. Effect of Variable Concentrations of Fungicides
a) Pyroxychlor. The pyroxychlor did not significantly affect the growth of corn roots although the highest rate did permit the longest root length (Table 7). The roots grown in soil treated with 4 pg/g of pyroxychlor were white and healthy in appearance. No lesions were noted at the high rate of fungicide. Root growth was probably inhibited by the pyroxychlor, which is phytotoxic in high concentrations (Rao 1976). The pyroxychlor treated roots appeared to have normal morphology so the re duction in root length occurred throughout the root system.
b) GA-1-62. The root length of corn plants grown over the
GA-1-62 treated soil increased in an asymptoxic fashion (Figure 3.). Near maximum root length, as estimated by the equation, occurs at the GA-1-62 concentrations greater than 110 ug of active ingredient per gram of soil. 51
Table 6 . Root lengths of five corn plants grown above three different
amounts of Pythium. infested soil.
Soil Weight Root Length Root Weight
Dry Weight Basis g m mg
50 k/ 9.2 1/ 98.9
100 8.5 103.3
150 12.0 131.4
1/No significant difference at the 25 percent level of probability
occurs between values in the same column. 52
Table 7. Effect of four concentrations of the fungicide, pyroxychlor, on root growth of corn plants exposed to Pythium infested soil.
Concentration Root Length Root Weight Ug Pyroxychlor per g Soil m mg
0 i/l6.1 1/108
1 16.1 142
2 16.4 139
4 19.0 136
1J No significant difference at the 25 percent level of probability occurs between values in the same column. Figure Root 3.— length plantsof m o c grown over Pythium Infestedsoil treated with six concentrations thefungicide GA-1-62. Each data point is an average ofthree replications.
ROOT LENGTH, 50 CONCENTRATION OF GA-l-62,pg/g -O.0277X 250
54
The root length at 250 ug/g was slightly less than at 125 Mg/g, but the difference is not significant, so the fungicide GA-1-62 is evidently not phytotoxic at effective concentrations.
The use of the vials to control the surface area of the soil exposed worked well, reducing the variability of the results to where
statistical and agronomic significance became apparent.
3. Soil Mass vs. Area of Soil Exposed
As shown in Table 8 , the length of the corn roots was not affected by changes in the weight of soil placed in the flask, but was affected by the area of soil exposed. As the weight of soil was increased from 10 to
20 grams, but with soil area held constant at 5.1 cm^, the root length did not decrease significantly. However, when the weight of soil was held constant at 20 grams and the surface area of soil doubled from 5.1 to 10.1 cm , the length of c o m roots decreased from 16.5 to 10.4 m. A further doubling of the soil mass with soil area held constant at 10.1 cm^ did not affect the length of the corn roots.
4. Infinite Thickness
The length of roots of ten-day-old c o m plants decreased curvi- linearly according to the exponential decay law (Figure 4). The root length at infinite soil depth was 12.0 m and the maximum reduction in root length caused by the Pythium was 9.0 m. The constant C was 1.45 cm”^. The soil depth at 95 percent of infinite thickness, found by multi plying the inverse of C by three is 2.1cm. The equation is significant at the 1 percent level of probability. The regression accounts for 86 percent of the sums of squares due to the treatment.
5. Surface Area Varied
a) Water. The root length of the c o m plants grown in flasks 55
Table 8 . Length of c o m root from five plants grown in flasks containing different numbers of vials filled to different depths of soil. Each value is the average of six replications.
Soil Weight Soil Depth Soil Area Root Length g cm cm'4 m
10 2 5.1 Xf18.8a
20 A 5.1 16.5a
20 2 10.1 10. Ab
AO A 10.1 12.3b
JL/ Values followed by the same letter axe not different at the 5 percent level of probability. 56
20
b
1 2
UJ
8
4 12.0 + 9.0e 1-45x
0 )i------1------1------1------0 12 3 4 SOIL DEPTH,cm
Figure 4.— Root lengths of 10-day-old com plants grown in flasks con taining vials with various depths of Pythium infested soil. Each experi mental value is the average of four replicates. 57
increased as the area of soil increased. The Increase in root length was linearly related to the increase in area of soil exposed (Figure 5).
The roots growing in the flasks containing no soil were short, thick, and obviously nutrient deficient. As the amount of soil exposed to the roots was increased, the effect of the nutrient deficiency was decreased* Equi librium, where increased amount of soil had no effect on the roots, was not reached as the roots growing in the flasks containing three vials of soil were larger than roots from flasks containing two vials. The roots grown in flasks containing soil did have lesions, probably caused by
Pythium. However, the effects of the nutrient deficiency were greater than the effects of the Pythium infection, even at the largest area of soil exposed.
b) Water plus fungicide. The root lengths of com plants grown in flasks with only untreated soil to supply nutrients increased as the amount of soil exposed increased, as in the previous experiment. Again, the plants grown in flasks without soil showed nutrient deficiencies at the end of the growing period. Where soil treated with fungicide was placed in the flasks such that all treatments had equal amounts of soil exposed to supply nutrients but the surface area of Pythium Infested soil was varied, excluding fungicide treated soil, the root length decreased as the amount of infested soil increased (Figure 6). The plants growing in flasks containing only fungicide treated soil were healthy without lesions. As the amount of infested soil was increased, the root length of the c o m plants decreased and visual signs of Pythium infection became apparent.
The root length of plants grown in flasks with only untreated soil to supply nutrients increased 0.37 m for every square centimeter of with distilled water as the rooting medium. Each data point is the average of six replications.six of theaverage is point data Each rootingmedium. the as water distilled with Figure 5.— Root lengths of 10-day-old c o m plants exposed to four different areas of Pythium infested soil,infested Pythium ofareas fourdifferent to exposed plants m o c 10-day-old of lengths Root 5.— Figure
ROOT LENGTH, 16 12 0 4 AREA OF EXPOSED,cm2 SOIL 5.1 10.1
24 E * X I— o
UJ
oh* o * X
o i o.i 15.2 AREA OF INFESTED SOIL EXPOSED.cm2
Figure 6 .— Root lengths of 10-day-old c o m plants exposed to four areas of Pythium infested soil, without treated soil to supply nutrients 0 and with soil treated with a fungicide to supply equal amounts of nutri ents O * The rooting medium is distilled water. Each experimental value is the average of three replications.
m 60 additional soil exposed* Conversely, when treated soil was used to keep the nutrient supply the same, root length was decreased 0.88 m/cm^ of infested soil. The effect of nutrient deficiency was much less than the effect of Pythium infection.
c) Nutrient solution. The length of corn roots decreased from
17.8 m when no soil was placed in the flasks to 10.4 m when the area of infested soil was 15.2 cm . The decrease in root length was linear
(Fig. 7) within the range of soil area in the experiment.
The 1/2 strength Hoagland's solution supplied sufficient nutrients to the plants as evidenced by the large amount of root growth
In flasks without soil. As the amount of soil was increased, more
Pythium Infection became evident, resulting in a decrease in root length.
6 . Discussion of Inoculum Potential Measurement
Any method used to quantify the inoculum potential of a zoospore producing fungus such as Pythium should meet the following criteria to be acceptable:
(1) The method should be rapid and easy to evaluate, without
depending on judgment of the technician.
(2) The method should use the host species in the laboratory
that is of interest in the field.
(3) The method should evaluate an agronomically important
parameter of plant growth.
(4) The method should have the capability to be adapted to a
wide range of inoculum potential of different soils. 20
17.3 - 0.57X
0 15.2 AREA OF SOIL EXPOSED,cm2 Figure 7.— Root length of 10-day-old corn plants exposed to four different areas of Pythium infested soil with 1/2 strength Hoagland's solution as the rooting medium. Each data point is the average of six repli
cations. 62
(5) The method should have the capability to reflect differences
in the physical properties of the soils.
(6) The method should be specific to the fungus of concern.
Of the laboratory baiting techniques developed to measure the
inoculum potential of Pythium graminicola, the crested wheatgrass sur
vival method (Knaphus and Buchholtz, 1958) and the corn root baiting method (Rao, 1976) have been used with some success. However, the crested wheatgrass method depends on Pythium Infection of the plant
root which is genetically different than the crop of interest in the present study, corn. Knaphus and Buchholtz (1958) found two varieties of
crested wheatgrass were infected in substantially different amounts.
The dangers of extrapolating the infection of crested wheatgrass to corn are many. For example, the fungus may develop a strain that is more effective in infecting c o m than other plants.
Also, the method is not necessarily specific to Pythium. Fungi other than Pythium can cause the death of crested wheatgrass. To insure
Pythium is the pathogen involved requires plating of root sections and identification of the fungus. The time and skill involved eliminate any contention that the method is rapid and simple.
The c o m root baiting method of Rao (1976) has several advantages over the crested wheatgrass method. C o m is used as the host plant so genetic differences can be eliminated by using the same variety in the lab as in the field. Also, because a liquid serves as the rooting medium rather than soil, any pathogen causing Infection probably has zoospores and must tolerate near anaerobic conditions. Pythium is one of only a few pathogens that infect corn and meets these criteria. The c o m rooting technique then is rather specific to Pythium. 63
In the method described by Rao, the soil to be examined was dumped into the bottom of a flask. Distilled water was used as the rooting medium. The parameter of infection was the number and size of lesions on the roots. Several disadvantages of the corn rooting technique as originally described have been uncovered by Rao and in the present study. First, the use of lesion counts as the parameter of measurement is unsatisfactory. The number or size of lesions on a root does not necessarily correlate with the damage done to the root. In the present study, uptake of phosphorus and potassium per unit length of root was the same for infected and uninfected roots (Table 11). The immediate effect of Pythium infection is to stop root elongation. Only later in the season does the infection spread to the vascular part of the root and interfere with nutrient translocation. In this study, some roots were stunted and curled by Pythium infection but the lesion causing the infection was not visible. As some time is required for lesions to become large enough to be seen, a significant lag time exists between actual and visible numbers of lesions. The counting and scoring of lesions is a time consuming task and requires a skilled observer.
The second disadvantage of the corn root baiting technique as originally described is that the degree of damage based on lesion counts or root length, does not relate to the amount of soil in the flask. Rao found no differences in the number of lesions on the roots when 100, 200, or 300 grams of soil were added to each flask. In the present study, the root length of the corn plants was the same when the flasks contained 50, 100, or 150 grams of soil (Table 6). 64
A third disadvantage of the original method Is the lack of control
of the physical condition of the soil. The soil was dumped into the
bottom of the flasks without any attempt to control the surface area
of the soil, surface roughness, the bulk density, or thickness of the
soil. The lack of control over the geometry of the soil placement
Increased the variability of the results, and thereby lowered the probability of detection of differences between treatments or soils.
The fourth disadvantage of the original root baiting technique is
that all nutrients must come from the soil. While nutrient supply is probably not a problem with fertile soils such as Hoytville when 200 grams of soil are used, root damage could be caused by nutrient de
ficiency rather than Pythium infection if soils of low fertility were studied.
To overcome the disadvantages of the corn root baiting technique, several changes were made in the method during the course of the present study. Root length was measured rather than the number of lesions.
Root length has been shown to be highly correlated with nutrient uptake
(Newman and Andrews, 1973) (Table 4). Conversely, no evidence was
found to indicate that lesions affect nutrient uptake by mechanisms other than by reduction or root growth.
To overcome the second and third disadvantages of the original method, the soil was packed into vials which allowed control of the surface area exposed, bulk density, sample thickness, and surface roughness. Use of the vials also expands the adaptability of the method to a wider range of Pythium inoculum potentials. By use of a preliminary study in which the number of vials is varied, the amount of root infection can be regulated to give maximum change in root length with minimum variability. Because of physical limitations of 65 the apparatus used in the present study, the maximum surface area exposed to the roots was 15.2 cm . With the soils used in this study, the re lationship between root length and area of soil exposed was linear.
However, the experiments where soil covered the entire bottom of the flasks (Table 6 ), about 70 cm^, indicate that an equilibrium is reached where more soil surface will not reduce root length. The data points on
Figure 7 indicate a near equilibrium at 15 cm. By use of more vials and larger flasks, the surface area exposed could be increased to find the point where root length is no longer linearly related to the area of soil exposed. In actual studies the area could be adjusted to stay within the linear part of the curve.
The packing of the soil into vials also allowed control of the thickness of the soil sample. Two hypotheses can be used to explain the results, zoospore penetration or diffusion of root exudates to oospores.
If the soil sample Is very thin, zoospores would be able to swim out of the entire soil mass, or root exudates will reach all oospores. As the thickness of the sample is increased, zoospores in the lower part of the sample would be unable to penetrate the overlying soil and reach the c o m roots or root exudates will not diffuse to oospores in the lower part of the soil. In radioisotope studies, where an analogous problem occurs, selfabsorption is solved by using infinitely thin samples, samples of intermediate thickness, or infinitely thick samples (Chase and Rabinowitz,
1967). Use of infinitely thin samples requires carrier free material which is not applicable to the study of the inoculum potential of Pythium in soils. The volume of soils is much greater than the volume of zoo spores so selfabsorption occurs even when the soil sample is very thin
(Figure 4.). Use of samples of intermediate thickness requires the 66
knowledge of a selfabsorption coefficient to normalize the data. The
variability of root lengths are too high to allow accurate determination
of the coefficient. Above the point of infinite thickness, addition of
more soil does not increase the number of zoospores that can reach the
roots or root exudates do not diffuse deeper into the soil. The point
of infinite thickness of the soil used in this study when the bulk den-
sity was 1.0 g/cm was about 2.0 cm. Use of samples somewhat thicker
insure that sample thickness is no longer a variable that affects the
dispersion of zoospores or diffusion of root exudates and hence root
length of the plants.
All samples used in the present study were packed to a bulk density of 1.0 g/cm . The effect of different degrees of compaction was not studied. Higher bulk densities would probably decrease the ability of zoospores to penetrate the soil mass. Therefore, as bulk density in creases, the value of infinite thickness would be expected to decrease.
The fourth disadvantage of the original corn root baiting technique arose from the use of distilled water as the rooting medium. The plants are dependent on the soil for nutrients. Any difference in the ability of the soil to supply plant nutrients would be reflected in the root growth, confounding root length reduction caused by Pythium infection and nutrient supply (Figures 5 and 6). The amount of soil nutrients can be standardized by the use of soil samples treated to kill Pythium (Figure
6), or the use of nutrient solution (Figure 7). Both methods give satis factory and equivalent results. Because the growth chamber temperatures were different for the data in Figures 6 and 7, root growth differed. If the two equations are normalized by rearranging from Y - a-bX to Y/Ymax *= l-b*X, the slope of the two experiments (b') are both -0.033 meters of 67 2 root per cm of soil exposed. Of the two methods the use of nutrient solution is more satisfactory. The higher nutrient levels do not enhance or inhibit the ability of the zoospores to infect the corn roots or the two equations would not agree so well. The use of a fungicide is not as desirable as the material may be phytotoxic as in the case of pyroxychlor
(Table 7), or the fungicide may be absorbed by the roots and reduce the infection by zoospores.
The results in Table 8 show conclusively that the amount of reduc tion in root length caused by Pythium is related to the area of soil exposed at infinite thickness and not the mass of the sample. With preliminary experiments to find the infinite thickness and to Insure that the relationship between root length and area of soil exposed is in the linear range, the revised corn root baiting technique should be use ful to determine the inoculum potential of a wide range of soils.
C. Field Core Experiments
Core samples of undisturbed soil were collected from the Hoytville and Wooster sites. Bulk densities and water contents at -0.06 bar poten tial were determined. Various laboratory treatments were applied to the soil cores and then corn was grown into the cores. Clods were also used to determine bulk density and corn plants were harvested from the field to relate lab results to field growth.
1. Autumn Core Experiment
Of the parameters of co m growth measured, only root length, total phosphorus content in the tops, and phosphorus uptake per unit of root length were significantly affected by the field or laboratory treatments in a consistent manner. The average of the three replications of all the variables are shown in Appendices B and C. 68
The root length of corn plants was not significantly affected
by the field tillage or rotations. The root length of plants grown in
cores from the plowed Hoytville plots averaged over the three laboratory
treatments was 19.6 m and the notillage continuous c o m averages were
17.6 m. The differences in root length caused by the field treatment
were about the same for the Wooster soils except roots grown in cores
from the no-tillage continuous corn plots were 3.8 m longer than the roots
grown in cores from plowed continuous c o m plots. However, the varia
bility between experimental units was too high for the differences be
tween field treatments to be statistically significant.
The laboratory treatments did cause significant changes in the
root length of c o m plants (Table 9). The sieving treatment increased
root length from 14.9 m for the untreated cores to 24.6 m for the Hoytville
cores that were sieved. The response to sieving was about the same for
Wooster soil. Steaming had no significant effect on root growth in either
soil, although steaming Increased root length by about 1.5 meters in both
soils.
Pythium lesions were observed on roots grown in untreated cores
of both Hoytville and Wooster soils. No lesions were found on roots from
cores that had been steam treated but the roots had the same general mor
phology as the roots grown in untreated cores. The roots followed struc
tural cracks in the untreated and steamed cores of Hoytville soil. The weaker structure of the Wooster soil did not seem to affect the root
configuration.
The roots grown in the sieved samples of Hoytville and Wooster
soil had the same morphological appearance. The roots were white with
few lesions noted. Secondary roots were profuse and about 5 cm long each. 69
Table 9. Root lengths of corn plants grown in cores of Hoytville and
Wooster soils that had been subjected to three field and three laboratory treatments.
Lab Treatment Hoytville Wooster
None — ^14.9a 13.6a
Steamed 16 .4a 15.9a
Sieved 24.6b 2 1 .2b
1/Values followed by the same letter in each column are not significantly different at the 5 percent probability level. 70
In both soli series* the roots grown in the sieved cores were straight and showed no effect of excessive soil resistance.
Sieving Hoytville soil did not affect the phosphorus content of the plant tops (Table 10). Sieving the Wooster soil conslstantly reduced the phosphorus content of the tops although the difference was significant only in cores from no-tillage continuous c o m plots.
Steaming Hoytville soil had variable effects on the phosphorus in the c o m tops. In plants grown on cores from no-tillage corn-soybean rotation plots* steaming reduced phosphorus content compared to plants grown on cores from no-tillage continuous c o m plots that were steamed.
The phosphorus uptake per unit of root length is shown in Table 11 for each of the treatments applied to the Hoytville and Wooster soils.
Steaming cores from plowed plots of both soils did not affect the amount of phosphorus absorbed per unit of root length. Also* the steam treat ment did not affect efficiency of uptake in the Wooster or Hoytville soils from the rotation plots. In the cores from the no-tillage continuous c o m plots at Wooster* the steam treatment decreased the uptake from 0.085 to
0.060 mg/m. In cores from the Hoytville notillage continuous corn plots* the steam treatment increased phosphorus uptake per unit of root length from 0.057 to 0.079 mg/m* although the difference was not significant.
Sieving consistently reduced phosphorus uptake per unit of root length in both soils. As the sieving treatment allowed greater root growth* and as sufficient phosphorus was probably available to the plants* the effect was not meaningful but simply a dilution effect. Evidently* the steam treatment caused a variable release of potassium as evidenced by the erratic results of potassium uptake shown in Appendices B and C.
For instance, the K/P ratio of plants grown in steam treated soil from Table 10. Total phosphorus content of corn plant tops grown in Hoytville and Wooster soil cores.
Total Phosphorus Uptake U Field Treatment Lab Treatment Hoytville Wooster
Continuous Corn, plowed None 0.66ab 0.70ab
Steamed 0.58ab 0.61a
Sieved 0.74ab 0.37a
Continuous Corn, no-tillage None 0.79ab 1.19b
Steamed 1.32b 1.22b
Sieved 0.68ab 0.57a
Com-Soybean, no-tillage None 0.61ab 1.02b
Steamed 0.42a 1.23b
Sieved 0.89ab 0.66ab
1/ Values followed by the same letter in each column are not significantly different at the 5 percent probability
level. Table 11. Phosphorus uptake per unit length of root of corn plants grown in Hoytville and Wooster soil cores.
Phosphorus Uptake Per Unit of Root Length Field Treatment Lab Treatment Hoytville Wooster
Continuous Corn, plowed None ^0.039ab 0.048bc
Steamed 0.042ab O.OAlab
Sieved 0.028a 0 .021a
Continuous Corn, no-tillage None O.057ab 0.085e
Steamed 0.079b 0.060cd
Sieved 0.031a 0 .021a
Corn-Soybean, no-tillage None 0.05lab 0.077d
Steamed 0 .021a 0.077d
Sieved 0.035a 0.030ab
1/ Values followed by the same letter in each column are not significantly different at the 5 percent
probability level. 73 the rotation plots at Hoytville was 35.2 while the K/P ratio for the other steamed cores from rotation plots was 10.6 to 17.6. The ratios are probably artifacts of the treatment rather than real effects.
The steaming and sieving treatments both changed the aeration porosity (air filled porosity at -0.06 water potential of the soil). As expected, the sieving treatment reduced the aeration porosity from 18.2 percent to 13.9 percent in Hoytville soil and from 18.0 percent to 16.8 percent in Wooster soil. However, the steam treatment increased the aeration porosity by 2-3 percent in both soils. The steam treatment probably changed soil strength and other physical properties as well.
2. Bulk Densities
The bulk densities of the Hoytville soil, determined by the core and clod method, and of the Wooster soil, determined by the core method, are shown in Table 12. The samples were collected from plots represen ting three field treatments. The bulk densities were not affected by the tillage or rotation treatments. The bulk densities of the Hoytville soil at the field water content of about 30 percent were 1.50 g/cm-* when determined by the clod method and 1.20 g/cm^ when measured with the core method. Large cracks and pores were observed in the cores taken from the Hoytville plots while the clods were uniformly dense.
3. Spring Core Experiment
The same basic experiment as the autumn core experiment was repeated with three changes. The plots that were plowed and planted to the corn-soybean rotation were sampled as well as the three field treat ments included in the autumn sampling. Pythium was killed using pyroxy- chlor rather than by steaming. Also rather than using the fourth sample Table 12. Bulk densities of Hoytville soil as determined from core and clod samples and of Vooster soil as
determined by core samples. The samples were taken in October 1975.
Bulk Density Hoytville Hoytville Hoytville Wooster Field Treatment (core) (moist clods) (oven-dry clods) (core)
g / cm J
Continuous Corn, plowed — ^1.21 1.49 1.61 1.27
Continuous Corn, no-tillage 1.18 1.S0 1.64 1.28
Corn-Soybean, no-tillage 1.18 1.53 1.64 1.31
If No significant difference at the 5 percent probability level occurs between values in the same column. 75 from each plot for the bulk density determination, the cores were sieved and treated with pyroxychlor.
The root length of the corn plants grown in the cores is shown in Table 13. The results are essentially the same as in the experiment using the samples collected in the autumn. The root lengths were not significantly affected by the field treatments, but the average root lengths of each of the field treatments are in the same relative order as for the fall experiment. Root lengths were the shortest in the cores from no-tillage continuous corn plots. As in the fall experiment, sieving the soil allowed twice as much root growth. For the non-fungicide treated cores, sieving Increased root lengths from 8.7 to 16.7 m. The effect of the pyroxychlor treatment was not statistically significant and was erratic. Generally the pyroxychlor increased root growth in cores not sieved in the lab and reduced root length In cores that were sieved. The fungicide was evidently phytotoxic in sieved soil or allowed
Infection by other CRP as some lesions were present.
Phosphorus uptake parameters are shown in Table 14 with the values representing the average of replications, field rotations, and fungicide treatments. The phosphorus content in the tops differed significantly between field tillage treatments. Plants growing in cores from no-tillage plots contained the largest amount of phosphorus. The total amount of phosphorus in the plant tops was 0.61 mg for the plowed soil and 1.76 mg for plants grown in cores of untilled soil. Evidently the phosphorus fertilizer applied to the no-tillage plots remained concentrated near the surface where the cores were taken, and roots were able to absorb the phosphorus without penetrating the entire core. The decrease in phos phorus uptake when no-tillage cores were sieved confirms this. The differential fertility levels between the field treatments at the sample Table 13. Root lengths of corn plants grown on Hoytville soil cores representing four field treatments and subjected to four laboratory treatments. The cores were collected in April 1976. Each value is the average of three replications.
Root Length Plowed No-tillage Continuous Corn- Continuous C o m - Lab Treatment Manipulation Fungicide C o m Soybeans Corn Soybeans Averages
None None 7.7 7.3 6.5 5.9 6.8
None Pyroxychlor 8.3 10.1 6.6 6.6 8.1
Sieved None 19.9 14.8 16.8 15.6 16.7
Sieved Pyroxychlor 15.7 11.7 9.0 16.0 13.1
Field Treatment Averages 12.9 11.1 9,7 11.0
______LSD at 5X probability level______
Between field treatment averages 4.6 Between lab treatment averages 5.1 Between two lab treatment averages at the same field treatment 9.7 Table 14. Phosphorus content of corn tops and uptake per unit of root length from Hoytville soil cores representing two field treatments and two laboratory treatments. Each value is the average of 12 determinations.
Tillage P Cone, Total P P Uptake/Unit Field Laboratory In Tops In Tops Root Length mg/g mg mg/m
Plowed None y 2.91a 0.61a 0.098b
Plowed Sieved 2.56a 0.53a 0.041a
No-tillage None 4.56b 1.67b 0.303c
No-tillage S ieved 3.27ab 1.25ab 0.095b
1/Values followed by the same letter in each column are not significantly different at the 5 percent probability level. 78
depth used confound the effects of physical properties, and make valid
conclusions about Phythium infection impossible. While root length may
have been shortened by Pythium infection, phosphorus uptake was not
similarly affected because of the variable concentrations of phosphorus
in the soil. The correlation coefficient between root length and total
phosphorus or potassium uptake was less than 0.30 for this experment.
The bulk densities were not significantly affected by tillage or
rotation treatments. The average bulk density of cores taken from the 3 plowed plots was 1.28 g/cm and the average bulk density of the no-tillage 3 cores was 1.32 g/cm . The water content of the soil cores after 14 days
of plant growth was 34 percent on a weight basis, or approximately -0.06
bars water potential.
4. Samples of C o m Plants From the Field
To evaluate the early growth of c o m in the field, 57-day-old plants were harvested from the plots at Hoytville on June 12, 1976. The dry weights and total uptake of potassium and phosphorus are shown in
Table 15. The weights of c o m plants from plowed plots were significantly greater than from the no-tillage continuous corn plots. Plants grown on no-tillage cornrsoybean plots contained more phosphorus than plants grow ing on no-tillage continuous corn plots. Plowing increased phosphorus uptake regardless of crop rotation.
In order to relate the field grown plant weights to laboratory experiments, the plant weights were compared to root weights of plants grown in cores of varied bulk densities (Figure 8). The plant weight data were normalized to the root length measurements by Hatching the mean of the experiments at the mean bulk density of 1.33 g/cm . The ratio used was 10 meters of plant root length per 3.2 grams of top growth. 79
Table 15. Dry plant weights and total potassium and phosphorus contents of corn plants collected from the field plots at Hoytville 57 days after planting. Each value is the average of nine determinations.
Field Treatment Plant Parameter Dry Total K Total P Tillage Rotation Plant Weight Content Content
g mg mg
Plowed Continuous Corn A/6.42b 249b 28.2d
Plowed Corn-Soybeans 6.20b 265b 25.6cd
No-tillage Continuous Corn 4.69a 176a 19.1a
No-tillage Corn-Soybeans 5.6lab 224ab 22.1b
17 Values followed by the same letter in each column are not different at the 5 percent level of probability. 30 "N* -----j ~ ------1------
m x 80 H o 20 2 so i LU 111 p 5 □ *- - ' 40h O 10 • Q plowed, continuous c o m _ O 0 plowed, com-soybeans < cc 20 _ i O notill, continuous c o m a. Bnotill, com-soybeans 0 — -- 1------i 0 10 1.25 1.40 I,55 BULK DENSITY, g/cm3
Figure 8,— The effect of bulk density on the root length as predicted from a growth chamber experiment— with 95 percent confidence intervals— plus actual dry plant weights from field plots treated with two rotation and two tillage variables.
00 o 81
The plant weights were then plotted vs. the bulk densities of the core
samples taken in April 1976. Each data point in Figure 8 is the average
of three plant weights and four bulk densities.
The plant weights fall into the 95 percent confidence Interval
of the prediction equation with the exception of two of three no-tillage
continuous c o m plots. The weights of the plants from the two plots of
no-tillagp continuous corn fell below the interval predicted for plant
weights at the measured bulk density.
Bulk density seems to be a major factor limiting plant growth In
the plowed plots and In the no-tillage corn-soybean plots. However, some
other factor besides bulk density limits the c o m growth on the no-tillage
continuous corn plots.
5. Discussion of Field Sampling
The field core experiments were conducted to gain further under
standing of the variables involved in reducing c o m yield. As with all
field sampling studies, the variability between samples was high. In
the core studies, the variability was high enough to make many of the
results statistically insignificant. The bulk density measurements 3 made in the fall showed only a 0.03 g/cm difference between treatments
and a standard deviation of 0.08 g/cm^. Shear and Moschler (1969) found
no-tillage plots to have a bulk density of 0.05 g/cm greater than tilled
plots but variability between the samples within treatments was too high
for the difference to be significant.
The substantial difference between the bulk densities as deter
mined by the core and clod methods emphasized the difficulty of measuring bulk density in cracking soils. The true bulk density was probably closer to the 1.20 g/cm^ as measured by the core method as the core method 82
sample includes the cracks and other voids. However, the density
determined by the clod method does give an indication of the dense
material root must penetrate if they are to do more than just follow
cracks.
An unexpected response occurred when the corn was grown directly
into cores collected from the field* Fertilizer applications on the surfece
and residue decomposition at the surface had concentrated plant nutrients
near the top of the cores collected from no-tillage plots. Roots growing
into the cores were able to extract nutrients without penetrating
the entire core. For reasons which are not apparent, the nutrient
difference between the field plots was more pronounced in the samples
collected in the spring than in the autumn samples.
Another problem was encountered in trying to eliminate the
Pythium from the soil. The steam treatment used on the autumn samples
evidently released nutrients such as potassium as evidenced by the high concentration of K in the tops of the plants grown on steam
treated cores. Also, the physical properties were changed substantially
as Bhown by the increased volume of water drained at -0.06 bar water potential. Pyroxychlor was used in the samples collected in the
spring to eliminate the changes in the soil properties caused by the
steam treatment. The use of pyroxychlor also introduced problems.
The fungicide is phytotoxic as evidenced by the decrease in root
growth in sieved samples treated with the material. The alternate
fungicide, GA-1-62, was not available when the experiment was estab
lished, but since the material was supplied as a dust, application
to undisturbed cores would not have been possible. 83
Even with all of the difficulties, some conclusions can be drawn
from the field core experiments. Apparently sieving of the samples re
duced soil resistance to root growth or increased aeration so the length of roots increased dramatically. The increased root growth was probably
caused by a combination of factors including decreased soil strength and possibly a dispersion or dilution of Pythium inoculum. While sieving did
increase root length, uptake of nutrients was not improved by the treat ment. Sufficient phosphorus and potassium were available to the plants wtth the shorter root systems, especially the no-tillage cores collected
in the spring. The longer root systems in the sieved samples absorbed
the same total amount of nutrients so the uptake per unit of root length was reduced. Root length was probably not a limiting factor to nutrient uptake in either case.
The pyroxychlor added to the sieved samples collected in the spring reduced root growth. However, on undisturbed samples, the pyroxy chlor treatment increased root growth. If the reduction in growth caused by the pyroxychlor in the sieved samples is added to the root growth in undisturbed samples, the fungicide treatment shows a significant increase in root length. The conclusion then would be that Pythium did reduce root length and that sieving the soil prevented Pythium infection.
Although the inoculum potential of Pythium of the Wooster soil was not measured roots growing in the untreated Wooster cores were in fected. The water potential used in this experiment, -0.06 bars, was much higher than normally found in the field. The Pythium is present but the lower water contents and lack of saturated conditions in the field prevent the fungus from limiting root growth.
The comparison of field plant weights at the respective field bulk densities with the predicted root length at those bulk densities 84 offers an interesting insight into the influence of physical properties
on corn growth. With the exception of the samples from the no-tillage
continuous c o m plots, all of the plant weights fall within the predicted
range. The relationshop should be expected as top weights and root lengths are highly correlated (r ** 0.41 to 0.81). However, the plant weights of two out of three samples from the no-tillage continuous c o m plots fall below the predicted interval. A factor other than bulk density must be limiting c o m growth. The factor is probably the increased root infection by Pythium (Rao, 1976) which was found only in the no-tillage continuous c o m plots.
D. Evaluation of Specific Hypotheses
1. Bulk Density by Pythium Interaction
a. Testing the Hypothesis
Soils packed at two bulk densities, equilibrated at two water contents and treated with and without pyroxychlor were used to evaluate the hypothesis that root infection increased at the higher bulk densities.
The plant growth and nutrient uptake parameters are shown in Appendices
G and H. The water potential of saturated and -0.06 bar did not affect root infection or root growth as measured by root length. The water contents on the weight basis were 37 percent for the saturated treatment and 21 percent for the -0.06 bar treatment. The expected water contents at these potentials would be 43 and 36 percent respectively, so both treatments were drier than desired.
Treatment of the soil with pyroxychlor increased root length when the soil bulk density was 1.20 g/cm^ (Table 16), but the difference was not significant. The fungicide had no effect at the bulk density of
1.50 g/cra^. The root length in cores packed to a bulk density of 1.50 85
Table 16. Effect of bulk density of Hoytville soil and pyroxychlor
fungicide treatment on root length and uptake of potassium and phosphorus
per unit length of root. Each value is the average of 12 determinations.
Soil Treatment Root and Nutrient Parameters Bulk Root K Uptake per P Uptake per Fungicide Density Length Unit of Root Unit of Root
g/cm^ m mg/m mg/m
None 1.50 y 9.8a 1.04bc 0.061ab
None 1.20 12.lab 0.96ab 0.058ab
Pyroxychlor 1.50 9.9a 1.19c 0.071b
Pyroxychlor 1.20 14.9b 0.79a 0.051a
1/Values followed by the same letter in each column are not different at
the 5 percent probability level. 86 g/cm-* was 9.9 m which was significantly less that the average root length
of 13.5 m of plants grown in cores packed to a bulk density of 1.20 g/cm-*.
Potassium uptake per unit length of root was increased from
1.04 mg/m to 1.19 mg/m when the high density cores were treated with pyroxychlor. However, the difference is not statistically significant and is contradicted by a reduction on potassium absorption per unit root length when fungicide was applied to the low density cores.
Phosphorus uptake per unit of root length showed a similar pattern (Table 16). For both nutrients, increasing bulk density in creased nutrient uptake per unit of root length when the soils were treated with pyroxychlor. For untreated soil, the bulk density did not affect nutrients absorbed per unit of root. The highest efficiency of nutrient uptake was in the 1.50 g/cm^ soil treated with pyroxychlor and the lowest efficiency was in the 1.20 g/cjn^ cores treated with the fungicide,
b. Evaluation of the Hypothesis
In the study of a possible soil density by Pythium inter action, high bulk densities of 1.50 g/cm^ did limit root growth but treatment with pyroxychlor did not allow greater root growth at the higher bulk density. The fungicide treatment improved root growth at the lower bulk density of 1.20 g/cra^, although the difference was not significant at the 5 percent level. The cores with the higher bulk density probably contained insufficient large pores for the zoospores of
Pythium to travel through to infect the roots or root density was too low for Infection so the fungicide treatment had no effect.
Kerr (1964) found root infection of peas by Pythium ultimuro was greater at bulk densities of 1.50 than at 1.30 g/cm^. The mechanism was thought to be increased diffusion of exudates from the roots through 87
the denser soil. Pythium ultimum does not have zoospores so increased
density would not have an inhibitory effect on the dispersion of the
fungus. Pythium ultimum probably spreads by hyphae extension which would be able to follow the exudate diffusion. While the root exudates may have diffused more rapidly through the higher density soil in the present study, zoospores, which are the dispersal mechanisms of PythiuiD graminicola. were not able to penetrate the smaller pores, or roots did not grow close enough to Pythium for infection to occur.
Fulton et al. (1961) also found an Increase in soybean root infection in compacted soils. The fungus involved, Fhytophthora tnegaserma does have mobile zoospores and the soil used was a clay loam. The authors did not speculate about the mechanisms of zoospore dispersal in their study.
The increased uptake of phosphorus and potassium per unit of root length in the high density soils (Table 16) was caused by the de creased root length rather than actual efficiency in uptake. The total amount of potassium and phosphorus in the plant tops was not significantly affected by any of the treatments.
The hypothesis that c o m root infection is increased by a combination of wet soil, uniformly high bulk density, and the presence of Pythium is not valid. In fact the results of the experiment show the opposite occurs. Root infection determined by the difference between treated and untreated samples at the same density, was greater at the low bulk densities where pores were large enough for the zoospores of Pythium graminicola to swim through.
2. Effect of Organic Amendments
A series of experiments were developed to find the effect of 88
organic amendments on CRP infection under various environmental condi
tions. The hypothesis was that c o m stalks buried by plowing increased
microbial competition of CRP reducing the amount of infection,
a. Testing the Hypothesis
1) Flask experiment. The flask experiment was utilized as
a rapid method of determining the effect of various amounts of added
organic residue and time of incubation. Nitrogen was added in proportion
to the residue to keep the C:N ratio at about 10:1.
The length of the radicle root systems is shown in Table
17. As noted earlier, the steam treatment killed CRP and allowed greater
root growth. The organic amendments together with the nitrogen added
allowed greater root growth. The addition of 1 percent c o m stalk mate
rial on a weight basis increased the root length from 9.0 to 14.2 m
averaged over the three time periods.
The root lengths of the plants are shown as a percent of
the steam treatment in Figure 9. Addition of c o m stalk residue to the
infested soil increased the root length to about 65-70 percent of the
root length of plants grown over steam treated soil. The effect of added
residue decreased with time, although the incubation period had little effect. The fitted equation shown in Figure 9 was significant at the
95 percent level of confidence. Maximum root growth over unsteamed soil was 1.0 percent added residue at the 0 time of incubation, when compared
to the steamed treatment at the same time.
2) Organic residue additions to soil cores
a) Without nitrogen. The data collected on com plants grown on soil cores treated with and without organic additions and pyroxy chlor are shown in Appendix F. Root length was Increased from 14.5 m to 89
Table 17. Root lengths of corn plants grown In flasks containing
infested soil, treated with various amounts of corn stalk residue, or steamed soil incubated for different times.
Soil Treatment Residue Added to Infected Soil Time of % by Weight Average of Incubation 0 0.5 1.0 1.5 Steamed Time
weeks
0 9.0 12.7 12.9 12.4 16.2 12.6
2 6.5 10.7 10.6 11.9 15.4 11.0
4 10.4 13.3 19.0 13.6 23.3 15.9
6 12.1 9.5 8.6 13.4 16.8 12.1
Average of Soil Treatments 8.6 12.2 14.2 12.6 18.3 90
Z = 53.5 + 563X - 27.AX2 - 3.2Y
-60
-40
x ROOT LENGTH.% ROOT OF STEAMED
Figure 9.— Effect of added c o m stalk residue and time of incubation on root length, expressed as percent of roots grown in steam treated soil. 91
25.4 m by the pyroxychlor treatment when no residue was mixed with the
soil (Table 18). The increase was somewhat greater than for the compa
rable treatments in the bulk density by Pythium interaction experiment
(Table 16) but temperatures in the growth chamber were higher in the present experiment. The root length of c o m plants grown in soils treated with 1.0 percent ground c o m stalk material was 6.3 m regardless of fungi cide treatment. The added organic matter, which had a high C:N ratio probably Induced a nitrogen deficiency in the corn plants which prevented more extensive root growth. Removal of the fungus by pyroxychlor did not increase root growth in the residue amended cores, but as the roots were stunted by the nitrogen deficiency, reduction by Pythium would be minor in comparison. The soil water content of the soil at harvest was 53 percent on the volume basis which is near saturation at the bulk density of 1.20 g/cm^.
b) With nitrogen. To prevent the added organic residue inducing a nitrogen deficiency in the corn plants, the same experiment was repeated but 330 pg/g of nitrogen as NH^NO^were added to the soil before packing the soil into cores. The nitrogen treatment was equivalent to 660 Kg/ha.
None of the treatments affected root growth (Table 18).
While the data is not directly comparable to the previous experiment as the two were done at different times, the root lengths in the present experiment are substantially shorter than the best treatment in the pre vious experiment. All the roots in the present experiment showed signs of Pythium infection. Evidently the nitrogen enhanced root growth, out stripping the reduction caused by the Pythium infection. 92
Table 18. Root lengths of c o m plants grown in Hoytville soil cores treated with and without an organic amendment, pyroxychlor, and ammonium nitrate. Each value is the average of four replications.
Soil Treatment Root Length Organic No Ammonium Fungicide Amendment Nitrogen Added Nitrate Added
Z m m
None 0 -4.4.5b 16.5a
None 1.0 6. 3a 13.9a
Pyroxychlor 0 25.4c 12.7a
Pyroxychlor 1.0 6. 3a 13.9a
Values followed by the same letter in each column are not significantly different at the 5 percent probability level. 93
3) Carbon dioxide evolution. The average rate of carbon
dioxide evolution from the soil during the 48-144 hour period is shovm
in Table 19. The rate of evolution of the soil material from the crack
area of the no-tillage continuous corn plots was 22.9 mg/(g day) which
was significantly greater than the rate of evolution from the soil from
the crack area of the no-tillage com-soybean rotation plots, or the
plowed plots. Organic matter from the c o m stalk residue moves
down the cracks of the no-tillage plots, enriching the organic matter
content in the cracks. Also, more roots growing in the cracks would
increase the organic matter content. The root length of c o m plants
grown over the soil from the crack area of the no-tillage continuous corn
plots was 5.8 m which was significantly less than the root length of
plants grown over other soils. Therefore, the soil which had the highest
rate of carbon dioxide evolution allowed the shortest root growth or had
the highest Pythium inoculum potential.
The values for carbon dioxide evolution and root length
for the individual samples are shown in Figure 10. The data points are
spread randomly with the correlation coefficient being only 0.17. As
the same soil was used to determine the carbon dioxide rate of evolution
and the inoculum potential, the variability should be at a minimum. The
conclusion is that no relationship exists between rate of carbon dioxide
evolution from the soil and the root length of the c o m plants. Presum ably then, no relationship exists between the rate of carbon dioxide
evolution and the inoculum potential of Pythium. Table 19. Water contents, carbon dioxide evolution, and root length growth in soil collected from tillage- rotation plots at Hoytville in April 1976. Each value is the average of six replications.
Microsite Water CO2 Tillage Rotation In Soil Content Evolution Length
% Ug/(g day) m
Plowed Continuous C o m Matrix U 27.0b 13.5a 6.9b Plowed Corn-Soybeans Matrix 25.9ab 12.8a 6. 6ab
No-tillage Continuous C o m Interior 23.5a 18.0ab 7.4b No-tillage Continuous Corn Crack 23.7a 22.9b 5.8a
No-tillage Corn-Soybeans Interior 25.5ab 16.Sab 6.Sb No-tillage Com-Soybeans Crack 24.7ab 14.6a 6.9b
1/ Values followed by the same letter in each column are not different at the 5 percent level of probability. 95
O
LU
3-
= 0.17
20 30 40 C02 EVOLUTION, mg(g day)
Figure 10.--Relationship between rate of CO2 evolution of soil and length
of corn roots grown in flasks containing the same soil. 96
The water contents on a weight basis are shown in Table
19. The no-tillage plots in continuous corn were the driest at the
time of sampling and the plowed plots in continuous c o m contained the
most water. However, neither water content approaches any agronomically
significant levels such as field capacity,
b. Evaluation of the Hypothesis
Plowing buries c o m stalk residue where soil microbes use
the carbon as an energy source. Conversely, in no-tillage culture, the
residue is left on the surface where decomposition takes place and the
material does not supply as much energy to the soil microbes. The
second hypothesis arose from these observations. If c o m stalk residue
is plowed under, microbial competition or substances toxic to Pythium
increase and limit the effective population of the pathogen. The use
of organic amendments to control soil borne plant pathogens has been
successful in many crops under a wide range of cultural conditions.
Cook et^ al. (1976) states there are few soilbome diseases that cannot be reduced by timely incorporation of organic matter.
In the present study, corn stalk residue and nitrogen were
mixed with Pythium Infested soil and exposed to c o m roots. The amount
of root growth increased as the amount of residue and nitrogen increased.
However, the increased root growth could have been caused by less infec
tion because of biological control from the added residue or by the added nitrogen. The effect was probably due to the added nitrogen because as
the time of incubation increased, the root growth of the corn plants
decreased when compared to the steam standard at the same incubation time. 97
The longer incubation time allowed more nitrogen to be immobilized by microbes using the residue as an energy source. Another possibility is that the soil in the flasks became more anaerobic because of the large residue additions. If this were the case, the effect of the residue would be reduced with time, as shown in Figure 9.
When corn stalk residue was mixed with soil and formed into cores, the root length of plants grown on the cores was the same regard less of fungicide treatment. However, a severe nitrogen deficiency reduced root growth more than the Pythium infection which the treatment was designed to control. When nitrogen was added to similar cores in the next experiment, the organic matter did not have any effect on root length. Also pyroxychlor did not affect root length, either, when nitrogen was supplied in excess. Either the nitrogen interefered with the pyroxy chlor, or Pythium was not present in any cores, or the roots were able to outgrow the Pythium because of the nitrogen. The latter is probably true as some lesions were noted. Okpala (1975) found that organic amendments with high C:N ratios reduced the infection of lettuce seedlings by
Corticium pratlcola. However, growth of the lettuce seedlings was not measured so nitrogen deficiency may have limited growth in Okpala*s studies as well. In the present study, root length was measured to provide an agronomically important parameter of growth which percent of
Infection does not necessarily measure.
In the final experiment to evaluate the biological control hypothesis, an attempt was made to relate carbon dioxide evolution rate of the soil to root length and thus to Pythium inoculum potential. As shown in Figure 10, no relationship existed. The hypothesis that corn stalks buried by plowing affects the amount of Pythium infection is 96
rejected because of lack of firm positive evidence. However, specific
negative evidence was not apparent from the study.
3. Pythium Buildup Along Cracks
a. Testing the Hypothesis
a) Inoculum potential along cracks. The amount of infection
of corn roots, as measured by root length, was not related to the rate of
carbon dioxide evolution in the previous experiment. However, root length
was significantly affected by field treatment and microsite from which
the soil was collected. The results are in agreement with the hypothesis
that Pythium accumulates along structural cracks in the soil when the
soil is not plowed for several years and planted to continuous corn.
The no-tillage continuous c o m plots at Hoytville were resampled in July 1976 to confirm the results of the previous experiment.
Soil material was collected from the ped interiors and ped surfaces or
crack region. The c o m root baiting technique was used to evaluate the
Inoculum potential of the soils.
The roots of c o m exposed to the soil material from along
the structural cracks were 9.2 m long and significantly less (93 percent level of probability) than the roots exposed to the soil from the ped
interiors which were 13.0 m long. The roots exposed to the soil from the crack areas had the characteristic appearance of Pythium infection-
Leslons were common and the secondary roots appeared stunted. Main roots were curled near the tips. Roots exposed to soil material from the ped interiors also showed the effects of Pythium infection but the severity of infection was visibly less.
b) Root growth along cracks. Root length Increased from
8.9 m for plants grown in cores without internal cracks to 12.8 m for 99
0 3 plants grown in cores with 0.66 cm'Vcm of internal crack area (Figure
11). The increase in root length was linearly related to the amount of the internal crack area. The difference in geometry probably accounts for some of the discrepancy between actual values and the linear predic tion equation.
The other parameters of c o m growth and nutrient are shown in Appendix J. Fresh top weight also increased linearly with internal crack area. The relationship was expected as root length is highly correlated with top growth. Concentration of phosphorus in the plant tops decreased as the internal crack area Increased. However, the decrease in phosphorus was probably a dilution effect caused by the
Increased top growth. Total phosphorus uptake was not affected by the internal crack area. Roots growing along cracks evidently were less able to absorb phosphorus as uptake of phosphorus per meter of root length decreased from 0.097 mg to 0.053 mg as internal crack area increased from
0 to 0.66 cm^/cm^.
Observation of the crack surfaces showed several roots following the horizontal cracks, being unable to penetrate the soil cylinders once they began to follow the cracks. More roots, especially the larger ones, were noted exiting the cores from the cracks than from the soil matrix.
c) Root growth along Pythium infested cracks. The top and root growth in the blocks with Pythium infested cracks were significantly less than in the uninfected blocks (Table 20). Fresh top weight of the c o m was reduced from 6.2 to 3.1 grams by the inoculation of the cracks.
Root growth in the interior of the A horizon segment of the blocks (in cluding cracks between cubes) was reduced from 25.2 ra to 11.6 m by the d) w. o \o E Xm o 2 LU_I f- O 8.A + 6.2X o 4 QC
0.26 0.39 0.52 0.66 INTERNAL CRACK AREA,cm2/cm3
Figure 11,— Effect of internal crack area of soil cores on root length of corn plants grown in the soil,
Each value is the average of eight determinations. 100 101
Table 20. Growth of corn plants in blocks of soil constructed from 5 cm cubes of Pythium infested soil. The surfaces of the cubes of the infested treatments were Inoculated with a Pythium infested medium. Each value is the average of four replications.
Pythium Treatment Plant Of Cube Surfaces Parameter Infested Unlnfested Probability A
Fresh Top Weight, g 6.2 3.1 0.05 Dry Top Weight, mg 635 342 0.04
Dry Root Weight (Interior), mg 510 275 0.04
Root Length (Interior), m 25.2 11.6 0.005 Root Length (B Horizon), m 0.4 0.005 0.07 Root Length (Exterior), m 3.0 1.2 NS
Total Root Length, m 28.6 12.9 0.005
1/Probability level at which the growth difference between the treatments is equal to zero. NS means no difference (P > 0.10). 102
pythium inoculation. Length of the roots protruding from the blocks
(exterior) was also reduced by about 50 percent, but the difference was
not significant because of the high variability between replications.
The differences in corn growth were visible (Figure 12).
The number of roots growing along the sides of the blocks were reduced by inoculation treatment (Figure 13). Larger roots (greater than 2 mm in
diameter) followed the cracks between the cubes of soil while the smaller
roots were able to penetrate the cubes.
Because of difficulty in breaking the blocks apart along
the cube surfaces, the percentage of roots growing along the cracks and
the amount of infection of the roots in the cracks could not be determined.
As the cubes were constructed from Pythium infested soil, root infection would be expected and was found in both treatments,
b. Evaluation of the Hypothesis
The root length of the corn plants grown over soil from dif
ferent microsites of the no-tillage plots show that Pythium inoculum potential of the soil material along cracks was greater than the inoculum potential of any of the other soil sampled. The measurement of Pythium
inoculum potential along the cracks was repeated using only soil from the no-tillage continuous c o m plots at Hoytville. The root length of the c o m plants grown over the soil material from cracks was about 71-78 percent of the length of roots grown over soil from the ped interiors.
The soil samples were collected for the last evaluation in July, when
Pythium activity approached a low level according to Hampton and Buchholtz
(1959). The results of the present study indicate that Pythium infection of roots is probably reduced because of factors such as lack of new feeder root growth (Foth, 1962) rather than a reduction in the activity of Pythium. 103
Uninfected Infectedju
Figure 12-— C o m growth In soil blocks constructed from 5 cm cubes of
Pythium Infested soil. The edges of the soil cubes of the Infected treatment were inoculated with a Pythium infested medium. Note the dif ference in the top growth of the corn plants. Uninfected
Figure 13.— Com growth in soil blocks constructed from 5 cm cubes of Pythium infested soil. The edges of the infected treatment were inoculated with a Pythium infested medium. Note the difference in the number of roots growing out of the blocks and that large roots follow cracks.
H* O 105
The Inoculum potential was higher in the better aerated
cracks than in the ped Interiors where aeration would be low because of
the high bulk density. The results are opposite of the predicted situa
tion if ethylene production was a factor related to the inoculum potential
(Smith, 1976). Ethylene production was not measured in the present study,
but the results show that ethylene production by anaerobic bacteria is
probably not involved in causing the increased CRP infection in no-tillage
corn in Ohio.
When Hoytville soil was compacted to a bulk density of 1.50
g/cm , roots did have a tendency to follow the cracks. In the experiment
where cracks were formed in cores, root growth increased as internal
crack area Increased (Figure 11). Other factors such as increased aera
tion could have caused the increase, but roots were found following the
cracks.
Pythium infested soil was used to construct cubes of soil
which were inoculated with a Pythium infested medium. The uninfected blocks reflect the field condition in no-tillage corn-soybean rotation
culture. The crack surfaces and the ped interiors have about the same
inoculum potential (Table 19). The infected blocks resemble the no
tillage continuous corn environment where the inoculum potential of the ped interior is about the same as in the corn-soybean rotation, but the
inoculum potential in the cracks is significantly greater. The reduc
tion of c o m growth in the infected blocks shows that Pythium located in
cracks can cause significant root infection.
The blocks were saturated for 24 hour periods. One day of saturated condition is probably sufficient for the corn roots to excrete 106 ethanol (Bolton and Erickson, 1970) and for zoospores to follow the ethanol back to the roots, if zoospore mobility is important in infection.
Two hypotheses are feasible in explaning the increased in fection of the c o m roots in the cracks. First, the concentration of roots in one location year after year leads to a buildup of Pythium inoculum potential. Probability of infection is Increased because of the higher concentration of roots and Inoculum potential in the relatively small volume of the cracks. Secondly, movement of root exudates and zoospores would be much faster through the cracks than through the dense ped interiors. Therefore the maximum distance that roots can be from a source of iritfculum potential and still be infected is much greater. The experiments conducted during the present study were not designed to eliminate either hypothesis and both will be discussed. Most likely, both high inoculum potential and Increased movement of root exudates and zoospores are important in fhe field.
The following sequence of events probably occurs in the field when no-tillage culture is started. The year before no-tillage practices are used, the soil has been plowed and the CRP spread through the soil, with no particular zones of accumulation. Structural cracks have been broken up and the clods or aggregates are randomly orientated. Dexter
(1976) measured orientation of aggregates in plowed soil and found no favored orientation. Therefore, the soil does not shape the rooting pattern of the c o m into the same location each year.
As the structural cracks have been destroyed by plowing, tortuosity of pores was high. Mobility of Pythium zoospores has been restricted to a reduced volume. Movement of root exudates to oospores 107 has been retarted. Pythium infects roots only If roots happen to grow close to sources of Pythium zoospores.
In the first year of no-tillage, cracks form in the soil and c o m roots follow the cracks to a degree. However, Pythium population along the cracks Is the same as in the ped interiors, so that corn roots are only moderately infected. Infection is higher in the roots in the cracks because the zoospores have greater mobility in the cracks than in the soil matrix; or, because root exudates diffuse further, more zoospores germinate (Stanghellini, 1974). Root exudate production may be greater in the cracks because of physical damage to the roots caused by soil shrinkage (White, 1975). Also the damaged roots would be more suscep tible to zoospores infection (Hickman, 1970).
In the second year of no-tillage, the roots will again follow the same cracks. As the inoculum potential along the cracks is high, the infection rate of the roots is high and a yield reduction occurs as noted by Van Doren et al. (1976).
The uptake of phosphorus by roots growing in the cracks is lower than by roots growing in the ped interiors (Appendix 10).
Vanterpool (1940) found phosphorus deficient plants to be more suscep tible to Pythium infection because of less vigorous root growth. If roots growing in the cracks are phosphorus deficient, they would not be able to elongate as rapidly and infection would be more severe.
If soybeans rather than c o m is planted the second year, the population built up along the cracks may die back because of lack of substrate or for other reasons not yet established (Knaphus and Buchholtz,
1958). Soybeans are not a host of Pythiuia graminicola (Middleton, 1943).
The growth of corn in plowed plots and In no-tillage corn-soybean rotated 108 plots was related to the bulk density of the soil (Figure 8) while the growth in the no-tillage continuous c o m plots was significantly lower at similar bulk densities. Therefore, factors besides soil compac tion must be causing the yield reduction. The buildup of Pythium along the cracks can explain the results shown in Figure 8.
In monocultures, pathogen populations often increase until after several years the host crop is decimated by the disease. As noted by Van Doren £t al., (1976) the yields of c o m in no-tillage plots remain at about 85 percent of the yields of the plowed plots.
In the present study, the inoculum potential of the ped in teriors in no-tillage continuous corn plots never increased above the levels found in rotated or tilled plots. Evidently the concentrations of roots are low enough in the dense ped interiors to prevent the buildup of inoculum potential of CRP. The Interior of the peds is uniformly dense so that root growth would not be concentrated in the same location year after year. Another possible explanation is that the dense ped interiors do not have pores which are large enough for the formation of fungal fruiting bodies, thus preventing reproduction (Griffin, 1963). Roots can penetrate the dense soil material and these roots are not highly infected because of the low inoculum potential. Evidently enough roots grow into the peds and along the crack surface to maintain the yields at the 85 per cent level, with no noticeable difference in top growth.
As Pythium Is ubiquitous in midwestem corn fields, a popula tion buildup cannot be prevented when no-tillage continuous c o m culture is used on soils such as Hoytville. Practices which decrease the inoculum potential such as rotating c o m with soybeans or practices which will destroy the crack environment, such as plowing, prevent the yield decrease.
As the B horizon of the Hoytville soil has a low hydraulic conductivity, 109 drainage to prevent the saturation of the A horizon is not feasible.
Fungicides would prevent the root infection by Pythium, but the material must not be as phytotoxic as pyroxychlor.
The present study is by no means complete and leaves many questions unanswered. In fact, CRP has not been positively identified as the cause of yield reduction of continuous corn grown with no-tillage methods on Hoytville soil. Field work with fungicides should clarify matters. Further work needs to be done with the root baiting technique to quantify the inoculum potential. The use of vials to control soil density and area exposed to the roots removed much of the variability so that differences between treatments can be found. However, lesion counts and organism identification are needed to test and evaluate the method.
Further work needs to be done on the distances inoculum can be from roots and still cause infection in soil under various conditions. For instance, increased bulk density will increase the diffusion rate of root exudates, decrease the movement of zoospores, and have little effect on the rate of hyphae growth. The importance of each of these factors needs to be evaluated. IV. SUMMARY AND CONCLUSIONS
Studies were conducted to find the causes of increased root infec tion by Pythium graminicola of corn grown under no-tillage culture in
Hoytville soils in northwestern Ohio. Preliminary studies showed that root length was a sensitive parameter of soil bulk density and the presence of Pythium.
A c o m root baiting technique was evaluated and improved so quanti tative measurements of Pythium inoculum potentials could be made. The major Improvements were:
1) The amount of damage caused by Pythium was determined by measuring the length of the roots, rather than counting the number of lesions.
2) The effective volume of the test soil exposed to the roots was con trolled by packing the soil in vials of known area and to an infinite depth.
3) Nutrient solution rather than water was used as the medium for root growth.
Three hypotheses to explain the increased root infection in no tillage continuous c o m were advanced, tested, and evaluated. Laboratory packed and undisturbed field cores were used to grow c o m under various levels of bulk density, rate of organic amendments, water potentials, and fungicide treatments. Blocks of soil, 15 cm square and 25 cm high, made of 5 cm cubes of soil, were used to grow corn with and without high levels of Pythium inoculum along the cracks.
110 Ill
The first hypothesis evaluated was the theory that high bulk den
sities and Pythium together increased root infection. However, experi
mental evidence showed that the root length reduction caused by Pythium
at bulk density of 1.50 g/cm^ was much less than at 1.20 g/cm^. The
lower porosity of the higher density soils reduced the mobility of
Pythium zoospores. Therefore, the first hypothesis was rejected.
The second hypothesis was that corn stalk residue turned under by
plowing enhanced the activity of competitors of Pythium. Laboratory measurement did not provide a conclusive answer. Root growth was reduced
more by nitrogen deficiency than by Pythium infection in one experiment.
When excessive nitrogen was added, all treatments allowed intermediate root growth. The inoculum potential of the soil was not related to the rate of carbon dioxide evolution of the soil. The second hypothesis was
rejected for reasons of lack of positive evidence rather than firm nega
tive evidence.
The third hypothesis incorporated the physical properties of the soil, the reaction of the c o m root to its environment, and the ecology of Pythium. If untilled Hoytville soil cracks in the same locations each year, if the corn roots follow the cracks preferentially, and if
Pythium accumulates along the cracks, the root infection is higher than
in plowed soil. The literature shows that soil cracks, once formed, do not move substantially. The present study showed that root growth is greater along cracks when the soil bulk density is 1.50 g/cm^. Also,
Pythium inoculum potential was found to be higher along cracks in no tillage continuous corn plots. Finally, in blocks of soil which had been inoculated along crack surfaces with Pythium, corn growth was reduced by about 50 percent compared to similar blocks without the Pythium 112 treatment. The third hypothesis was accepted as the major cause of increased root infection in no-tillage continuous corn plots. APPENDICES Appendix A. Effect of bulk density of Hoytville soil on the early growth of c o m and nutrient uptake*
Bulk Density
Plant Parameters 0.95 1.10 1.25 1.40 1.55 1.70 g// cmJ 1 ---- C o m height* cm 36 33 34 30 29 27 Fresh top weight, g 4.5 3.4 3.6 2.6 2.3 1.9 Dry top weight, mg 485 357 412 295 255 228
Dry root weight, mg 172 162 205 117 113 778 Root length, m 41.4 27.1 24.1 11.2 10.1 7.4
Potassium concentration In tops, mg/g 40.3 43.3 40.8 32.2 43.5 38.1 Phosphorus concentration in tops mg/g 2.04 2.21 2.02 2.38 2.33 1.90
Total potassium in tops, mg 18.9 15.4 16.8 8.6 10.5 8.4 Total phosphorus in tops, mg 0.97 0.78 0.83 0.61 0.52 0.41 Appendix B. Parameters of corn growth and nutrient uptake in Hoytville soil cores representing three field
treatments and three laboratory treatments. Each value is the average of three replicates.
Continuous C o m _ „ , „ _« Com-Soybean, No-Tillage LSD 5* Plowed No-tillage___ Proba- Plant Parameters None Steamed Sieved None Steamed Sieved None Steamed Sieved blllty
C o m height, cm 27 29 28 26 32 28 26 32 29 7 Fresh top weight, g 1.7 2.1 2.1 1.6 2.8 2.3 1.7 2.8 2.1 1.6 Dry top weight, mg 230 310 284 228 339 295 257 378 287 201
Dry root weight, mg 149 150 181 98 172 130 116 198 164 79 Root length, m 18.3 13.3 27.2 14.8 16.7 21.4 11.5 19.2 25.2 9.1
Potassium concentration in tops, mg/g 35.3 32.6 39.6 35.8 41.3 43.2 33.9 38.3 39.9 10.7 Phosphorus concentration in tops, mg/g 2.76 1.91 2.63 3.71 3.47 2.25 2.79 1.12 3.33 2.65
Total Potassium in tops, mg 8.2 9.8 11.3 8.1 16.6 12.8 7.6 14.4 11.5 9.6 Total Phosphorus in tops, mg 0.66 0.58 0.74 0.79 1.32 0.68 0.61 0.42 0.89 0.87
Potassium uptake per unit of root, mg/m 0.47 0.71 0.42 0.55 0.96 0.66 0.66 0.74 0.46 0.44 Phosphorus uptake per unit of root, mg/m 0.039 0.042 0.028 0.057 0.079 0.031 0.051 0.021 0.035 0.043 Po ta s s ium/Pho spo rus ratio 15.5 17.6 15.1 13.4 12.7 29.3 14.5 35.2 13.9 16.0 Appendix C. Parameters of corn growth and nutrient uptake in Wooster soil cores representing three field treatments and three laboratory treatments. Each value is the average of three replicates.
Continuous; Corn Corn/Soybean LSD 5% Plowed No-tillage No-Tillage Proba bility Plant Parameters None Steamed Sieved None Steamed Sieved None Steamed Sieved
Corn height, cm 225 24 21 25 27 25 24 27 24 5 Fresh top weight, g 1.7 1.4 1.1 1.8 1.9 1.7 1.4 2.1 1.5 0.8 Dry top weight, mg 238 190 148 250 254 241 185 289 199 97
Dry root weight, mg 119 97 88 122 139 168 103 152 128 76 Root length, m 13.9 14.2 17.0 14.0 17.4 25.0 12.9 16.1 21.5 7.5
Potassium concentration 38.6 42.9 46.2 41.4 41.4 38.2 44.1 43.1 42.7 7.9 in tops, mg/g Phosphorusconcentration 2.83 3.02 2.65 4.76 4.66 2.23 5.55 4.11 3.31 1.5 in tops, mg/g
Total potassium in 9.3 8.8 6.8 4.8 10.9 9.3 8.2 12.7 8.5 4.9 tops, mg Total phosphorus in 0.70 0.61 0.37 1.19 1.22 0.57 1.02 1.23 0.66 0.60 tops, mg
Potassium uptake per 0.66 0.58 0.40 0.74 0.61 0.37 0.62 0.81 0.41 0.17 unit of root, mg/m Phosphorus uptake per 0.048 0.041 0.021 0.085 0.060 0.021 0.077 0.077 0,030 0.021 unit of root, mg/m 116 Potassium/Phosphorus 14.5 14.0 20.1 8.8 8.9 19.9 8.0 10.6 13.8 9.7 ratio 117
Appendix D. Corn plant growth In Hoytvllle soil cores representing four
field treatments and four laboratory treatments. The soil was sampled
in April 1976. Each value is the average of three replicates.
Plant Growth Parameter Top Fresh top Dry top Dry root Root Treatment Code height weight weight weight length
cm g mg mg m
ACNI* 24 1.5 170 60 7.7 ACNU 26 2.0 230 116 8.3 ACSI 27 2.5 260 140 19.9 ACSU 25 2.1 210 144 15.7
ARNI 28 2.0 190 102 7.3 ARNU 28 2.5 280 101 10.7 ARSI 26 2.2 230 87 14.8 ARSU 2k 1.8 180 52 11.7
CCNI 30 3.2 330 96 6.5 GCNU 30 3.2 320 62 6.6 CSC I 32 4.3 460 110 16.8 GCSU 27 2.4 250 118 9.0
CRN I 34 4.0 420 75 5.9 GRNU 34 4.0 420 61 6.6 GRSI 32 3.8 360 133 15.6 GRSU 32 4.1 400 156 16.0
LSD (5% probability) 8 1.9 200 87 9.7
* Treatment codes:
Field treatment Lab treatment Tillage Rotation Tillage Fungicide A - plowed C - continuous c o m N - none 1 - none G - no-tillage R - com/soybeans S - sieved U - pyroxychlor Appendix E. Nutrient uptake by corn from Hoytville soil cores representing four field treatments and four laboratory treatments. The soil was sampled In April 1976. Each value is the average of three replicates.
Nutrient Uptake Parameter Cone, of K Cone, of P Total K Total P K uptake/ P uptake/ K/P Treatment Code in tops in tops in tops in tops root length root length ratio
mg/g mg/g mg mg mg/m mg/m
ACNI 31 3.5 5.4 0.6 0.7 0.08 9.7 ACNU 30 2.9 7.5 0.7 2.0 0.18 10.5 ACSI 40 2.5 10.0 0.6 0.7 0.05 17.2 ACSU 41 2.3 8.6 0.5 0.5 0.03 18.6
ARNI 32 2.6 5.8 0.5 0.8 0.06 12.4 ARNU 32 2.6 9.7 0.7 0.8 0.07 13.7 ARSI 36 2.2 8.2 0.5 0.6 0.04 16.6 ARSU 44 3.2 8.0 0.6 0.7 0.05 14.9
GCNI 43 4.3 14.1 1.3 2.9 0.26 10.5 GCNU 45 5.1 13.8 1.6 2.1 0.24 8.9 GCSI 32 3.1 16.2 1.7 0.9 0.09 10.7 GCSU 38 3.8 10.0 1.1 1.4 0.13 10.8
CRN I 46 4.9 19.2 2.0 3.4 0.36 9.7 GRNU 44 4.4 17.9 1.7 3.4 0.35 10.4 GRSI 36 3.3 12.4 1.1 0.9 0.08 11.2 GRSU 45 2.9 17.2 1.1 1.2 0.08 15.5
LSD (5% probability) 14 1.6 8.4 1.0 2.0 0.19 6.2 118 Treatment Codes: See Appendix D. Appendix F. Corn growth In Hoytvllle soil cores, with and without pyroxychlor treatment, packed at two bulk densities and equilibrated at two water potentials. Each value Is the average of three replications.
Soil Treatments Plant Parameters Bulk Water Plant Fresh top Dry top Dry root Root Fungicide density potential height weight weight weight length
g/cmJ bars cm S mg mg m
None 1.50 -0.06 23 2.1 290 178 10.0 None 1.50 0 24 2.2 271 107 9.6
None 1.20 -0.06 23 2.0 302 256 12.0 None 1.20 0 25 2.4 294 157 12.0
Pyroxychlor 1.50 -0.06 23 1.8 256 153 8.7 Pyroxychlor 1.50 0 26 2.5 287 138 11.1
Pyroxychlor 1.20 -0.06 24 2.1 307 231 15.6 Pyroxychlor 1.20 0 26 2.6 306 191 14.2
LSD at 5% level of probability 3 0.6 86 83 4.4 119 Appendix G. Nutrient uptake by corn grown in Hoytvllle soil, with and without pyroxychlor treatment, packed at two bulk densities and equilibrated at two water potentials. Each value Is the average of three replicates.
Soil Treatments Nutrient Uptake Bulk Water Cone, of Cone, of Total Total K uptake/U P uptake/U K/P Fungicide density potential K In tops P in tops K uptake P uptake of root of root ratio
g/cm bars mg/g mg/g mg mg mg/m mg/m
None 1.50 -0.06 36.6 1.99 10.8 0.54 1.05 0.055 19.4 None 1.50 0 34.9 2.25 9.4 0.61 1.02 0.067 15.6
None 1.20 -0,06 41.9 2,39 12.4 0.69 1.05 0.060 18.0 None 1.20 0 33.9 2.14 10.1 0.63 0.88 0.057 16.0
Pyroxychlor 1.50 -0.06 39.4 2.14 10.0 0.54 1.28 0.071 18.9 Pyroxychlor 1.50 0 37.4 2.35 10.8 0.67 1.11 0.071 16.1
Pyroxychlor 1.20 -0.06 37.7 2.18 11.7 0.66 0.78 0.046 17.8 Pyroxychlor 1.20 0 36.4 2.51 11.3 0.74 0.79 0.055 15.0
LSD at 5% level of probability 5.8 0.32 3.8 0.20 0.29 0.020 3.2 Appendix H. Corn growth and nutrient uptake from soil cores treated with and without pyroxychlor and corn stalk residue. Nitrogen was not added. Each value Is the average of four replications.
______Soil Treatments______No Fungicide______Pyroxychlor______LSD at 51 0% Organic 1% Organic OX Organic IX Organic level of Plant Parameters______residue residue residue______residue______probability
Top height, cm 34 25 49 24 7 Fresh top weight, g 2.8 1,3 5.6 1.3 1.8 Dry top weight, mg 290 160 590 170 180
Dry root weight, mg 90 43 120 33 59 Root length, m 14.5 6.3 25.4 6.3 8.0
Concentration of K in tops, mg/g 38.8 39.2 47.8 31.6 9.9 Concentration of P in tops, mg/g 2.91 2.68 2.75 2.32 0,55
Total K in tops, mg 11.1 6.1 28.2 5.7 7.7 Total P In tops, mg 0.86 0.42 1.57 0.41 0.41
Uptake of K/unit of root length, mg/m 0.78 1.09 1.11 0.90 0.37 Uptake of P/unit of root length, mg/m 0.059 0.077 0.064 0.068 0.032 Appendix I. Corn growth and nutrient uptake from soil cores treated with and without pyroxychlor and corn stalk residue. Nitrogen was added to all cores at 330 pg/g. Each value is the average of four replications.
______Soil Treatments______No Fungicide______Pyroxychlor______LSD at 5% 0% Organic \% Organic 0% Organic IX Organic level of Plant Parameters______residue______residue______residue______residue probability
Top height, cm 31 28 29 26 7 Fresh top weight, g 3.5 2.9 2.9 2,4 0.4 Dry top weight, mg 450 351 376 291 165
Dry root weight, mg 134 151 159 117 113 Root length, m 16.5 13.9 12.7 13.9 9.2
Concentration of K in tops, mg/g 34.5 39.0 33.3 35.3 4.8 Concentration of P in tops, mg/g 1.93 1,76 2.25 1.93 0.39
Total K in tops, mg 15.5 13.8 12.4 10.4 6.2 Total P In tops, mg 0.87 0.61 0.83 0.52 0.26
Uptake of K/unit of root length, mg/m 0.95 1,04 1.04 0.77 0.29 Uptake of P/unit of root length, mg/m 0.054 0.049 0.070 0.042 0.021 Appendix J. Corn growth and nutrient uptake from soil cores made with four different Internal crack areas.
Each value Is the average of eight determinations.
Internal Crack Area cm^/cm3 Plant Parameter 0 0.13 0.26 0.66 Probability!/
Height, cm 26 28 29 30 ns Fresh top weight, g 1.6 1.8 2.2 2.3 0.06 Dry top weight, mg 187 220 267 243 ns
Dry root weight, mg 45 45 44 54 ns Root length, m 8.9 9.4 8.7 12.8 0.04
Concentration of K in tops, mg/g 44.3 41.6 36.4 40.4 ns Concentration of P in tops, mg/g 3.10 2.56 2.32 2.18 0.02
Total K in tops, mg 8.2 8.9 10.0 9.8 ns Total P in tops, mg 0.53 0.54 0.55 0.51 ns
Uptake of K/unit of root length, mg/m 1.4 1.0 1.1 1.0 ns Uptake of P/unit of root length, mg/m 0.097 0.063 0.069 0.053 0.07
1/ Probability at which linear relationship is equal to zero. ns - Linear regression not significant. VI. LITERATURE CITED
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