soil dynamics and cropping systems

Soil & Tillage Research, 16 (1990) 143-152 143 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Soil Dynamics and Cropping Systems*

R.L. SCHAFER 1 and C.E. JOHNSON 2

~National Soil Dynamics Laboratory, ARS, USDA, Auburn, AL (U.S.A.) 2Department of Agricultural Engineering and Alabama Agricultural Experiment Station, Auburn University, Auburn, AL (U.S.A.)

(Accepted for publication 10 March 1989)

ABSTRACT

Schafer, R.L. and Johnson, C.E., 1990. Soil dynamics and cropping systems. Soil Tillage Res., 16: 143-152.

Any mechanical manipulation that changes the soil condition may be considered as tillage. Most often machines are used to apply forces to the soil to effect this change. Soil dynamics includes a description of the behavioral response of soil to applied forces and a description of soil-machine behavior. The state of development of soil dynamics (quantitative descriptions of soil behavior, soil-machine behavior, and resultant soil condition) is explored. Research needs and directions in soil dynamics related to tillage and to prediction of the resultant soil condition are discussed. The wisdom and vision of Professor Henk Kuipers are embodied in this discussion.

INTRODUCTION

Our inv i t a t ion to make a con t r i bu t i on to th i s special issue of Soil & Tillage Research in h o n o r of P ro fe s so r H e n k Ku ipe r s s t a t ed t h a t we were to wri te a " n o r m a l " paper , one no t specif ical ly ded ica ted to P ro fes so r Kuipers . We con- s ider this an o p p o r t u n i t y to h o n o r a m a n who has had a posi t ive inf luence on our profess iona l careers .

M a n y people make a las t ing con t r i bu t ion to the a d v a n c e m e n t of t echno logy t h rough science and engineer ing. In addi t ion , some people give us a vision of the fu tu re which we use as a basis for the deve lo p m en t of knowledge and technology. P ro fes so r H e n k Kuipe r s is well known for his t echnica l con t r ibu t ions to i n t e rna t iona l agr icul ture . However , pe rhaps his mos t i m p o r t a n t con t r ibu- t ion is the legacy of wisdom he has sha red wi th all of us an d which we are us ing as a basis for our research to develop new knowledge and technology.

T h i s pa pe r is no t abou t P ro fe s so r Kuipers . However , we w an t to share some of ou r t hough t s abou t his in f luence on deve lop m en t s in soil dynamics and crop-

*Alabama Agricultural Experiment Station No. 2-881908P.

144 R.L. SCHAFER AND C.E. JOHNSON

ping systems. We have known Professor Kuipers since we completed our un- dergraduate training and started post graduate training, and as we have pro- gressed through our professional careers. At first we knew Professor Kuipers as a person who wrote technical papers which represented cutting edge research. Then, as we were able to at tend international meetings and participate in discussions with him, we realized that he was a man with a vision of the future. He documented his vision by publishing "philosophical" papers in addition to technical papers.

Most of our professional careers have been spent in research and education in the discipline of soil dynamics. After 30 years of work, we realized that Pro- fessor Kuipers is one of those special people who is willing to share his wisdom of the past as well as his vision of the future. We also realized that our interaction with him had helped us to see the light at the end of the tunnel as we pursued our research. In research the tunnel seldom ends. But, if there is light at the other end, there is always the inspiration to continue to make your way along the path. We are grateful to Professor Kuipers for keeping that light shining and for keeping us inspired.

A REFERENCE OF WISDOM

In the first issue of the first volume of Soil & Tillage Research, Professor Kuipers wrote an article entitled "Reflections on the 8th Conference of ISTRO" (Kuipers, 1980). In reflecting on the conference and developing that article, Professor Kuipers expressed considerable wisdom about what should happen in the future. We will use quotes from that article to develop our perspective of the subject of this paper, soil dynamics and cropping systems. All references to Kuipers will be from this article unless otherwise noted.

Our intent in extracting quotes is to help us emphasize what we think is important and to a t tempt to develop a vision of the future for the discipline of soil dynamics. When sentences or phrases are extracted from a document, they may lose some of their meaning; this is not our intent. The main topic of the conference was direct drilling, and Professor Kuipers' remarks were addressing that subject. However, we believe his remarks are also relevant to other aspects of cropping systems.

T H E A R T O F T I L L A G E

In reviewing the papers presented at the 8th conference of ISTRO, Kuipers stated, "These contributions showed a wide variety in subject matter, clearly reflecting the fact that tillage problems are very much dependent on specific circumstances, just as are the research approaches for solving them." So, it seems that considerable effort has been exerted to solve specific problems with specific approaches, in many cases very site specific. In that regard one can

SOIL DYNAMICS AND CROPPING SYSTEMS 145

ask several questions. (1) What is the effect of a specific tillage tool (or vehi- cle) on a specific soil? (2) What are the best {if any) currently available predictive equations (models) for these effects? (3) What information is required to make the predictions?

Answering these questions would help in the solution of specific problems. Solving specific problems for specific circumstances is important. More important, building a knowledge base as a foundation for solving a broader spec- trum of problems is essential. In this regard, Kuipers observed, "In any case, tillage as a habit is a most interesting and obviously lasting phenomenon, but we are aiming at controlled and predictable effects and we should be aware that any solution to practical problems is only a best fit for the given circumstances. Therefore, technical knowledge on tillage will be needed more and more."

If one is to attain controlled and predictable effects, then the science of tillage (technical knowledge) must be more fully developed. Knowledge of the mechanics of the tillage process (mechanical soil manipulation) is an important aspect of attaining controlled and predictable effects in tillage. The mechanics of the tillage process include mechanical soil behavior and soil-machine behavior. In this regard, at least 3 questions must be addressed. (4) What is the soil physical response and/or change in soil condition caused by force systems? (5) What predictive equations (models) are available for describing this soil physical response and/or change in soil condition? (6) What information is required to make the predictions? Other questions may be equally . ,mportant; we will not try to document all important questions here. Rather, we will use these questions to develop further a rationale for the scientific investigation of mechanical soil manipulation.

As a basis for developing a viewpoint of soil manipulation, one can conceptually view tillage tools as applying forces to soil which cause soil motion that changes the soil condition for enhanced agricultural production, e.g. by creat- ing a soil physical condition for increasing emergence, improving plant rooting, increasing infiltration and controlling erosion. The active and passive behavioral response of soil to forces applied by machines influences the results of the tillage process. The investigation of soil behavior and soil-machine behavior is embodied in the discipline of soil dynamics. One may raise several questions. (7) What is soil dynamics? (8) How does soil dynamics relate to tillage? (9) Is an understanding of soil dynamics pertinent to answering Questions (4)- (6)?

Soil dynamics may be defined as the relation among forces, soil deformation and soil in motion. This definition does not restrict the type of force system or the purpose for applying the force system. However, in this paper we will restrict our discussion to the application of mechanical forces by machines to change soil condition for agricultural production purposes.

As a framework for discussion and a prospectus for the importance of relat-


ing soil dynamics to tillage and crop production, we will use an analogy. Con- sider the body of knowledge that defines aerodynamics (a segment of fluid mechanics and thermodynamics) which has greatly influenced developments in aircraft design, space travel and automotive design.

There is a contrast in the complexity of the medium, air, in aerodynamics compared with the medium, soil, in soil dynamics. An airfoil moves through air and a tillage tool moves through soil, but air is a much more continuous medium than soil. Furthermore, air can be considered a homogeneous and iso- tropic mixture of particles, whose sizes are very much smaller than an airfoil moving through them, in contrast to soil which may contain aggregates, clods, and foreign material whose sizes are nearer to the size of the tillage tool. In contrast to air, soil is non-homogeneous and often exhibits anisotropic behavior. In addition, tillage creates discontinuities (shear planes) within the soil. Thus, a description of soil behavior must necessarily be much more complex than a description of air behavior.

Air travel has advanced from an art in the Wright Brothers' era to a science in our present space travel. Aerodynamics has been a key element in that prog- ress during a time span of less than 100 years.

Soil dynamics could have a similar impact on tillage; unfortunately, the state of knowledge in soil dynamics is not as advanced as in aerodynamics, nor has the rate of knowledge increase been the same. However, much less scientific man-power has been devoted to the development of soil dynamics than to the development of aerodynamics; perhaps because of the complexity of soil behavior. Interestingly, a leading engineer who has made major contributions to soil dynamics and tillage (Walter SShne) was trained in aerodynamics.

RELATION OF SOIL DYNAMICS TO CROPPING SYSTEMS

In agriculture, we apply active force systems (tillage) for the purposes of preparing seedbeds and rootbeds, incorporating amendments, controlling weeds and pests, enhancing infiltration and controlling erosion. The state of the soil is changed from its initial condition to some final condition, as the result of the applied forces and of the resulting soil movement. Cooper and Gill (1966) illustrated that idea with the conceptual relation

Sf= f(Si, F) (1)

where: Sf = final soil condition; Si = initial soil condition; F = mechanical forces applied to the soil.

They expressed another simplified conceptual relation that may be of more interest from a crop production standpoint:

CP=g(S, E, P, M) (2)


where: CP=crop production; S---soil composition and condition; E -- environment; P = plant species; M - management practices.

The conceptual relations in eqns. (1) and (2) were stated in a simplified manner without mathematical rigor. This is analogous to a scientist selecting pertinent factors and then developing a factorial statistical design to explore main effects and interactions in data (Steel and Torrie, 1960). Development of those two conceptual relations into mathematical equations for predictive purposes is the real crux of our challenge. The part of this challenge that involves mechanical soil behavior and soil-machine behavior is embodied in the discipline of soil dynamics.

Gill and Vanden Berg (1967) and Vanden Berg and Reaves (1966) expressed two additional generalized relations which reflect aspects of the tillage machine system

F=h(Ts , Tin, Si) (3)

and

Sf=k(Ts, Tin, Si) (4 )

where: Ts = tool shape; Tm = manner of tool movement. They referred to these abstract relations as the force-tillage equation (eqn.

(3) ) and the soil-condition equation (eqn. (4)). Much of the past research on soil-machine relations and soil dynamics has been related to the concepts of eqn. (3), the force-tillage equation.

A change from Si to Sf is caused by soil movement. This change involves strain and yield of the soil. Therefore, force-movement relations of soil are important when soil condition is changed. Some research has been related to the concepts of eqn. (1), the basic processes of soil deformation, e.g. stress- strain behavior. Such research has been concerned with soil stress, stress distribution, strain, strain distribution, soil strength, soil yield (shear, compres- sion, tension and plastic flow), and rigid body movement (momentum, fric- tion, adhesion and abrasion).

Several different quantities or soil properties may be required for adequate quantification of each of the abstract entities, S, Si and Sf. Si in eqn. (3) must be quantified in terms of the resistance of the soil to deformation and movement, whereas S, Si and Sf in eqns. (2) and (4) must be quantified in terms of the soil's strength and of its resistance to water, air and heat flow.

Most objectives of past research have not been aimed at empirical or theo- retical definition of the concepts of eqns. (1)-(4) . Rather, they have been aimed at relating the differential change observed in a "dependent factor" as influenced by a change in one or more "independent factors." That is, differential changes in Sf, CP and F have been studied with respect to changes in individual "independent factors" or in combinations of "independent factors." Conceptually, the differential change in a "dependent factor" with respect to


an "independent factor" may be: (1) a constant; (2) a function of that independent factor; (3) a function of one or more of the other independent factors; (4) a function of that independent factor and one or more of the other independent factors; (5) a function of that dependent factor; (6) a function of that dependent factor and one or more of the other independent factors. Cases (3) and (4) are interactions, as defined in statistical analyses (Steel and Torrie, 1960). Interactions increase the difficulty of developing empirical relations.

With respect to eqn. (1), work by Dunlap and Weber (1971), Kumar and Weber (1974), and Grisso et al. (1987) suggested complicated interactions. They found that the final soil condition has some dependence on the stress path ( stress history) of the applied load. Their results indicated that eqns. ( 1 ) and (3) may be more complex than behavioral relations in other technologies, e.g. aerodynamics. However, their results suggested that the energy efficiency of one force system applied to create a final soil condition may differ from that of another force system applied to create the same final soil condition. So, the energy efficiency of the tillage process depends on how the tillage machinery applies force to the soil (manner of movement) .

What is the relation of soil dynamics to cropping systems? The relation of soil dynamics to cropping systems involves defining eqns. (1), (3) and (4) in rigorous mathematical terms, rather than developing conceptual expressions, to provide some fundamental foundations for eqn. (2).

SOIL DYNAMICS DEVELOPMENT

In reviewing the papers presented at the 8th conference of ISTRO, Kuipers considered the three aspects of tillage as "agronomic, technical and mechanical aspects of tillage" and stated: "In particular the field of mechanical soil properties may be regarded as a weak part of our knowledge" and "A well balanced, coherent body of knowledge on all three aspects of soil tillage and their interactions is essential for the development of a real science of soil tillage (Frese, 1960). Therefore, it seems appropriate to emphasize in the near future research on mechanical properties and on technical aspects of soil tillage."

The mechanical properties that Kuipers refers to are the behavioral properties that describe the soil reaction to a force system. When forces are applied to the soil by a tillage tool, the soil moves and its condition changes. Behavioral properties must be used to describe its action. Identification and measurement of these properties are important aspects in the development of soil dynamics.

What are behavioral properties? As a basis for definition, first consider state properties. State properties describe a material without regard to intended use. As an example, a wire may be characterized by its chemical composition and density; these are state properties. On the other hand, behavioral properties describe the reaction of a material to an applied force system. For example, if a voltage is applied across a wire, the amount of current flowing through the


wire depends on the resistance of the wire (Ohm's Law). Resistance is a behavioral property. Also, if the wire is stretched by force applied to its ends, the amount of deflection depends on the modulus of elasticity (Hooke's Law). The modulus of elasticity is a behavioral property. Both of these completely different behaviors, current flow and deflection, are important in the description of the wire based on its intended use, but they are described separately. Further, although it may be possible to relate behavior of the wire to the state properties, such as chemical composition and density, the state properties may not be rationally descriptive of the wire's behavior.

Unfortunately, in the past, state properties, particularly moisture content and bulk density, have often been used to correlate soil physical behavior and the results of tillage with parameters of the force system. However, unless the relations between behavioral and these or other state properties are unique and are known, the use of state properties to describe the dynamic tillage action is not a rational approach. State properties have probably been used because they are more obvious and more easily quantifiable than behavioral properties, or because the important behavioral properties have not been identified yet. Behavioral properties are often very difficult to quantify, but we must under- take and complete that task if we are to develop a soil dynamics.

T H E SOIL DYNAMICS C H A L L E N G E

The force system imposed on the soil by a tillage tool is more complex than the force system imposed on a soil sample in a soil strength test, such as a triaxial test. This complexity has been a major deterrent to the development of an adequate description of soil behavior. A force boundary condition, in particular a pressure boundary condition, is assumed to exist for the soil in a typical triaxial test, whereas a geometric boundary condition exists for the soil during tillage.

The force system, F, for a force boundary condition (triaxial test) is independent of soil behavior, but the geometry of the deformed sample is dependent on soil behavior. Thus, the soil's reaction to the force system can be studied and modeled.

On the other hand, the force system, F, that is associated with some geometric boundary condition (such as tillage tool, cone penetrometer, or shear vane), depends on soil behavior and cannot be defined without consideration of at least one or more soil behavioral properties. This fact is considered conceptually in eqns. (3) and (4) (Gill and Vanden Berg, 1967) since tool shape and manner of movement are independent factors. Thus, a tillage tool operated in the same manner in a soil with two initial soil conditions, Sil and Si2 (e.g. two strength conditions) will require two different force systems, F1 and F2, for movement of the tool through the soil. Tool shape and manner of movement are "imposed" on the soil.


The force system that is required to "impose" the tillage tool on the soil is of interest for several reasons. An engineer is faced with designing tillage tools for use in a wide range of initial soil conditions. Thus, the reliability and du- rability of the machine's framework and soil working parts are major concerns. Other design concerns include energy requirements, maintenance and adjustment.

The principles of engineering mechanics and the criteria for structural design are well developed. Structural design of a machine (framework and parts) requires knowledge of the force system imposed on the machine. Thus, it was natural for research engineers to pursue the development of predictive equations for the force systems generated by tillage tools to aid in structural design.

Engineers must also be concerned with the functional performance of the tillage tool. But the functional performance of a tillage tool, change in soil condition, is not well defined in quantifiable terms. Thus, as emphasized by Spoor (1975), no clearly defined goal has been established for developing equations that will predict soil condition. Consequently, most past research by engineers has been directed toward developing technology for predicting the force system created by a tillage tool (eqn. (3) ), and less research has been directed towards relating soil condition and tillage (eqn. (4)). Qualitative and quantitative descriptions of a tillage tool's functional performance are difficult because the final soil condition, Sf, should be adequately defined and quantified based on intended use.

STATE-OF-THE-ART

Some of the soil and soil-machine behaviors that are manifest in tillage have been identified. Quantitative descriptions that can be used in predictive equations have been developed for some behaviors, but not for others. Attempts to integrate the complex interaction of these behaviors into overall quantitative descriptions of the tillage process have not been very successful. Quantitative descriptions of soil condition in terms of behavioral properties are not well understood and developed.

In concluding his article, Kuipers observed: "However, it is fascinating to work in this ancient field which science has barely discovered." Although considerable effort has been expended towards the development of soil dynamics, particularly in the last 30 years, the task is far from completed. The wisdom of the past has demonstrated that the task is very difficult, and the number of people developing this knowledge base is small compared with those working on other aspects of tillage. In this regard, Kuipers said: "The number of these people will always be too small compared to the quantity of knowledge that is needed. Therefore, it will be the more necessary to improve and safeguard the quality of the knowledge."

Therefore, each of us involved in soil dynamics research must do better and


smarter work. Carefully planned experiments must be conducted in which all known variables are controlled and measured. Results must be carefully scru- tinized for evidence of influence by unknown variables, i.e. discrepancies from expected results that are not random. Research which is successful and meaningful will most likely be a series of conceptually related experiments, in contrast to experiments that include the "universe" and which will probably be too complex for meaningful interpretation of results. The most modern tools of research must be used.

C O N C L U S I O N

The efficiency and effectiveness of present-day tillage systems cannot be disputed. These systems have been developed based on astute observations and past research. The history of technological development has indicated that science and engineering often follow in the footsteps of invention. Someone determines how to make something work (invention), and then others deter- mine why it worked (science and engineering) and how to make it work better (development).

Current tillage systems have been developed based more on qualitative descriptions of soil and machine behavior than on quantitative descriptions. Per- haps further improvements (inventions), dictated by necessity, may be made without a soil dynamics that quantitatively describes tillage. However, science, engineering and development may lead to greater improvements.

New tillage concepts should and will be explored, because of necessity, before a soil dynamics is fully developed. Certainly, we can and must use the best knowledge available to enhance the effectiveness and efficiency of tillage systems, and we must continue to do so. However, one has to wonder what could be done and will be done in the design and use of tillage systems with a well- developed body of knowledge in soil dynamics. Certainly, we have excellent examples of the impact of the engineering sciences on the design of other physical systems. Why should soil-machine systems be any different? On the other hand, no one can expect any more from soil dynamics than the knowledge level that has been developed. We must remember that other engineering sciences are well developed and the challenge is to develop new applications or approaches. The science of soil dynamics is still being developed.

As we consider the current and projected world needs for food and fiber and the means for meeting those needs, we can observe, as Kuipers did, that: "Til- lage and traffic are constantly influencing the physical condition of agricultural soils." These influences must be determined and quantified. The current approach that is used to establish this influence is embodied in the discipline of soil dynamics. The development of a soil dynamics as related to crop production must be thoroughly studied until the advantages of a different approach become apparent. A well-developed body of knowledge in soil dynamics


is needed as t he bas i s for des ign ing a n d o p e r a t i n g t i l lage s y s t e m s t h a t will p roduce soil cond i t ions t h a t have b e e n p r e s c r i b e d for c rop p roduc t ion .

REFERENCES

Cooper, A.W. and Gill, W.R., 1966. Characterization of soil related to compaction. Grundfdrbiittr- ing, 19: 77-88.

Dunlap, W.H. and Weber, J.A., 1971. Compaction of an unsaturated soil under a general state of stress. Trans. ASAE, 14: 601-607,611.

Frese, H., 1960. Haben wir ein Konzept ~ r eine Wissenschaft vonder Bodenbearbeitung? Trans. 7th Int. Congr. Soil Sci., Madison, WI, Vol. 1: pp. 54-66.

Gill, W.R. and Vanden Berg, G.E., 1967. Soil Dynamics in Tillage and Traction. USDA Agricul- ture Handbook no. 316. U.S. Government Printing Office, Washington, DC, 511 pp.

Grisso, R.D., Johnson, C.E. and Bailey, A.C., 1987. Soil compaction by continuous deviatoric stress. Trans. ASAE, 30: 1293-1301.

Kuipers, H., 1980. Reflections on the 8th conference of ISTRO. Soil Tillage Res., 1: 7-10. Kumar, L. and Weber, J.A., 1974. Compaction of unsaturated soil by different stress paths. Trans.

ASAE, 17: 1064-1069, 1072. Spoor, G., 1975. Fundamental aspects of cultivations. In: Soil Physical Conditions and Crop Pro-

duction. Ministry of Agriculture, Fisheries and Food, London, Technology Bulletin 29, pp. 128-144.

Steel, R.G.D. and Torrie, J.H., 1960. Principles and Procedures of Statistics. McGraw-Hill., New York, NY, 481 pp.

Vanden Berg, G.E. and Reaves, C.A., 1966. Characterization of soil properties for tillage tool performance. Grundfdrbtittring, 19: 49-58.

soil dynamics and cropping systems

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