. ha! in. W WWW!Nliilifliimflfifliii """"" , ‘ - ..._.... 29300103 2709 liq- - o P 1 fl .4 .‘ (it“a’ba-a . PHJN'M'.‘£.; I . ““héw- ~‘qu '1 u. -‘ ’fif n. v! ”I. 4, r U 9. . 10' ' : . h - ‘—‘ ~43 'v c Q. g .I This is to certify that the dissertation entitled EFFECT OF CROPPING SYSTEM 0N SOIL STRUCTURE AND GROWTH AND DEVELOPMENT OF SUGAR BEETS (BETA VULGARIS, L.) presented by Nour i Moussa Momen has been accepted towards fulfillment of the requirements for Ph.D. degree in Soil Science me (2.0% Major professor DateW 8; {q 85 MS U is an Afl’mnativc Action/Equal Opportunity Institution 0-12771 MSU LIBRARIES “ RETURNING MATERIALS: Place in book drop to remove this checkout from your record. FINES will be charged if book is returned after the date stamped below. .3 3553;: 21991 SEP? ‘3 NO? ‘9‘}‘f 'i ‘ .- -. E753“ U 0 “A5 EFFECT OF CROPPING SYSTEM ON SOIL STRUCTURE AND GROWTH AND DEVELOPMENT OF SUGAR BEETS (BETA VULGARIS, L.) BY Nouri Moussa Momen A DISSERTATION Submitted to Michigan State University in partial fullfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Crop and Soil Science 1985 ABSTRACT EFFECT OF CROPPING SYSTEM ON SOIL STRUCTURE AND GROWTH AND DEVELOPMENT OF SUGAR BEETS (BETA VULGARIS, L.) BY Nouri Moussa Momen The effect of cropping system on the structure of a Charity clay soil was studied in 1982 and 1983 in an established experiment initiated in 1972 at the Saginaw Valley Bean and Beet Research Farm. Field studies were conducted for sugar beet growth and deve10pment to determine the influence of cropping system on the modification of soil structure. The soil structure indices measured were bulk density, saturated hydraulic conductivity, total porosity, air porosity, mean-weight diameter of soil aggregates and their distribution at different size ranges. All the indices were affected by the soil depth within the plow depth (0 to 0.23 m). The magnitude of changeiwas different among the sampling dates due to freezing and thawing, wetting and drying and/or the root system effects of sugar beets. A cropping system with 50% or more corn contained Nouri Moussa Momen high.percent of aggregates with diameters between 5 to 0.5 mm. However, during the period of this study, the corn- beets system showed more dynamic changes in soil structure. The positiverlinear correlation found between mean- weight diameter and soil organic carbon and the C/N ratio indicates the importance of the continuous additions of organic matter in modifying soil structure. Sugar beet growth and development were measured by leaf area index, taproot to leaf weight ratio and the fibrous root length density. The corn-beets system had lower leaf area index and taproot to leaf weight ratio and higher fibrous root length density than the corn-beans-beans-beets system. That could be the reason for the lower sugar beet yield in the corn-beets system where most of the assimilates were probably used by the fibrous roots. The fibrous root length density was significantly correlated with the leaf area index and with the final crop yield at specific root sampling positions below the taproot. This will help locate the optimum sampling positions for root studies which will save time and energy and assist in the best placement of fertilizer. DEDICATION To my parents and grandfather ii ACKNOWLEDGMENTS I wish to express my sincere gratitude and appreciation to Dr. D. R. Christenson, my major professor, for his kindness and valuable guidance throughout this study. He has provided both friendship and encouragement which I admire. Sincere appreciation is extended to the members of my guidance committee, Drs. A. J. M. Smucker, L. S. Robertson, 8. A. Boyd and K. L. Poff for their comments and advices. Special thanks are due to Dr. R. J. Kunze for his aid in some of the computer programs. Special thanks are also extended to Mr. Calvin Bricker for his friendship and help in the field, laboratory and greenhouse work. I would also like to thank Mr. Richard Johnson for his friendship and assistance in taking soil-root core samples and root washing. Also gratefully accepted and deeply appreciated were the efforts of many friends who helped in field soil sampling. Sincere appreciation is extended to the Michigan Sugar and Monitor Sugar companies for their financial support. Appreciation is also extended to Garyuonis iii University for the scholarship given enabling me to do my graduate study. I am very grateful to those who are very special to me, my wife Salima, daughter Assma and son Mohannad, for their understanding and sacrifice made during the course of this study. Finally, I would like to thank the Crop and Soil Sciences Department for making my study at Michigan State University worth-while. iv TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES INTRODUCTION LITERATURE REVIEW I. II. III. IV. VI. Introduction Soil Structure in the Field of Soil Science 2.1. Soil Structure as a Physical Property 2.2. Importance of Soil Structure 2.3. Some Indices for Soil Structural Studies Stability of Soil Structure 3.1. Definition of Soil Aggregates 3.2. Aggregate Formation and Stabilization 3.2.1 Effect of Swelling and Shrinking Characteristics of Clay Minerals 3.2.2. The Role of Organic Matter 3.2.3. The Role of Soil Microorganisms 3.3. Mechanisms of Aggregate Stabilization 3.3.1. General 3.3.2. Clay-Organic Interactions Effects of Seasonal Variations on Soil Structure 4.1. The Action of Freezing and Thawing 4.2. The Action of Wetting and Drying The Impacts of Cropping Systems and Rotational Lengths on Soil Structure 5.1. Cropping Systems Effect 5.1.1. Beneficial Effects 5.1.2. Deleterious Effects 5.2. Effect of Length of the Rotations Effect of Soil Structure on Root Growth and Crop Yield 6.1. Effect on Root Growth 6.2. Effect on CrOp Yield V page viii xii H mflxl O‘U‘I'U'l-b huh 10 11 11 ll 13 13 14 16 16 17 19 20 21 21 VII. Improvements of Soil Structure MATERIALS AND METHODS I. II. III. IV. VI. VII. Description of the Experimental Area 1.1. General Properties of the Soil 1.2. Climatological Aspects Field Experiment Methods of Soil Sampling 3.1. Undisturbed Core Samples 3.2. Disturbed Soil Samples Soil Measurements 4.1. Undisturbed Samples 4.2. Disturbed Samples Methods of Plant Sampling and Measurements 5.1. Leaf Blades and Taproots 5.2. Fibrous Root Systems 5.3. Sugar beet Yield and Quality .Greenhouse Experiment 6.1. Soil Sampling and Preparation 6.2. Harvesting and Root Sampling Separation of Roots and Their Length Measurements RESULTS AND DISCUSSION ,1. II. Description of the Study Soil Structural Studies 2.1. Undisturbed Samples 2.1.1. Bulk Density 2.1.2. Saturated Hydraulic Conductivity 2.1.3. Total Porosity 2.1.4. Air Porosity 2.2. Disturbed Samples 2.2.1. Mean-Weight Diameter 2.2.2. Aggregate Stability 2.3. Changes Over Time 2.3.1. Bulk Density 2.3.2. Saturated Hydraulic Conductivity 2.3.3. Total Porosity 2.3.4. Air Porosity 2.3.5. Aggregate Stability 2.4. Discussion 2.4.1. October 1982 Sampling Date 2.4.2. May 1983 Sampling Date 2.4.3. November 1983 Sampling Date vi 25 29 29 29 31 34 36 36 37 37 37 39 40 40 41 42 44 44 47 2.4.4. Effect of Time on Soil Structure III. Plant Indices and Crop Yield 3.1. Leaf Area Index 3.2. Taproot-Leaf Weight Ratio 3.3. Root Length Density 3.4. Discussion 3.4.1. Cropping System Effect on the Measured Indices 3.4.2. Relationship Among Some Plant Parameters, Crop Yield and Quality 3.4.3. Optimal Positions For Root Sampling Studies IV. Greenhouse Experiment 4.1. Changes in Bulk Density and Root Length With Soil Core Sampling Depth 4.2. Influence of Cropping System on Root Length and Shoot Dry Weight 4.3. Discussion SUMMARY CONCLUSIONS LITERATURE CITED vii 95 103 103 103 106 116 117 119 121 125 127 127 132 136 141 144 LIST OF TABLES Table 1. Monthly maximum and minumum temperatures one meter from soil surface at the Saginaw Valley Bean-Beet Research Farm during 1982 and 1983. Monthly precipitation as measured by the US Weather Bureau Rain Gauge at the Saginaw Valley Bean-Beet Research Farm during 1982 and 1983. Effect of soil sampling depth on bulk density (BD) in a cropping systems study for the three sampling dates. Effect of cropping system and depth on soil bulk density (BD) in a cropping systems study for the October 1982 sampling date. Effect of soil sampling depth on saturated hydraulic conductivity (SHC) in a crepping systems study for the three sampling dates. Effect of soil sampling depth on total porosity (TB) in a cropping systems study for the three sampling dates. Effect of soil sampling depth on air porosity (AP) in a cropping systems study for the three sampling dates. Effect of soil sampling depth on mean-weight diameter (MWD) of soil aggregates in a crepping systems study sampled on October 1982 and November 1983. Cropping system effect on mean-weight diameter (MWD) of a Charity clay soil in a cropping systems study sampled on October 1982 and November 1983. viii page 32 33 53 55 56 58 60 61 62 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. Effect of soil sampling depth on aggregate stability (AS) of a Charity clay soil in a cropping systems study sampled on October 1982 and November 1983. Aggregate size ranges and their distribution in a Charity clay soil in a cropping systems study sampled on October 1982 and November 1983. Probability of a significant F test for various soil structural indices combined over the different sampling dates. Effect of sampling date and depth on bulk density (BD) of a Charity clay soil in a cropping systems study. Effect of sampling date and cropping system on bulk density (BD) of a Charity clay soil in a cropping systems study. Effect of sampling date and depth on saturated hydraulic conductivity (SHC) of a Charity clay soil in a cropping systems study. Effect of sampling date and depth on total porosity (TP) of a Charity clay soil in a cropping systems study. Effect of sampling date and cropping system on total porosity (TP) of a Charity clay soil in a cropping systems study. Effect of sampling date and depth on aeration porosity (AP) of a Charity clay soil in a cropping systems study. Effect of aggregate size range and sampling date on aggregate size distribution of a Charity clay soil in a cropping systems study. Probability of a significant F test for various soil structural indices for the October 1982 sampling date. Probability of a significant F test for various soil structural indices ix 64 71 73 75 76 79 81 82 83 85 88 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. for the May 1983 sampling date. Probability of a significant F test for various soil structural indices for the November 1983 sampling date. Probability of a significant F test for various structural indices combined over the different sampling dates . Effect of cropping system on leaf area index (LAI) for sugar beets (US H20) in a crepping system study sampled on August 26, 1983. Effect of crOpping system on taproot- leaf weight ratio (TLWR) of sugar beets (US H20) on wet and dry basis in a cropping systems study sampled on different dates. Effect of sampling depth on soil-root core bulk density (BD), moisture content (MP) and root length density (RLD) of sugar beets (US H20) in a crapping systems study sampled on October 1983. Root Length (RL) distribution of sugar beets (US H20) grown on a Charity clay soil in a cropping systems study sampled in October 1983. Influence of cropping system on fibrous root length of sugar beets (US H20) in a cropping systems study sampled in October 1983. Influence of cropping system on soil- root core bulk density (BD) and root length density (RLD) of sugar beets (US H20) in a cropping systems study sampled on October 1983. Effect of cropping systems on the ratio of root length (RL) to taproot-leaf weight ratio (TLWR). yield and recoverable white sugar (RWS) of sugar beets (US H20). Simple correlations among some plant parameters and root length density (RLD) of sugar beets (US H20) sampled on October 1983 at different sampling positions. X 92 94 96 104 105 108 110 111 112 120 124 32. 33. 34. 35. 36. 37. Influence of cropping system on fibrous root length and yield of sugar beets (US H20) in a cropping systems study sampled in October 1983. Changes in bulk density (BD) with sampling depth of soil cores for the greenhouse experiment. Influence of soil core sampling depth for the greenhouse experiment on root length (RL) of navy beans (Seafarer). Cropping system effect on root length (RL) of navy beans (Seafarer) grown in the greenhouse. Cropping system effect on shoot dry weight of navy beans (Seafarer) grown in the greenhouse. Effect of cropping system on root length (RL) required per unit weight of dry shoot of navy beans (Seafarer) grown in the greenhouse. xi 126 128 129 130 131 133 LIST OF FIGURES Figure page 1. Processes for taking the soil-root core samples. A, driving the core sampler into the soil. B, the core pulling frame mounted on the tractor. C, the core being subsectioned by a 9-cell fractionator. 43 An assembly of a soil core with its plastic ring and their dimensions for the greenhouse experiment. 46 Instruments for sampling soil-root cores for the greenhouse experiment. A, a harvested soil core with a thin aluminum cylinder at the top. B, a plunger to remove the sample from the thin aluminum cylinder. 48 Part of the hydropneumatic electriation system for root separation from the soil- root sample. A through E are explained in the text. (Taken from Smucker, McBurney and Strivastava, 1982). 50 Influence of cropping systems on aggregate size distribution sampled from 0 to 0.1 m soil depth on October 1982. Bars with the same letter are not significantly different at 0.05 probability level based on Duncan's Multiple Range Test (for comparison of two cropping system means within an ASR) 66 Influence of crOpping systems on aggregate size distribution sampled from 0.1 to 0.2 m soil depth on October 1982. Bars with the same letter are not significantly different at 0.05 probability level based on Duncan's Multiple Range Test (for comparison of two cropping system means within an ASR). 67 xii 10. 11. 12. 13. Influence of cropping systems on aggregate size distribution sampled from 0 to 0.1 m soil depth on November 1983. Bars with the same letter are not significantly different at 0.05 probability level based on Duncan's Multiple Range Test (for comparison of two cropping system means within an ASR). Influence of cropping systems on aggregate size distribution sampled from 0.1 to 0.2 m soil depth on November 1983. Bars with the same letter are not significantly different at 0.05 probability level based on Duncan's Multiple Range Test (for comparison of two cropping system means within an ASR) Effect of cropping system X ASR interaction on the size distribution of aggregates sampled on November 1983. Bars with the same letter are not significantly different at 0.05 probability level based on Duncan's Multiple Range Test (for comparison of two cropping system means within an ASR). Variations in BD among the different systems as affected by depth for the three sampling dates. Aggregate size distribution as affected by aggregate size range for three cropping systems, averaged across the October 1982 and November 1983 sampling dates. Bars with the same letter are not significantly different at 0.05 probability level based on Duncan's Multiple Range Test (for comparison of two cropping system means within an ASR). Aggregate Mean-weight Diameter (MWD) as affected by percent organic C and C/N ratio of the soil. Influence of sampling date on saturated hydraulic conductivity for the different cropping systems. Bars with the same letter are not significantly different at 0.05 probability level based on Duncan's Multiple Range Test (for comparison of two cropping system means within an ASR). xiii 68 69 72 78 86 90 100 14. 15. 16. 17. 18. 19. 20. 21. Influence of sampling date on air porosity at 600 mm water tension for the different cropping systems. Bars with the same letter are not significantly different at 0.05 probability level based on Duncan's Multiple Range Test (for comparison of two cropping system means within an ASR). Comparisons of mean aggregate size distribution from three cropping systems (C-B, O-Be-B and C-Be-Be-B) sampled on two dates with that of an adjacent virgin soil sampled on October, 1982. The inter-row space soil profile sampled with its 54 soil-root core subsamples after fractionation. Diagramatic illustration for taking average measurements for each corresponding pair of soil-root core subsamples. Distribution pattern of root length density of sugar beets (US H20) as affected by soil depth below the taproot sampled from the C-B crOpping system. The three soil-root core samples (RIGHT, MIDDLE and LEFT) represent the fibrous roots of one plant. Distribution pattern of root length density of sugar beets (US H20) as affected by soil depth below the taproot sampled from the O-Be-B cropping system. The three soil-root core samples (RIGHT, MIDDLE and LEFT) represent the fibrous roots of one plant. Distribution pattern of root length density of sugar beets (US H20) as affected by soil depth below the taproot sampled from the C-Be-Be-B cropping system. The three soil-root core samples (RIGHT, MIDDLE and LEFT) represent the fibrous roots of one plant. Sampling positions for sugar beet RLD measurements beneath the taproot. Significant correlation between RLD and either LAI, TLWR or Yield are indicated at the corresponding sampling positions. xiv 101 102 107 107 113 114 115 123 INTRODUCTION Depending on the amount of crap residues produced, different cropping systems may benefit or deteriorate the soil structure. The depth to which soil structure is affected is very important for plant growth and development. The effect of different craps on soil structure with depth will be reflected by soil aeration, bulk density, water conductance and aggregate stability. Beneficial effects will result in water and nutrients being available to unimpeded roots which proliferate with minimum exerted pressures. Therefore, the maximum capacity for crop yields will not be restricted as far as soil structure is concerned. For a given soil, many factors which act simultaneously are involved in the modification of soil structure. They include physical, chemical, biological and environmental factors which are integrated over time. The most important is the availability of organic matter upon which the magnitude of the other factors will depend. As the amount and characteristics of organic matter varies, the effect of microbial activities, freezing and thawing, and wetting and drying on soil structure will vary accordingly. Soil compaction and wind and water erosion, which are the consequences of poor soil structure, will result in diminishing crap yields. To preserve the soil and increase its productivity, researchers have tried several techniques including different tillage practices and the addition of chemical conditioners. Good results have been obtained, but the applicability of these techniques are limited due to their high costs, especially in the developing countries. A well known practice, which has been studied for many years, is the use of different cropping systems to maintain or improve soil structure and hence its. productivity. Several observations have suggested that some plant species have greater ability than others to overcome mechanical stress in the soil. Plants which display this characteristic may improve soil conditions for crops which are planted subsequentlyu To what extent cropping systems can modify soil structure, especialty at different depths, is not completely understood. Variations in soil types, the factors involved and the indices used are contributing components to be understood. Cropping system effects in modifying soil structure with depth will be reflected upon root growth and development. Root distribution studies under field conditions as related to cropping systems and soil structure are limited, especially for root crops. Including root densities at different soil depths with the other indices will help in characterizing soil structural modifications by the different crepping systems. With this background, the working hypothesis is that cropping systems have differential effects upon soil structural modification and root distribution with depth. Evaluating structural changes with soil and plant indices make yield correlation studies with these parameters more pronounced. The objectives of this study were: 1. To evaluate the effect of different cropping systems after 11 years on the following soil physical indices: a) saturated hydraulic. conductivity, b) bulk density, c) total porosity, d) air porosity at 600 mm water tension and e) aggregate mean weight diameters and their stability. To relate selected phenological measurements to the above indices. 3. To relate root growth measurements in the greenhouse to field measurements as they are affected by soil structure with depth. 4. To relate the measured soil physical indices to sugar beet yield. LITERATURE REVIEW I. Introduction Soil structure controls many of the soil properties which have been studied extensively. In order to understand this phenomenon it is important to know what factors could affect its modification as reported in the literature. The next three sections deal with the concept of soil structure where its definitions and its stabilization. as affected by several factors will be explained. As far as the effect of different cropping systems on soil structure are concerned, their beneficial and deleterious effects will be reviewed in sections V and VI. The last section explains some ways of identifying good soil structure and how to maintain and/or improve it. II. Soil Structure in the Field of Soil Science Researchers have worked with soil structure for more than two centuries in order to understand and improve this phenomenon. Unfavorable soil structure could limit the plant roots from reaching water and nutrients which otherwise could be beneficial to plant growth (Baver, Gardner and Gardner, 1972). From this prospective they suggested that soil structure should be considered as a parameter of soil fertility. 2.1. Soil Structure as a Physical Property The soil physical conditions under which plants grow and develop are very important. For the scientific evaluation of various agricultural and reclamation practices, parameters describing these physical conditions must be estimated. Such estimates are particularly needed for direct regulation of physical prOperties during soil cultivation in intense agricultural systems. The physical conditions for a given soil can be expressed by its structure. Marshall (1962) defined soil structure as the arrangement of the soil particles and the pore space between them. It includes the size, shape, and arrangement of the aggregates formed when primary particles are clustered together into large separable units. 2.2. Importance of Soil Structure The general agreement in the literature is that good soil structure can improve, directly or indirectly, some physico-chemical properties of a soil. Page and Willard (1948) stated, "The future of agriculture on many of our soils depends largely on how well favorable soil structure can be maintained or increased". Structural improvement might influence soil-water relationships because aggregate stabilization promoted increased water infiltration (Jamison, 1953) The dynamic changes in soil structure, due to the interrelated factors involved, make it difficult to define exactly what would be the Optimum soil structure (Danilson, 1972). At the same time, well aggregated soils were found. to exhibit increased infiltration, reduced erosion and runoff, increased seedling emergence, and would help to maintain a favorable soil-air-water regime for plant growth and microbial activities (Thien, 1976). 2.3. Some Indices for Soil Structural Studies Since the soil matrix consists of solid, liquid and gaseous phases, their interaction is considered to be very important in describing the physical state of the soil profile. Nikolayev (1975) formulated three equations to be used as indices for each phase and a general equation for the interaction among the three phases. He suggested that using this kind of modulation would reflect the soil productivity conditions. Leamer and Shaw (1941) reported that Schumacher recognized the importance of the distribution of pore size for plant growth as early as 1864. Schumacher introduced the terms “capillary” and "non-capillary” pore space to designate the small and large pore space, respectively. Russell (1971) used the term structural pores, which include coarse, medium and fine sizes. He stressed that the fundamental problems in soil structure management are concerned with the creation of these pores and their stabilization when formed. In order to characterize the structural behavior of a soil, other indices have been used. They include bulk density, hydraulic conductivity, aggregate stability and soil strength. Even though correlations may not exist between all of these indices (Sorochkin, 1975 and Voznyuk, Kuzmich and Volkova, 1980), scientists have used one or several of them. III. Stability of Soil Structure From the stand point of plant growth and develOpment, good soil structure is usually measured by the ability of the soil to form water-stable aggregates. 3.1. Definition of Soil Aggregates A broad definition of soil aggregation which can be related to soil structure was given by Woodruff (1939). He defined it as “that physical property which affects the functional behavior of the soil with respect to water absorption, aeration and root penetration". A more precise definition of soil aggregates was given by Martin 33 1., (1955). They defined a soil aggregate as "a naturally occurring cluster or group of soil particles in which the forces holding the particles together are much stronger than the forces between adjacent aggregates". 3.2. Aggregate Formation and Stabilization As mentioned previously, aggregate formation involves primarily the orientation of soil particles, and bringing them together closely so that when allowed to dry the physical forces hold them firmly (Baver, Gardner and Gardner, 1972) Chemical and electrical forces may also be involved in at least a minor way (Martin 35 31., 1955; Stefanson, 1968 and Greenland, 1971). A130, the size and stability of dry soil aggregates may be influenced by CaCO3 and Al and Fe hydroxides (Siddoway, 1963 and Bond and Harris, 1964). Tisdall and Oades (1982) concluded that aggregates < 0.25 mm are stabilized by organo-mineral complexes and polysaccharides. Meanwhile, the stabilization of aggregates > 0.25 mm depend largely on the amount of roots and hyphae present in the soil. 3.2.1. Effect of Swelling and ShrinkingyCharacteristics of Clay Minerals The stability of soil aggregates will be influenced to a great extent by the swelling and shrinking ability of clay minerals. Mazurak (1950) predicted that the order of greater water-stability of any aggregate size would be smectite > hydrous ndca > kaolinite. Soils containing smectite showed an almost reversible swellingiand shrinking on rewetting and redrying. On the other hand, soils containing kaolinite or hydrous mica showed an initial large volume decrease on drying with only a limited swelling on rewetting (Yong and Warkentin, 1966) . Uehara and Gillman (1981) found that the swelling ability of clays depend on their activity. An activity factor, which was defined as the ratio between plasticity index and percent clay-sized particles, was related to the surface area of the clay' mineral. 312.2. The Role of Organic Matter The significant role of organic matter in aggregate stabilization results from their reduction of swelling, reduction of destructive forces by entrapped air, decrease in wettability, and by their strengthening of the aggregates (Robinson and Page, 1950). During the cultivation of arable lands, the organic matter between aggregates becomes more exposed to further decomposition by soil microorganisms which will reduce soil aggregation (Stoneman, 1973). Miller and Kemper (1962) found that incorporation of alfalfa into the soil affected aggregate stability only for a short period of time. Sufficient amounts of organic matter should therefore continuously be available. For unstable and poorly permeable soils,‘Williams and Cooke (1961) suggested that 10 more organic matter might be needed than was given by the residues of arable crops grown. 3.2.3. The Role of Soil Microorganisms Under suitable environmental conditions of nutrient and water availability, optimum soil temperature and pH, soil organisms efficiently breakdown fresh organic materials (Harris, Chesters and Allen, 1966). Further decomposition of the partially decomposed organic matter will result in more complex compounds such as fulvic and humic acids. Synthesis of these organic compounds requires the presence of some enzymes which can be produced by other microorganisms (Hamblin and Greenland, 1977) . Filamentous microorganisms can directly stabilize soil aggregates. The adherence of soil particles to the mucilage which covers the hyphae was found to be an effective process in aggregate formation (Aspiras 35 31., 1971). Some negative effects on soil structure by some microorganisms have been speculated. McCalla (1951) suggested that production of gases or highly hydrated organic materials might interfere with water movement in the soil as a result of decomposition and/or a change of aggregate stabilizing agents. Fahad 23 31., (1982) also related their low infiltration data after 6 years of soybeans to some microorganisms clogged the soil pores. 11 3.3. Mechanisms of Aggregate Stabilization 3.3.1. General Several mechanisms have been proposed for the stabilization of soil aggregates. They include the linkage of clay particles by water-dipoles, cross-bridging and sharing of intercrystalline forces and interaction of exchangeable cations between oriented clay plates. The involvement of soil particles by precipitated and irreversibly dehydrated colloids such as silicates, sesquioxides and humates are also considered. Aggregates may be stabilized against water entry by the presence of hydrophobic organic materials as fats and waxes. The interparticle linkage by organic polymers that form bonds through their functional groups with the surface of two or more clay particles is another important mechanism. A comprehensive review of the theories involved in aggregate formation and stabilization was given by Harris, Chesters and Allen (1966). 3.3.2. Clay-Organic Interactions Clays were considered to associate in parallel alignments to form quasi-crystals or domains. Then these crystals were formed into micro-aggregates which were held together by organic polymers (Tabibudeen, 1981). Mortland (1970) concluded that the dominant factors determining the 12 nature of clay-organic interactions are the properties of the organic molecule, the water content of the system, the nature of the exchangeable cation on the clay surface and the unique properties of the clay mineral structure. Since clay particles are negatively charged, the adsorption mechanism of organic polymers will vary according to their electrical nature. For example, Van der Waals forces and hydrogen bonding are thought to be important "mechanisms in the bonding of uncharged polysaccharides to clay particles (Clapp, 1972). The effect of these interactions depends mainly on the molecular weight of the polymer. As the molecular weight increases, the additive effect of the physical adsorption by Van der Waals force will be very significant. . Stability of soil aggregates by clay-organic interactions will be highly affected, among other factors, by the types of organic polymers and exchangeable cations in the soil. For example, humic and fulvic acids were found to be absorbed by clay minerals via ionic linkages with di-and tri-valent cations (Schnitzer, 1969; Greenland, 1971 and Theng, 1976). Meanwhile, adsorptions of polysaccharides was found to be primarily through physical forces, such as Van der Waals forces (Greenland, 1965 and Hamblin and Greenland, 1977). At high concentration of polycations, as is the case of Ca and Mg in calcareous soils, the strong aggregation may be explained by the strong bonding of the 13 carbonyl group of polysaccharides to these cations (Clapp, 1972). IV. Effects of Seasonal Variations on Soil Structure Characterization of soil structure by aggregate formation and stabilization indicates that it is a complex and a dynamic process. The combination of the different factors involved and their changes over time influences the structural behavior of a given soil. Aggregation is highly affected by the activity of soil microorganisms. At the same time, microbial activities are controlled by the soil's environment which in turn is closely related to seasonal variations. Wilson and Browning (1946) concluded that variations in the degree of aggregation should be expected between samples collected over a period of time. A definite seasonal trend on aggregate stability was found by Strickling (1950). 4.1. The Action of Freezing and Thawing The effect of freezing and thawing appears to be one of the important factors on aggregate variations between seasons. Bisal and Nielsen (1967) indicated that the reduction in the percentage of erodible aggregates at high moisture content was due to freezing and thawing. Depending on the aggregate sizes and their moisture content, soil structure can be highly affected by the 14 processes of freezing and thawing. For example, Hinman and Bisal (1968) concluded that freezing and thawing and subsequent drying might increase or decrease the prOportion of aggregates less than 0.1 mm in diameter. Also, Chepil (1953) found that the breakdown of aggregates greater than 0.84 mm during the winter was associated with an increase of water-stable aggregates less than 0.01 mm in diameter. Other indices which may be related to soil structure also can be affected by freezing and thawing. Benoit (1973) found that freezing and thawing decreased the hydraulic conductivity at high soil moisture while increased it at low water content. Bolton, Dirks and Findlay (1979) related the variation in pore space observed between years to the level of winter freezing and to the moisture conditions in the spring. 4.2. The Action of Wetting and Drying In conjunction with freezing and thawing, wetting and drying also have a large effect on soil aggregates. Water enters between aggregates at different rates depending on the pore-size distribution. The result will be an uneven degree of wetability among aggregates, hence the water will act unequally on the binding agents. Baver, Gardner and Gardner (1972) summarized the causes of crumb breakdown as a result of wetting: 1) dispersion of the cementing material 15 2) reduction in Cohesion with increasing moisture content 3) compression of entrapped air and 4) stresses and strains set up by unequal swelling due to soil heterogeneity and non-uniform wetting. Some of the above processes may be reversed upon drying resulting in a stabilization of soil aggregates. The significant effect of wetting and drying on soil structural modification will depend on soil type and its constituents, moisture of the aggregates at the time of wetting and the intensity of wetting. Generally, it has been found that maximum aggregate ‘ stability occurs in the summer, fcdlowed by a gradual decline during the fall and early winter with an increase in the spring and early summer (Stefanson, 1968,1971). During the spring time the availability of organic matter and the favorable environmental conditions for microbial activities will result in an increased aggregate stability. During the fall and winter months the soil will be more exposed to weather changes. For example, the effect of intensive rain on poorly structured dry soil may result in unfavorable soil conditions for the next growing season. 16 V. The Impacts of Cropping Systems and Rotational Lengths on Soil Structure Different agricultural cr0ps have different rooting patterns and produce different amounts of biomass. Therefore, it will be expected that similar crops may have specific impacts on soil structural modifications. From these perspectives, different cropping systems have been studied extensively to find answers for increasing soil productivity. With the increasing demand for food production, scientists considered soil structural improvement as one of the achievable approaches. Concerns about the possibility of modifying soil structure by different cropping systems have led researchers to study this phenomenon on already existing experiments. In making conclusions from such studies, we have to keep in mind the interrelationships among all factors involved. 5.1. Cropping Systems Effect Modifications in the soil productivity have been observed by the use of different crops and cultural practices. Baver (1949) made an excellent review for the relations between some soil physical changes and crop production. He stated ”There are numerous studies in the literature showing the effect of certain cropping and soil management practices upon the changes in the physical 17 properties of the soil. These studies provide evidence of the beneficial or detrimental effects of a given practice upon the soil. From such results, one can deduce what will probably happen to crop yields". Data available in the literature indicate the ngnificance of some agricultural practices, including cropping systems, on soil structure modifications. Cary and Hayden (1974), meanwhile, indicated on a silt loam soil that cropping history did not show any effect on either pore-size distribution or on the soil hardness. The divergent results reported in the literature concerning the effect of crap rotation on soil structure can be attributed to several reasons. Most importantly, methods of manipulating the soil under which the experiments were run could lead to different results. The lack of consistency in the cultural practices and in the methods of soil sampling and analyses may also lead to some conflicting results. 5.1.1. Beneficial Effects Generally, a cropping system which includes plants with massive and extensive root systems and good vegetative cover may be beneficial to soil structure (Harris, Chesters and Allen, 1966; Ojeniyi and Dexter, 1979 and Tisdall and Oades, 1980) . The beneficial effects from these crops may 18 be due to the action of their roots and from providing enough organic matter to the soil. The presence of extensive root system plants in the soil can stabilize the aggregates in several ways. First, during plant growth, the roots can stabilize the aggregates physically and chemically. Adherence of fine soil particles to living root hairs and production of organic complexes constitute the major mechanisms. Second, when plant roots die, they become an additional source of organic matter in the soil (Allison, 1973) . Greenland, Lindstrom and Quirk (1962) found small reduction in aggregate stability of Red Brown soils which was in pasture for many years. It seems that good soil structure can be restored either by permanent grasses or by the inclusion of some beneficial crops in a rotational system. For example, Low (1955) reported that periods between 5 and 50 years, depending on the soil texture, were required to restore the stability of old arable soils to levels comparable to those under permanent grass. Also, Grieve (1980) found a significant reduction in aggregate stability after only 2 or 3 years the soil was out of grass. Winter wheat has been found to stabilize larger aggregates more than sorghum or soybean (Armbrust £3 22,, 1982) . However, Siddoway (1963) showed that inclusion of grasses and legumes in rotation with winter wheat and fallow 19 resulted in lower stabilization of larger aggregates than under the more common wheat-fallow rotation. Beneficial crops can provide part of the nutrients required by soil organisms as well as their physical and chemical functions in stabilizing the aggregates. Close growing crops with deep and well developed root systems, such as alfalfa, might increase soil porosity and permeability (Uhland, 1949). Van Bavel and Schaller (1950) found that aggregation was approximately twice as high under corn of corn-oats-meadow rotation as under continuous corn. 5.1.2. Deleterious Effects In planning long term cropping systems we have to realize that some creps if planted continuously may not preserve the structural stability of soils. For example, Browning, Russell and Johnston (1942); Browning (1945); Strickling (1950) and Fahad gt 31. (1982) indicated that corn and soybeans had the same negative effect on building stable soil structures. Also, Armbrust gt 31. (1982) found that aggregate stabilities from soybean plots were lower than those from either grain sorghum or winter wheat plots. It is apparent that corn and soybeans and probably other crops having similar growth habits and residue return lack the characteristics of the beneficial creps in stabilizing soil structure. Therefore, a rotation system which includes both kinds of crops may preserve the good 20 soil physical conditions. Robertson (1955) found that cash crap rotations which did not have nitrogen supplying legumes resulted in poor soil structure while inclusion of green manure crOps in the rotations improved those soils. Also, .Asrar (1978) indicated that on a Charity clay soil the structure improved only in alfalfa-bean rotation. In other rotations, where alfalfa was not included, he found that in order to improve the soil structure, organic residues must be applied to the soil. 5.2. Effect of Length of the Rotations Longer rotations which included legumes were found to maintain or improve soil structure (Newton and Drover, 1956). Also, Toogood and Lynch (1959) showed that a rotation of grains and legumes which lasted for 5 years had almost double the mean-weight diameter of soil aggregates than a wheat-fallow sequence. Bolton, Dirks and Findlay (1979) found, on a Brookston clay soil, that a 4 year rotation with 2 years of alfalfa had more total pore space than rotations with only one year of alfalfa. Studies have been done to measure the length of time required for grass to produce adequate aggregation under certain soils. Low (1955) found that structure restoration of a clay loam soil took place rapidly in 2 to 4 years, with a gradual decreasing improvement in a parabolic manner up to 100 years. In a coarse textured soil Barber (1959) showed 21 that aggregation increased linearly when measured in grass plots for 4 years. When grasses were involved in long term rotations (20 years) the geometric mean diameter of water- stable aggregates, hydraulic conductivity and air permeability increased curvilinearly with the age of grass (Mazurak and Raming, 1962). VI. Effect of Soil Structure on Root Growth and Crop Yield Soil physical properties generally deteriorate if the soil is intensively cultivated (Skidmore, Carstenson and Banbury, 1975). This deterioration will be followed by. reductions in the soil productivity and hence crop production will be affected. Low (1973) concluded from a long term yield experiment (100 years) that changes in the state of soil structure over the years resulted in crop yield differences. 6.1. Effect on Root Growth The amounts of water and nutrients absorbed by the plant root systems are highly influenced by the soil physical conditions. The effects could be through those properties which govern the soil's ability to retain and conduct water and nutrients or through the effects on root growth and functions (Eavis and Rayne, 1969). Greacen, Barley and Farrell (1963) reported a wide range of soil physical conditions that caused cessation of root elongation 22 depending on texture, bulk density, soil water suction and plant species. Even though soil bulk density may not be a good indication of soil permeability (Mason, Lutz and Peterson, 1957), it has been found that this parameter proved to be a good index of soil compaction. The ability of plant roots to penetrate different soil layers depend on the degree of compaction and on plant species. Bertrand and Kohnke (1957) found that corn roots did not penetrate a subsoil compacted to a bulk density of 1.5 Mg m-3. When the bulk density was reduced to 1.2 Mg m-3, the roots grew profusely. Barley . roots were found to penetrate aggregates with bulk densities of 1.4 Mg m"3 but were restricted to the priphery at bulk densities of 1.8 Mg m-3 (Voorhees 95 9A.. 1971). In order for plant roots to function properly they require soil pores larger than their diameters (Russell, 1977). At the same time, the volume and geometric arrangements of voids in the solid matrix of the soil will affect gaseous and liquid diffusion (Baver, Gardner and Gardner, 1972) . But, positive effects of soil aeration on root growth will occur only if the roots are able to penetrate the soil. Tackett and Pearson (1964) found that at low bulk densities the elongation rate of cotton roots decreased as CO2 concentrations increased to 24% even though 0 concentration was 21%. At high bulk densities, C02 2 concentration did not affect root elongation rate. They 23 suggested that soil strength was the limiting factor in the elongation rate. Gooderham (1977) related the decreased elongation rate of pea seedling roots in compacted soils to the lack of some growth regulating chemical compounds produced by the roots. When he added the growth active compound 3,5-diiodo-4-hydroxybenzoic acid to the soil the root elongation rate increased by up to 25%. It is apparent that soils must be friable and well aerated, especially for root crops. Baver and Fransworth (1940) found that sugar beet yield decreased by about 4.5 to 9 Mg ha”1 when the soil air porosity was about 2% (v/v). They concluded that maximum beneficial effects of fertilizers could not be expected unless the soil structure was improved to permit adequate aeration for the growing beet. When the soil porosity increased by planting sugar beets on ridges the yield increased from 3.4 Mg ha.1 to more than 26.9 Mg ha"1 (Baver, 1949). Evidence for the importance of soil aeration for sugar beet root proliferation was provided by Wiersman and Mortland (1953) . They supplied the beets with a source of oxygen by mixing Ca-peroxide with the soil and found increased root length of the beets. 24 6.2. Effect on Crop Yield The ultimate effect of the factors which affect shoot and root growth will finally appear on the crap yield. If we assume a crOp has an optimum leaf area index (LAI), and can utilize the solar radiation efficiently (Shih and Gascho, 1980 and Mengel and Kirkiey, 1982), then what factors may control its yield? As far as the soil is concerned, all the factors which are involved in its productivity will have some influence. Therefore, improvement in the soil's physico~chemical properties associated with crop rotation and proper soil management may also improve yields. Schuurman (1965) concluded that the beneficial effect of improved soil productivity on root growth will enhance the development of the whole plant. In making yield correlations with soil structure, other growth controlling factors should also be considered. DeBoodt, DeLeenheer and Kirkham (1961) suggested that correlations between yield and soil structure often depend on the weather. Also, Low (1973) found correlation between the yields of cereals, peas and red beets with the stability of soil structure. But those correlations were based on the condition that the quantity of soil nutrients or disease was not a limiting factor. Aggregate sizes and their stability as related to different crop rotations have been used for yield correlation studies. In.a corn-oats-meadow rotation, corn 25 yield was highly correlated with aggregation when expressed by mean weight diameter (Van Bavel and Schaller, 1950). Odland, Bell and Smith (1950) found the yields of onions grown in rotation with mangels, buck wheat, corn and red top were directly correlated with the amount of water-stable aggregates. On a silt loam soil, Salomon (1962) concluded that increased potato yields were due to improved soil aggregation rather than to other factors associated with organic matter. Beneficial effects of soil structure modifications could result in improvements of soil fertility and tilth _ (Black, 1973) . Therefore, in studying yield responses to soil and crop managements, crop yields can be related either to the physical or the fertility soil parameters or both. For example, in an experiment under different fertility levels and crop rotations, Bolton, Dirks and Aylesworth (1976) found that differences in corn yield were due to addition of N via a legume crop. On the other hand, Dirks and Bolton (1980) found that corn yield decreased in continuous corn plots. They related that to increased soil compaction which reduced nutrient and water availability as well as restriction of root growth. VII. Improvements of Soil Structure Variations in bulk densities within the soil profile may limit the vertical and horizontal distribution of plant 26 roots. Therefore, knowing these restrictions may provide clues for the need of profile modifications. This knowledge can also lead to develOpment of the most effective placement of fertilizers in the soil (Mengele and Barber, 1974 and Chaudhory and Prihar, 1974). To insure good seed germination, root growth and crop yield the soil should be in suitable and stable physical conditions, provided other growth factors are not limited. Hagin (1952) found that larger aggregates promoted higher yields in a greenhouse experiment. Kuznectsova (1980) indicated that the plow layer would have a stable. make up if aggregates >0.25 mm had a stability of 40% or more. Although these findings stress the importance of soil‘ aggregation, the dependence of plant performance on a single-sized aggregates is doubtful. Instead, a mixture of granules of varying sizes showed the best effects on plant stand (Baver, Gardner and Gardner, 1972). In.fields typically planted for row crops, Hillel (1982) indicated that at least two zones should be considered in a soil structure management. First, a planting zone where the structure should be favorable for seed germination and seedling establishment would be needed. Second, a management zone in the interrow areas where soil structure should be coarse and open for water and air economy of the growing crep. 27 The complexity of the soil system and the different factors involved make it difficult to rely upon a specific procedure to maintain and/or improve soil structure. For example, a wide range of aggregate sizes and spatial arrangements (depending on soil texture and water content) can be obtained in a seedbed as a result of various tillage operations (Allmaras £2 31., 1965). Also, Henderson and Haise (1967) suggested the use of soil ammendments using appropriate tillage and crop sequences or sometimes adding organic manures. Recently, various synthetic chemical~conditioners have been proposed. Some improvements in soil physical. conditions have been established by the use of poly- vinylalcohol (PVA), polyvinylacetate (PVAc), dimethylaminoethylmetocrylate (DAEMA) and polyacrylamide (PAM) (DeBoodt, 1972). Meanwhile, McGuire, Carrow and Troll (1973) added (PVA) and (PAM) to a compacted sandy soil in greenhouse and field experiments and did not find any beneficial effects in the soil physical conditions. Restriction of these conditioners to certain soil types, skills required for their applications and their high costs limited their use. Throughout this review evidence has been presented for the beneficial and deleterious effects of certain cropping systems on soil structure. In order to assess these effects, Gerard, Sexton and Shaw (1982) suggested the 28 use of periodic evaluation of mechanical impedance. We chose the use of both soil and plant indices to express the magnitude and dynamic changes of soil structure and to explain in more details the role of a specific cropping system. MATERIALS AND METHODS I. Description of the Experimental Area The research area in which this study'was conducted is located in the center of Saginaw County in Swan Creek Township (Section 9, T11N, R3E), at the corner of Swan Creek and Thomas Roads. Saginaw County is in the east-central part of Michigan, a few miles south of Saginaw Bay. The county is part of the Saginaw lowland, a smooth - low-lying plain which represents the old beds of glacial lakes preceding the present lake Huron. The surface geological formations were laid down by ice and water during the Wisconsin stage of the glacial period and subsequently were smoothed over by waves of glacial lakes and by shallow' Lacustrine deposits. The predominant soils of the county have fine textures and slow drainage and are adversely affected by too much rain in the spring which may reduce crop production (Moon, 1938). 1.1. General Properties of the Soil The soil type of the experimental area was a Charity clay (Aeric Haplaquept; fine, illitic (calcareous) mesic). Particle size analysis indicated that this soil contained; 29 30 6.4% sand, 39.8% silt and 53.8% clay (Asrar, 1978). The clay fraction is dominated by vermiculite while smectite, chlorite, hydrous micas and quartz were presented in smaller amounts (Zielke, 1983). The Charity series consists of naturally poorly drained soils developed in highly calcareous stratified lacustrine clay and silty clay materials (soil management group lc-c). As far as agricultural crOps are concerned, these soils have certain limitations. These include moderate to high water table where artificial drainage is needed for. good crap production. The soils have low permeability, have a poor workability when wet and poor bearing capacity for farm machinery during wet periods. Also, they are commonly deficient in Mn and/or Zn for susceptible crops (Mahjoory and Whiteside, 1976). The general research area showed the following soil test results; pH, 7.7; Bray P1 phosphorous, 17 mg kg-l; exchangeable K, 226 mg kg-l; exchangeable Ca, 5030 mg kg-l; exchangeable Mg, 827 mg kg.1 and organic -li/ matter, 439 kg . 1See MSU, Department of Crop and Soil Science mimeo of Saginaw Valley Bean-Beet Research Farm and Related Bean- Beet Research, 1983 Research Report. 31 1.2. Climatological Aspects The common features of the climate of Saginaw county are long cold winters, mild pleasant summers, well distributed moderate precipitation and little wind. Generally, the climatic conditions are about the same for .all parts of the countyy as local variations in elevations are negligible (Moon, 1938). The air temperature is somewhat modified because of the proximity to Lake Huron. The difference between the winter and summer mean temperatures is about 80 C. For the research area, maximum and minimum temperatures were‘ measured each month during the period of study (1982-1983) are summarized in Table 1. The average frost-free season is 157 days from May 3 to October 7 which is an ample period for the growth and maturity of many crop species (Meon, 1938). Rainfall is almost evenly distributed throughout the growing season and is normally sufficient for good crop production. However, due to the nature of the soils, short drought periods would result in reduction of crOp yield (Moon, 1938). Monthly precipitation for the years 1982 and 1983 are shown in Table 2. As a result of the generally low wind velocities and high relative humidity, evaporation.is moderately low. Of the possible amount of sunshine, 65 to 70% is the range expected during summers and only about 25% during winters (Moon, 1938). 32 Table 1. Monthly maximum and minumum air temperatures one meter from soil surface at the Saginaw Valley Bean-Beet Research Farm during 1982 and 1983. 1982 1983 Month Maximum Minimum Maximum Minimum °c January 7 -27 7 -17 February 7 -24 15 -17 March 15 -22 19 -19 April 24 -13 25 —6 May 23 -1 26 -3 June 29 4 36 3 July 33 8 34 6 August 30 0 34 8 September 31 0 33 -1 October 29 -6 28 4 November 19 -9 18 -9 December 18 -16 4 ~22 33 Table 2. Monthly precipitation as measured by the US Weather Bureau Rain Gauge at the Saginaw Valley Bean-Beet Research Farm during 1982 and 1983. Precipitation Month 1982 1983 mm January 60 23 February 12 23 March 25 84 April 32 116 May 84 156 June 78 90 July 67 49 August 65 64 September 77 130 OCtober 19 75 November 102 78 December 83 51 34 II. Field Experiment Field plots for this study were part of a larger crapping systems experiment initiated in 1972 at the Saginaw valley Bean-Beet Research Farm. The crepping systems were selected to study their differential effects upon soil structural modification and its relation to sugar beet yield. The choice of sugar beets as the indicator crop was based on its sensitivity to changes in the soil physical conditions. The experiment was arranged as a randomized complete block design with six cropping systems and four replications, giving a total of 24 experimental units. Each unit was 20.1 m long and 5.7 m wide making an area of 1.15 x 10.2 ha. As far as tillage is concerned, the units received the same practices every year since 1972. The methods involved fall plowing with a mold board plow to a depth of 0.2 to 0.25 m, where the soil would be exposed to weather variabilities. In the spring prior to planting, the soil was harrowed once with a spring and a spike tooth harrow combination. The cropping systems used involved combinations of the following crops; corn (Ega_may§ L.), oats (£1223 sativa L.), alfalfa (Medicago sativa L.), navy beans (Phaseolus vulgaris L.) and sugar beets (Beta vulgaris L.). This study involved the following cropping systems; (1) corn-beets (C- B), (2) beans-beets (Be-B), (3) oats-beans-beets (O-Be-B), 35 (4) corn-corn-corn-beets (C-C-C-B), (5) corn-beans-beans— beets (C-Be-Be-B), and (6) oats-alfalfa-beans-beets (O-A-Be- B). The entire experimental area received an application of 130 kg P205 ham1 prior to planting in 1972. Thereafter, fertilizers were added based on the crop grown and the soil test level. During the growing season of 1983 all the experimental units under study were planted with sugar beets. Fertilizers applied in a band below and to the side 1 of the seed composed of 336 kg ha- of 11-53-0 plus 1% B and 3% Mn, no K was added due to its high soil level (532. 1). Nitrogen rates were 56 kg ha.1 for each cropping kg ha- system plus an additional amount of 28 kg ha-1 was added to the beans-beets system. Additional N was broadcast as required in May. Sugar beets (variety US H20) were planted on May 11 at a row spacing of 0.71 m and were thinned to 0.2 m between pflants about five weeks after planting. Preemergence application of 6.7 kg ha.1 trichloro-acetic acid (TCA) and 4.5 kg ha"1 5-amino-4-chloro-Z-phenyl-3(2H)-pyridazmone (pyramin) were used to control weeds. 36 III. Methods of Soil Sampling 3.1. Undisturbed Core Samples Undisturbed soil samples were taken to characterize the field soil physical conditions as they were affected by the different cropping systems. The soil cores were sampled by a double cylinder hammer driven core sampler described by Jamison, Weaver and Reed (1950). The inner aluminum cylinder which contained the sample had an inside diameter of 76 mm and a length of 76 mm. Cores were taken from positions in each plot as to, avoid tractor tracks and other obviously compacted areas. During the fall of 1982 and spring of 1983 samples were taken from all the experimental units (plots). During the fall of 1983 only corn-beets, oats-beans-beets and corn- beans-beans-beets systems were sampled. For each sampling date five subsamples per plot were taken at 0.0 to 0.08 m, 0.08 to 0.15 m, 0.15 to 0.23 m and 0.23 to 0.31 m soil depths. Therefore, with the cropping systems used and their four replications, 480 soil cores were sampled in 1982 and the same number was taken during the spring of 1983. In the fall of 1983 one-half of the cropping systems were sampled giving a total of 240 cores. Excess soil over the cylinder edges were trimmed off with a sharp knife and the core samples were put in paraffin-coated 0.473 x 10-3 m3 sized ice:cream containerS‘which were labelled and sealed. The 37 samples were stored in a cooler at 40 C for laboratory analysis. 3.2. Disturbed Soil Samples For aggregate analyses, disturbed soil samples were taken at two sampling dates. During the fall of 1982 samples from all the experimental units and from uncultivated area (virgin soil) located at the south end of the Research Farm were taken while during the fall of 1983 only the corn-beets, oats-beans-beets and corn-beans-beans- beets systems were used for sampling. A composite sample from 0.0 to 0.1 m and 0.1 to 0.2 m depths was taken from each experimental unit and the uncultivated area. Each composite sample consisted of 24 probes taken at random following a zigzag pattern across the plot, mixed in a plastic pail and passed gently through a 5 mm screen while moist. Screened samples were air-dried and sealed in labelled plastic bags until laboratory measurements could be conducted. IV. Soil Measurements 4.1. Undisturbed Samples Indices used in this study to evaluate field soil structure included saturated hydraulic conductivity (SHC) , air porosity at 600 mm water tension (AP), total porosity 38 (TP) and bulk density (BD). All the indices were measured in the laboratory on the same soil core for each subsample. Soil core preparation included covering the bottom with a Whatman filter paper number 2 and a piece of cheesecloth and secured with a rubber band. A 25 mm deep plastic ring having the same inside diameter as the aluminum cylinder was fastened to the top of the cylinder with masking tape. The prepared cores were placed in a~ large aluminum pan containing tap water for 24 to 48 hours for saturation by capillarity. Saturated cores were weighed carefully to. avoid water losses. Saturated hydraulic conductivity (SHC) was measured by the constant head method (Klute, 1965). A time of 60 minutes was used for the determination. At the termination of SHC experiment, the cores were allowed to drain for about 2 minutes before they were put on a tension table described by Leamer and Shaw (1941) and modified by Vomocil (1965). The samples were allowed to equilibrate with 600 mm water column for 24 to 48 hours. After equilibrium was reached, air porosity (AP) of each sample was determined by the method of Vomocil (1965) using an air-pycnometer system. Bulk densities were measured by the method of Blake (1965). 39 Using an average value of 2.65 kg mm3 for soil particle density, total porosities were determined by the procedure of Vomocil (1965). 4.2. Disturbed Samples For each sampling date, duplicate subsamples were used for aggregate analysis by the wet-sieving method of Kemper and Chepil (1965) with some modifications. Six sieves each with a diameter of 250 mm and a depth of 45 mm and had Openings of 4, 2, 1, 0.5, 0.25 and 0.106 mm were assembled in order of size (coarsest sieve on. top). The set of sieves was locked in a Yoder (1936) type of sieving machine, immersed in water and oscillated for 2 minutes to remove trapped air before soil was added. (The machine had a stroke length of 38 mm and a speed of 30 oscillations per minute). An air-dried subsample (50 g) with predetermined moisture (w/w) was placed on the top sieve when the machine was at its highest position. The soil was allowed to wet by capillarity for 10 minutes and then sieved for 30 minutes. Aggregates retained in each sieve were washed in 500 ml beakers, dried at 105° C for 24 hours for oven dry weight. The oven-dry aggregates were placed in a baffled mixer cup, 10 ml of sodium hexametaphosphate (5 g L-l) Kemper (1965) , 350 ml of water were added and the mixture was mixed with a mechanical stirrer for 5 minutes. (The stirrer is popularly referred to as a milk shake mixer). Sand was removed from 40 the dispersed soil by wet sieving on the Yoder machine, collected from the screens and the dry weight was determined. True aggregate weights were determined by subtracting the amount of sand retained on each sieve from the corresponding aggregate-size weight. Fraction weights of true aggregates were corrected for moisture content and used for calculations of aggregate stability (Kemper, 1965) and mean-weight diameter (MWD) (Van Bavel, 1949 and Youker and McGuiness, 1957). V. Methods of Plant Sampling and Measurements 5.1. Leaf Blades and Taproots In order to relate crop responses to soil structural modifications, sugar beet leaf blades and their taproots were sampled during the growing season of 1983. Leaf area was determined on leaf blades removed from 3 areas (0.71 x 0.86 m) within each plot. All blades were removed and area was determined by a Lambda leaf area meter, Model LI-3050 A. Based on the section area for leaf samplings and an average leaf areas per section per plot, leaf area index (LAI) was calculated (Leopold, 1975). Taproots were sampled on July 25, August 17, and 26, September 13 and October 9. Ten plants per plot were chosen randomly and were dug out by hand. Discarding the petioles, 41 taproots and leaf blades were separated and were put in separate marked plastic bags for further processing. Measurements of taproot-leaf weight ratio (TLWR) were based on both fresh and dry weights of leaf blades and taproots. Leaf blades of the 10 plants sampled for each plot were weighed fresh, dried at 60°C for 48 hours and reweighed. The taproots were cleaned from soil, weighed, sliced longitudinally to thin sections, dried at 60°C for 48 hours and reweighed. An average weight of 10 plants per plot was used to calculate TLWR (Snyder gt 31., 1979). 5.2. Fibrous Root Systems 'To investigate the relationships between sugar beet root distribution and soil structure at different depths, soil-root cores were sampled at the end of the 1983 growing season. Soil-root cores of one plant per plot were sampled on October 10 from the C-B, O-Be-B and C-Be-Be-B crOpping systems by a mechanical soil-root sampler (Strivastava, Smucker and McBurney, 1982). The following technique was followed so that the majority of the roots would be included in the samples. Two opposite beets which had about the same size were pulled from adjacent rows and their central positions were marked *with flags. The soil between the marked positions (between the rows) was removed to a depth of 0.23 m. This allowed 42 the soil-root cores to be sampled between a depth of 0.23 and 0.69 m. Figure 1 shows the procedure that was used to remove the cores from the soil. Picture A shows the core being uu >u«~«A-noun no. c an uauuouuuu haucuouuucnuu ya: on: nouuoa oe-a ozu sue: anon .muo— honau>oz :o :unuv Ado: I a. o On a aouu vougiua cauu:nquuu«v and. ousuuuwua :o usage». unannouo no ousoauucu Aalv swan; anew ouuwuumm< oo~.cv oo~.onn~.° n~.°In.c n.OI~ gnu are mumomo Q. 950 a mo a .n unauuh (Z) NOIlflfiIHISIG HZIS-ELVDEXSOV 69 .33 ca :23: 230! 3393 uni—mono 9.5 no soak-aloe no: :3. was 333:: 0.52:5 .5 v2.2. ~25.— huzununonn nod u: 33.33:. xquauuquucwdu no: on. noun-H «I: 2: :3: can: .32 boa-252 .8 52.0.. :0. I «6 3 To :03 nodal: :cuuaflnu-uu on... cu-uouumu :0 alone»: uni—no.3 no 028533 .m «usur— Allv 0:5 as; duuuouwm< 02 .ov on: D. In em momomo E as. E mo a mm on (X) NOImEIXLSIG 3215-1139339?! 70 cropping systems, aggregate size range and their interaction on aggregate size distribution within the entire plow layer. The simple effect of aggregate size range was significant for the October 1982 sampling (Table 11) and the interaction was significant for the November 1983 sampling (Figure 9). 2.3. Changes Over Time In order to evaluate changes in structure indices with time, a combined analysis was done on three cropping systems. These systems were selected on the basis of percentage corn in the crOpping system and were O-Be-B, C'- Be-Be-B and C-B or 0, 25 and 50% corn, respectively. Bulk density, SHC, TP and AP were evaluated for all three sampling dates and MWD and AS were evaluated for the October 1982 and November 1983 sampling dates. The data were analyzed as a split-plot design with sampling date as the main plot, cropping system as the subplot and depth as the sub-subplot. The probability of a significant F test is shown in Table 12. The 3-way interaction among date, cropping system and depth was not significant for any of the parameters measured. Only the simple effects or the 2-way interactions are presented. 71 Table 11. Aggregate size ranges and their distribution in a Charity clay soil in a crOpping systems study sampled on October 1982 and November 1983. Aggregate Size Distribution Aggregate size Range October 1982 November 1983 mm % 5 - 4 1.8a.“ 1.64“ 4 - 2’ 8.1b 9.0 2 - 1 9.1b 12.4 1 - 0.5 16.bc 21.2 2.5 - 0.25 24.5d 23.9 0.25 - 0.106 17.8c 13.4 (0.106 22.0e 18.6 1'Means followed by the same letter or letters in a column are not significantly different at a 5% probability level according to Duncan's Multiple Range Test. * Cropping system x aggregate size range interaction significant, see Figure 9. 72 mmmumofi Zoom»... 37.x «0% .22 :0 5:33 3303 Iouu». noun—noun on» no coon-ale". no: :8. ans; 3.53:: «.5355 so vac-A 13: 5:32.93 no.0 u. ugh-uuuv 33333.9: no: one .333 also 23 5:: 9:5 .33 non-.252 so boa-5a nounmouwma no Genusnuuuuav can. 2.? =0 Begun-03: nm< N Baum»: mama—on“. no uoouuu Alv can: ouum uuumouum< oo—dv oc—énaué n~.°In.o nof— TN «IQ . o 925: (Z) NOImmSICl SZIS-SJNSIHDDV 73 Table 12. Probability of a significant F test for various soil structural indices combined over the different sampling dates. Source of variation BD SHC TP AP MWD AS Date * NS * * NS NS Cropping System (T) NS NS NS NS * NS Depth (D) * * * * * t T x D NS NS NS NS NS NS Date x D *. * * * NS NS Date x T * NS * NS NS NS Date x T x D NS NS NS NS NS NS 74 2.3.1. Bulk Density The interaction between depth of sampling and date of sampling will be evaluated for the effects of date within a depth. The simple effect of depth within a single date has been presented in sub-subsection 2.1.1. The general pattern of increase in BD with depth was followed for all sampling dates (Table 13). At the same time, the magnitude of increase was different from one sampling date to the other depending on the sampling date. At the end of the first 7 month period (October 1982 to May 1983) ED increased by 6.5, 8.7, 11.8 and 6.2% for the 0 to 0.08, 0.08 to 0.15, 0.15 to 0.23 and 0.23 to 0.31 m depths, respectively. Changes in BD with depth were not consistent for the second 6 month period (May to November 1983). When soil samples were taken 13 months after the first sampling date (October 1982), ED increased by 12.2, 7.8, 6.8 and 5.5% for the consecutive depths, respectively. The interaction between cropping system and sampling date appeared to be4due to a larger change in BD from October 1982 to May 1983 for the C-B system than for the other two systems (Table 14). From October 1982 to May 1983, ED increased by 11.4 8.5 and 5% for the C-B, O-Be-B and C-Be-Be-B cropping systems, respectively. However, there was no change between the May and November 1983 sampling dates. 75 Table 13. Effect of sampling date and depth on bulk density (BD) of a Charity clay soil in a crOpping systems study. Depth of Sampling (m) Sampling Date O-0.08 0.08-0.15 0.15-0.23 0.23-0.31 Mg m-3 October 1982 1.07a* 1.15a 1.18a 1.28a May 1983 1.14b 1.25b 1.32b 1.36b November 1983 1.20c 1.24b 1.26c 1.35b +Means followed by the same letter in a column are not significantly different at a probability level of 5% according to Duncan's Multiple Range Test. 76 Table 14. iEffect of sampling date and cropping system on bulk density (BD) of a Charity clay soil in a cropping systems study. Cropping Systemf Sampling Date CB OBeB CBeBeB Mg m-3 October 1982 1.14a¥ 1.18a 1.19a May 1983 1.27b 1.28b 1.25b November 1983 1.27b 1.25b 1.27b * C a corn, B = sugar beets, O a oats, Be a navy beans ¥Means followed by the same letter in a column are not significantly different at a probability level of 5% according to Duncan's Multiple Range Test. 77 Even though the 3-way interaction was not ngnificant, a plot of those data shows an interesting pattern.(Figure 10). It is apparent that more convergence (of the cropping systems with respect to sampling depth occurred as the sampling dates progressed. This aspect will be discussed in sub-section 2.4. 2.3.2. Saturated Hydraulic Conductivity The swelling and shrinkage characteristics of Charity clay soil in combination*with boundary flow errors resulted in high values of SHC. The sampling date x soil depth interaction on SHC was significant (Table 15), while the simple effect of depth for each sampling date was presented previously (sub-subsection 2.1.2.). The major changes in SHC with sampling date occurred at the 0.08 to 0.15 and 0.15 to 0.23 m depths. Seven months after the first sampling date, SHC decreased by about 42% at 0.08 to 0.15 m soil depth, then increased by 36% six months later. For the same periods, SHC decreased by 55% and increased by 85% at 0.15 to 0.23 m depth. 2.3.3. Total Porosity Since TP was calculated on the basis of bulk density [ (1 - gggg ) x 100 ], the pattern of changes in TP with either depth or cropping system followed that of BD. The - nu nmsxn (a; {3) 78 ‘V Oct. 1982 Hey I983 04 in 4'9 2') M“ -v- c I -.- 0 De I -U- 6 lo I! I 0"" luv. I983 .+.- Y I I 04 .n .39 .3) son. harm a.) Figure 10. Verietioae in ID em; the different cropping eyetoee ee effected by depth for the three eeepl. in; datee. 79 Table 15. jEffect of sampling date and depth on saturated hydraulic conductivity (SHC) of a Charity clay soil in a cropping systems study. Depth of Sampling (m) Sampling Date 0-0.08 0.08-0.15 0.15-0.23 0.23-0.31 kg 3 mm3 October 1982 20.8af 22.2a 17.6a 7.1a May 1983 20.4a 12.8b 8.2b 7.6b November 1983 19.9a 18.4ab 15.2a 2.5a 1’Means followed by the same letter in a column are not significantly different at a probability level of 5% according to Duncan's Multiple Range Test. 80 interaction of sampling date x soil depth on TP was significant (Table 16). Most of the changes in TP occurred at the 0 to 0.08 m soil depth. Total porosity decreased by 4.5 and 4.4% from October 1982 to May 1983 and from May to November 1983, respectively at that depth. At 0.08 to 0.15, 0.15 to 0.23 and 0.23 to 0.31 m soil depths, TP decreased by 6.7, 8.5 and 5.4%, respectively from October 1982 to May 1983 with negligible changes between May and November 1983. From October 1982 to November 1983, TP decreased by 8.7, 6.0, 5.8 and 5.0% at the four consecutive depths. The'interaction of sampling date x cropping system on TP was also significant (Table 17). Total porosity' decreased by 7.6, 7.2 and 4.2% from October 1982 to May 1983 in plots from the C-B, O-Be-B and C-Be-Be-B cropping systems, respectively. Changes in TP between May and November 1983 were negligible for all the crOpping systems. Regardless of the interaction effect, all cropping systems had acceptable ranges of TP for cr0p production at all sampling dates. 2.3.4. Air Porosity The interaction effect of sampling date x soil depth on AP was significant (Table 18), while the simple effect of soil depth for each sampling date was presented in sub- subsection 2.1.4. 81 Table 16. Effect of sampling date and depth on total porosity (TP) of a Charity clay soil in a cropping systems study. Depth of Sampling (m) Sampling Date o-o.oa 0.08-0.15 0.15-0.23 0.23-0.31 % October 1982 59.7a* 56.6a 55.4a 51.6a May 1983 57.0b “52.8b 50.7b 48.8b November 1983 54.5c 53.2b 52.2b 49.0b fMeans followed by the same letter in a column are not significantly different at a probability level of 5% according to Duncan's Multiple Range Test. 82 Table 17. Effect of sampling date and cropping system on total porosity (TP) of a Charity clay soil in a cropping systems study. Cropping SystemT Sampling Date CB OBeB CBeBeB % October 1932 56.7a* 55.6a 55.2a May 1983 52.4b 51.6b 52.9b November 1983 51.9b 52.7b 52.0b T C a corn, B a sugar beets, O = oats, Be a navy beans 4: Means followed by the same letter in a column (for comparison of two sampling date means within a cropping system) are not significantly different at a probability level of 5% according to Duncan's Multiple Range Test. 83 Table 18. IEffect of sampling date and depth on aeration porosity (AP) of a Charity clay soil in a cropping systems study. Depth of Sampling (m) Sampling Date 0-0.08 0.08—0.15 0.15-0.23 0.23-0.31 % October 1982 27.3aT 18.8a 17.4a 9.8a May 1983 . 22.9b 13.6b 7.9b 5.1b November 1933 22.7b 19.7a ' 15.8a 9.2a TMeans followed by the same letter in a column (for comparison of two sampling date means within a depth) are not significantly different at a probability level of 5% according to Duncan's Multiple Range Test. 84 Air porosity decreased by 16.1, 27.7, 54.6 and 48.0% from October 1982 to May 1983 for the 4 consecutive depths. From May to November 1983, AP increased to values close to those from the first sampling date at 0.08 to 0.15, ().15 to 0.23 and 0.23 to 0.31 m depths with negligible change at O to 0.08 m. 2.3.5. Aggregate Stability The interaction effect of sampling date x aggregate size on aggregate size distribution was significant (Table 19). In November 1983, the distribution percentage was 3.5 and 4.6% higher than October 1982 samples for the aggregate size ranges of 2 to 1 and 1 to 0.5 mm, respectively. Meanwhile, in October 1982 the soil samplings contained more aggregates of smaller size (0.25 to 0.106 and < 0.106 mm). This means that part of the fine aggregates were regrouped to form larger aggregates during the thirteen month period. Combined analysis for the two sampling dates indicated that cropping system x aggregate size range interaction on aggregate distribution was also significant (Figure 11). The percentage distribution of aggregates between 5 to 1 mm seemed to be higher in plots from C-B than from O-Be-B or C-Be-Be-B cropping systems. The percentage of aggregate size distribution was reversed for aggregates <0.5 mm in diameter. That is, the percent of aggregates 85 Table 19. Effect of aggregate size range and sampling date on aggregate size distribution of a Charity clay soil in a cropping systems study. Date of Sampling Aggregate size Range October 1982 November 1983 mm % s - 4 1.6a* 1.6a 4 - 2 8.0a 9.0a 2 - l 8.9a 12.4b 1 - 0.5 16.6a 21.2b 2.5 - 0.25 24.6a 23.9a 0.25 - 0.106 18.2a 13.4b < 0.106 22.2a 18.6b 1"Means followed by the same letter in a row are not significantly different at a probability level of 5% according to Duncan's Multiple Range Test. 86 mumumog :3. m on o «n... mo.a .n¢m< sq saga“: one»: ecu->- ucanODU can no conuuuaaoo nouv nook omsam onnuuaax a.=uu==a no woman ~o>on huuaanunoun no.0 u: acououuuv sau:auquucmqn uo: and uouuoa 0am. osu saw: anon .uousv acuaaaan anon uoaau>oz u:- Nmo— uOAOuuo usu nachos touuuu>a .aaouu>0 unwanOHU ooucu uOu own-u and. ouumunnul he vauoouuc an acuu=nuuunuu cud. unencumw< Allv canal onum ouauouum< oo_.ov oo—.enn~.o mN.eIn.o n.oo~ uIN Nle .__ shaman (I) NOILQEIHLSIG HZIS‘ZLVDSHDSV 87 of size range 0.25 to 0.106 mm were higher in plots from O- Be-B or C-Be-Be-B systems than in those from the C-B systems. 2.4. Discussion The simple effect of depth for all indices was significant for each sampling date. On the other hand, the effect of cropping system or cropping system x soil depth interaction varied from one sampling date to the other. Therefore, each sampling date will be discussed separately to understand what factors might have affected soil structure. The interactive effects of sampling date x soil depth or sampling date x cropping system on some of the indices emphasize the dynamic process of soil structure modification. Therefore, in order to understand the changes in soil structure with time, the structural indices combined over all sampling dates, will also be discussed. 2.4.1. October 1982 Sampling Date The simple effect of depth on all measured indices was significant, while the effect of system and/or system x soil depth varied among the indices (Table 20). The data indicated that BD (Table 3), MWD (Table 8) and total AS (Table 10) increased with soil depth, while SHC (Table 5), TP (Table 6) and AP (Table 7) have decreased. 88 Table 20. Probability of a significant F test for various soil structural indices for the October 1982 sampling date. Source BD SHC TP AP MWD AS Depth(D) * * * t * * Cropping System (T) * NS * NS * NS T X D * NS NS NS NS NS 89 Degradation of larger aggregate sizes to smaller aggregates and/or alteration of pore size distribution probably were the reasons for these changes. Previous studies indicated that at a DD of 1.0 to 1.2 Mg m”3 and a TP of 55 to 60%, soil conditions will be most favorable for plant growth and development (Kusnetsova, 1979). Accordingly, results from the plow layer (0 to 0.23 m) in this study suggest that BD and TP are not limiting yields. Below the plow layer depth (0.23 to 0.31 m), values of AP (10% were observed. This value of AP was considered by Baver and Fransworth (1940) as a critical value for sugar beet growth. Reductions in SHC and AP below the plow layer were probably due to the formation of a plow pan by the mold board plowing operation. It has been found that the action of plowing presses soil aggregates together which results in a dense subsoil (Baver, Gardner and Gardner, 1972). The simple effect of cropping system on MWD was significant (Table 9). It seems that a system which had corn as 50% or more of the rotating crops had the largest MWD. Apparently the amount of organic matter returned to the soil from each treatment played a significant role in increasing the MWD of soil aggregates. When the soil percent carbon and C/N ratio from various cropping systems (Zielke, 1983) were plotted against the measured MWD, good correlations were obtained (Figure 12). This indicates that MEAN-WEIGHT DIAMETER (mm) 90 0.70 r o - MWD vs. 2 organic C o - MWD vs. C/N ratio 1 ' C-B 4 4 2 ‘ Be-B C/O 3 - O-Be-B 4 I C-C-C-B 0.65 - 5 - O-ArBe-B 0.60 - / ,é—Mwn - 0.59 (C/N ratio) - 4.38 1 r2 - 0.87 0.55 - O ' MWD - 1.09 (2 organic C) - 1.11 0-5 ' r2 - 0.92 .02 02 0.45 l J, I l ' 1.45 1.50 1.55 1.60 1.65 2 organic C 8.2 8.3 8.4 8.5 8.6 C/N Ratio Figure 12. Aggregate mean-weight diameter (MWD) as affected by percent organic C and C/N ratio of the soil. 91 more organic matter has been returned to the soil from cropping systems with high percent of corn (C-B and C-C-C-B) and consequently has resulted in a larger MWD than the other treatments. The cropping system x soil depth interaction on BD ‘was significant (Table 4). Most of the changes in BD among crOpping systems occurred within the plow layer (0 to 0.23 m). Even though BD increased with depth for all the systems, it is apparent that the percentage of corn in the system affected those changes. The C-C-C-B and C-B systems had lower BD than Be-B system at 0.08 to 0.15 and 0.15 to 0.23 m soil depths, respectively. The larger amounts of organic matter returned from the first two systems might have a significant role in decreasing BD. 2.4.2. May 1983 Sampling Date Only the simple effect of soil depth was significant fem the four indices measured at this sampling date (Table 21). Bulk density increased with increasing depth (Table 3) which resulted in decreasing SHC (Table 5), TP (Table 6) and AP (Table 7). An increase in BD by about 15% at 0.15 to 0.23 m soil depth relative to the first depth (0 to 0.08 m) coincided in a reduction of AP of 64%. Similarly, reductions by 58% and 12% were measured for SHC and TP, respectively. This indicates that more changes have 92 Table 21. Probability of a significant F test for various soil structural indices for the May 1983 sampling date. Source BD SHC TP AP Depth (D) * * * * Cropping System (T) NS NS NS NS T x D NS NS NS NS 93 occurred within the plow layer depth.during this sampling date. 2.4.3. November 1983 Sampling Date Table 22 illustrates statistically the effect of the different sources of variability on the measured indices. Bulk density, SHC, TP, AP, MWD and AS were all significantly affected by soil depth (Tables 3, 5, 6, 7, 8 and 10, respectively). Meanwhile, cropping system effect was only significant for MWD (Table 9). Sugar beets were grown during the 1983 season. Therefore, results from this sampling date could have been affected by the sugar beets root system. Changes in soil structural indices within the plow layer depth (0 to 0.23 m) followed a consistent trend with depth. That is, changes from one depth to another were not drastic. This means that modification of soil structure was uniform to a certain extent with depth. The deep tap roots of sugar beets and their numerous fibrous roots could have some effect. It is probable that the taproots pushed soil aside which resulted in a uniform BD, TP and SHC within the plow layer depth. At the same time, the fibrous roots would enhance the formation of a large number of small pores. Consequently, AP values 'were within an acceptable range for crop production (> 10%) as was suggested by Baver and Fransworth (1940). 94 Table 22. Probability of a significant F test for various soil structural indices for the November 1983 sampling date. Source BD SHC TP AP MWD AS Depth (D) * * * * * * Cropping System (T) NS NS NS NS * NS T x D NS NS NS NS NS NS 95 The significant effect of crOpping system on MWD indicated that systems with a high percentage of corn had Larger MWD than those with a lower percentage of corn or no corn at all in the cropping system. 2.4.4. Effect of Time on Soil Structure The probability of a significant F test for the combined analysis of variance is reproduced as Table 23 (also shown as Table 12). Interactive effects of date x crOpping system and/or date x depth were significant for BD, SHC, TP and AP while only simple effects of cropping system and/or depth were significant for MWD and AS. During the first seven month period (October 1982 to May 1983) ED increased significantly at all depths (Table 13). As a result, TP and AP decreased at all depths (Tables 16 and 18, respectively) while SHC decreased only at 0.08 to 0.15 and 0.15 to 0.23 m soil depths (Table 15). For the second six month period (May to November 1983) AP showed an increase to nearly the same values as the first sampling date (October 1982). The increase in AP occurred only below 0.08 m. During this period the slight decrease in BD and increase in AP could be the result of a combination of two factors. First, is the effect of sugar beet roots as explained in sub-subsection 2.4.3. The second factor might be the warmer temperature during that period. This in turn would enhance microbial activities 96 Table 23. Probability of a significant F test for various structural in ices combined over the different sampling dates . Source of Variation BD SHC TP AP MWD AS Date * NS * * NS NS Cropping System (T) NS NS NS NS * NS Depth(D) * t t t * * T x D NS NS NS NS NS NS Date x D * * * * NS NS Date x T * NS * NS NS NS Date x T x D NS NS NS NS NS NS TThis is Table 12 reproduced for easy reference. 97 which would help in improving soil structure, especially below 0.08 m depth. On the other hand, during the first period, freezing and thawing and wetting and drying would result in smaller size aggregates which would fill the large pores and increase the soil weight per unit volume. As a result TP and AP decreased at all depths while SHC decreased at 0.08 to 0.15, and at 0.15 to 0.23 m soil depths. The swelling and shrinkage characteristics of Charity clay soil could have resulted in differential changes in its structure. This would result in an uneven distribution of pore sizes which in turn would affect the . aeration porosity (Baver, Gardner and Gardner, 1972). .Asva result, fluctuation in AP occurred due to the changes in aggregate size distribution which was indicated between October 1982 and November 1983 (Table 19). The resultant change in aggregate size distribution was an accumulation of finer aggregates below the plow layer (0.23 to 0.31 m) which decreased AP at that depth as well as reducing its change among the sampling dates. Changes in SHC within the plow layer followed the same pattern as that of AP. This was expected since the hydraulic conductivity would be affected somewhat by soil characteristics such as distribution of pore sizes (Hillel, 1982). The results indicated that the MWD was larger in cropping systems which had a higher percentage of corn. 98 Also, the C-B system contained a higher percent of aggregates with diameters ranging from 5 to 0.5 mm (Figure 11). The MWD is an average value and therefore gives a larger weight to large size aggregates. Therefore, in relating cropping system effects on soil structure care should be exercised not to depend solely on MWD alone. During the first seven month period, all cropping systems showed an increase in 80 (Table 14) and a decrease in TP (Table 17). The magnitude of change in 30 from the three systems followed the order; C-B > O-Be-B > C-Be-Be-B while for TP the order was C-B = O-Be-B > C-Be-Be-B. Six months later, no significant differences occurred. Thirteen months after the first sampling date, the magnitude of change in BD and TP was higher in C-B system than either 0- Be-B or C-Be-Be-B systems which were almost equal. It seems that changes in BD and TP followed that of AS. The percent of aggregates (0.05 mm in diameter were higher in October 1982 than in November 1983. As a result, lower BD and higher TP were measured in October 1982. Thirteen months later, small size aggregates were regrouped to larger aggregates in the C-B system. Meanwhile, the percentages of smaller aggregates increased in the O-Be-B and C-Be-Be-B systems. The formation of larger aggregates in the C-B system resulted in more changes in BD and TP. However, the breakdown of larger aggregates in the O-Be-B and C-Be-Be-B systems into smaller aggregates reduced those 99 changes. These results indicate that the C-B system had the ability to modify soil structure (as measured by BD and TP) more than O-Be-B or C-Be-Be-B systems. The interactive effect of sampling date x cropping system was not significant for either SHC or AP (Table 23). However, changes in SHC and AP among the three sampling dates appeared to be greater for the C-B and C-Be-Be-B systems than in the O-Be-B system (Figures 13 and 14). As a conclusion for this section, the general effect of cultivation on soil structure was found to increase the percent of smaller aggregates ((1.0 mm in diameter) as shown . in Figure 15. Even though statistical analysis was not made, differences among aggregate sizes from the cultivated and virgin areas are evident. The extent to which cultivation could affect soil structure would depend upon among other factors the types of crOps grown. Data from the present study indicated that cropping systems which had a high percentage of corn tended to increase the percent of larger aggregates. This in turn resulted in more fluctuation in soil structural indices with time. On the other hand, cropping systems which had a low percentage of corn (C-Be-Be-B) or no corn at all (O-Be-B) increased the percent of smaller aggregate size ranges. Consequently, fewer changes were observed on the measured indices from these systems. 100 .uuou. «mama aux—guns: c.5355 no woman aged 5.33.39:— mod an 2553:. hnucuouuucmam uo: one uouuoa also on“. :3: and: ileum: 95.530 ugh—0:20 of new 5.1533330 033333 1333...: no «use 9:33am mo ouuuaamcu .na «Mamas nonflno non—O .....\ S... w .v .V .../2 x . ., . I. \ ...? 4. .. ....x. . , 1 . ., x In 9.2/4 \,\.k\.\. “I... .... .../I.../ \\ H... ..l . . )1 ,r./ . \\\ . . z 2....“ \\\. .ox. / / ... .,\ . : ... t . .. A ....V... J, 2.7 \M... a. ..i/ \x \1 tr... \ \ a ‘ :/ ...U\ . . I §\ . fr" /' ,-" _/ , , .' .- I“ ' . / _. f‘ / ' ’/;'///.”/ \ V \ x >.\ . :33? .. 8.. I I // /- / .\ ". . ‘.‘ /I .41. - ' \ I f r ' .' .’ .r / \\ .>l\ . '. 'x \ . ‘ .' \. K ‘0 f/ . \\\ . - V. ”‘0' “O 22 z’f/ 9’, . .' / 9 / Y‘ - \ \ iv 2', / / o J J “”222 \‘ \ '3' ° ' Z/ >\ '7 43:. .. 1, ’ ‘ NC . mV/ :4 ;. . .\ \ ///, \\‘ 0’0 93" 0 0?. of 1 ' x /, 6 ‘x 9 D' 2: ‘\ Er. ‘ o “722// .. zw AN. 32 .82 .W\ x/ x u ... 32 .3: g ./ v NOS .uuo .... I LO 1 O -m— (£3! 83’!) umnommoo axmvmm anvumvs .uuuh wanna oamuuuax m.:aoc=a no canon ~u>a~ headwaaaoum no.o u: uaououuuv zuuauuuuacwuu no: one panama also onu sag: upon .nlaunAu unannouo ucuuuuuue ecu new 101 deanGOu noun: El coo an >uqa0uom has so ouav ucuuaauu uo aunoanucn .cu ouswqh o n.10 M. V.“ MQ¢~ .>oz \ nag an: Q «can .uoo r _ V\ \\ \.. .\ .x x .x1 / .feww t..«.qnw.. / / \‘-\- \ V' ‘\ -\ \\\§\\ \ \2\ {\i x Q >5: K ‘1‘ Q . s t ... ,. ///h w/h/x t. ..4. .//1 *qip‘ "\ ‘\.\ b! .X‘ h \ ‘\ [D- -m— (Z) ALISOXOJ XIV E3; Oct. 1982 1132 Virgin Soil, Oct. 1982 A—L'_.l 'LAL' g:‘:j:\_A ‘L4'1.1\_4' I.A .A. :, .2..k,a a;&fif(,t t.« )1 «3(3- 4 A.;‘-V v5.55.) . ..a<‘.--‘.rE/(é(¥ ”.‘1 [.9 ..','."....' {fluff/"1"!fi/f'f’ ’::r.o'/_ !/ .//.: fir/’l’,:”,"/’I/:/;/fif . . . . I. I. . . I 4" 995. ’~ ,2 SJ 1929?. 5?? :29 (0.106 ' ‘- -‘ .- 01—" if p. “"35" I ,1. :/ '0 I .J' l.) ff...- ;:/2 2’4 I ,n . ' " 9” ‘ 'i 4" .a' If .1”) AL 11:5“. ii\\‘\ \5 5 5. L4L WWW-9' 3:949 5.9.945 3 “919.2329. “M 'T ; ’44? 4 / 44 //4 {/2 744/424. A\\\\\\\\\\ \‘x. ‘ \\: 'o arfla‘.’\i 2'». .Qfiiv: 3;. /(\)({ 1((/<’A A if; {53‘ V/Z/ / Z//Z: ‘1. I. L ‘ s. N); I 0' .r 5‘ ' ~11” j/Z/Zm ‘\\* “ \ ‘ fig: ' zihtmLz’ \ Ir 4-2 7]7///// 30 9 U? c4 TV I I l CD U3 C3 U3 (‘1 v— 1— (%) NDIIOEIEISIU EZIS-EIMSEHSSV 1-0.5 0.5-0.25 0.25-0.106 Aggregate Size Range (mm) 2-1 5-4 Comparisons of mean aggregate size distribution from three cropping systems (C-B, O—Be-B and C-Be-Be-B) sampled on two dates with that of an adjacent virgin soil sampled on October, 1982 Figure 15. 103 III. Plant Indices and Crop Yield Sugar beets (US H20) were grown during the 1983 season. Several plant parameters were measured at different dates to assist in studying cropping system effects in modifying soil structure. The plant indices included leaf are index (LAI) and taproot-leaf weight ratio (TLWR) where all the six cropping systems were sampled. After sugar beet harvesting for yield and quality determinations, fibrous root lengths were measured at 6 different soil depths sampled from C-B, O-Be-B and C-Be-Be-B systems selected previously (section VII of the MATERIALS AND METHODS). 3.1. Leaf Area Index The effect of cropping system on LAI was significant (Table 24). The lowest value was measured in the C-B system which was significantly different from those measured in either Be-B, C-Be-Be-B, or O-A-Be-B systems. 3.2 Taproot-Leaf Weight Ratio The cropping system effect on TLWR calculated on dry and wet basis was significant for two sampling dates (Table 25). For July 25th sampling date, C-Be-Be-B system had a lower value of TLWR on wet weight basis than the C-B and C- C-C-B systems. For August 26th sampling, the C-Be-Be-B system also had the lowest value of TLWR on the wet weight 104 Table 24. Effect of cropping system on leaf area index (LAI) for sugar beets (US H20) in a cropping systems study sampled on August 26, 1983. CroppingT System LAI C-B 1.68 a* Be-B 2.31 b O-Be-B 1.82 ab C-C-C-B 1.82 ab C-Be-Be-B 2.32 b O-A-Be-B 2.16 b t C = corn, B = sugar beets, Be = navy beans, 0 = oats and A = alfalfa * Means followed by the same letter or letters are not significantly different at a probability level of 5% according to Duncan's Multiple Range Test. 105 Table 25. Effect of cropping system on taproot-leaf weight ratio (TLWR) of sugar beets (US H20) on wet and dry basis in a cropping systems study sampled on different dates. Sampling Date July 25, 1983 August 26, 1983 Cropping T System Wet Dry Wet Dry C-B 0.878 ab* 2.02a 3.12 a 5.10 ab Be-B 0.760 bc 1.86a 2.76 ab 4.34 b O-Be-B 0.760 bc 1.94a 3.07 a 4.76 ab C-C-C-B 0.935 a 2.23a 3.28 a 5.42 a C-Be-Be-B 0.730 c 1.94a 2.72 b 4.19 b O-A-Be-B 0.828 abc 1.85a 3.08 a 4.77 ab * C a corn, B = sugar beets, Be A = alfalfa. navy beans, 0 . oats, and * Means followed by the same letter or letters in a column are not significantly different at a probability level of 5% according to Duncan's Multiple Range Test. 106 basis. Meanwhile, on dry weight basis, the C-C-C-B system had a higher TLWR than the Be-B and C-Be-Be-B systems with the latter having the lowest value. 3.3. Root Length Density In order to determine root length density (RLD), bulk density (BD) and soil moisture percentage (MP), three soil cores (right, middle and left) between two sugar beet plants adjacently located in two rows were sampled. Each soil core taken was from a depth of 0.23 m to 0.69 m with a thickness of 0.08 m and then fractionated into subsamples 0.08 by 0.08 by 0.08 m. Figure 16 shows the soil profile which was sampled from the inter-row space between the two plants and its 54 soil-root core subsamples. Bulk density, MP and RLD were measured in all the subsamples, however, the results were used in two ways. For the first one, measurements from all the subsamples were included in the statistical analysis. The second way utilized average values of the corresponding subsample numbers for the statistical analysis while the inner subsamples of the middle soil-root core were discarded to minimize the overlapping effect of the fibrous roots (Figure 17). The simple effect of soil depth on soil-root core bulk density, soil moisture content and RLD was significant (Table 26). It is apparent that RLD decreased with depth. Beet position R --- I I I 107 Beet position "-“\ ’ 08 m ’ \ ’ §_JC;/'\ 1 2 3 4 25 4 3 2 l 5 6 7 8 26 8 7 6 5 9 10 11 12 27 12 11 10 9 13 14 15 16 28 16 15 14 13 17 18 19 20 29 20 19 18 17 21 22 23 24 30 24 23 22 21 LEFT MIDDLE RIGHT Figure 16. The inter-row space soil profile sampled with its 54 soil-root core subsamples after fractionation. 1 2 3 4 4 3 2 1 S 6 7 8 A/ 8 7 6 5 9 10 11 12 \l 12 ll 10 9 13 14 15 16 j\ 16 15 14 13 l7 18 19 20 l \ 20 19 18 17 21 22 23 24 24 23 22 21 Figure 17. Diagramatic illustration for taking average measurements for each corresponding pair of soil-root core subsamples 108 Table 26. Effect of sampling depth om.soil-root core bulk density (BD), moisture content (MP) and root length density (RLD) of sugar beets (US H20) in a cropping systems study sampled on October 1983. Sampling Depth 80 MP RLD m Mg m'3 %(VIv) mun)-3 0.23-0.31 1.22 aT 32.4 ab 1.46 a 0.31-0.38 1.40 b 35.3 c 0.95 b 0.38-0.46 1.46 be 34.5 c .0.74 c 0.46-0.53 1.51 cd 33.8 be 0.61 cd 0.53-0.61 1.56 cd 33.4 abc 0.56 cd 0.61-0.69 1.52 cd 31.6 a 0.47 d T Means followed by the same letter or letters in a column are not significantly different at a probability level of 5% according to Duncan's Multiple Range Test. 109 Relative to the first depth, RLD decreased by about 35, 49, 58, 62 and 68% at 0.31 to 0.38, 0.38 to 0.46, 0.46 to 0.53, 0.53 to 0.61 and 0.61 to 0.69 m soil depth, respectively. Also, relative to the first depth, BD increased by 15, 20, 24, 28 and 25% at the five consecutive depths, while moisture content changed slightly with the lowest value at 0.61 to 0.69 m depth. The changes in RLD among soil depths below the taproots were the result of the significant changes in the total root length distribution with depth (Table 27). Although the data in Table 27 included all the fibrous roots across the inter-row space they indicate that the longest roots were measured at the first depth (0.23 to 0.31 m) and that root length decreased with increasing depth. The simple effect of the cropping system on net root length per plant (all the roots across the inter-row space) was significant (Table 28). This was reflected on RLD even after the overlapping of roots was minimized as shown in Table 29 where the cropping system effect on BD was also significant. The highest BD was measured in the C-Be-Be-B system where the RLD was the lowest relative to the other systems. Although the effects of the cropping system and the sampling depth on RLD for the individual subsamples were not analyzed statistically, RLD distribution presented in Figures 18, 19, and 20 illustrate interesting patterns. It 110 Table 27. Root Length (RL) distribution of sugar beets (US H20) grown on a Charity clay soil in a crOpping systems study sampled in October 1983. Soil Depth Root Lengtht' m m 0.23-0.31 58.9 a* 0.31-0.38 37.7 b 0.38-0.46 29.4 c 0.46-0.53 24.7 cd 0.53-0.61 22.4 cd 0.61-0.69 18.6 d T Root length was measured in 3.95 x 10"3 m3 soil below the taproot. * Means followed by the same letter or letters are not significantly different at a probability level of 5% according to Duncan's Multiple Range Test. 111 Table 28. Influence of cropping system on fibrous root length of sugar beets (US H20) in a cropping systems study sampled in October 1983. Croppingf Net Root Length System m C-B 231 a* O-Be-B 181 ab C-Be-Be-B 163 b T C a corn, B = sugar beets, O = oats, and Be = navy beans * Means followed by the same letter or letters are not significantly different at a probability level of 5% according to Duncan's Multiple Range Test. 112 Table 29. 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