Soil dynamics is the branch of knowledge that considers the motion of soil. Soil dynamics, a phase of soil science and mechanics concerned with soils in motion, may be defined as the relation between forces applied to the soil and the resultant soil reaction. This definition does not restrict the source of the force applied to the soil; consequently, the dynamic reactions that result from the natural forces of wind, water, and other sources are also included in the definition. Reactions due to wind and water are of paramount importance in erosion and hydrology and the mechanics of these reactions are being studied. However, reactions caused by mechanical forces applied directly to the soil are considered in this course. The dynamic reactions of soil in tillage and traction affect the design and use of machines that handle soil. Because of the primary interest in the interrelations, the tool (or traction device) and the soil must be considered together.

Soil movement results from man's attempts either

 i. to change prevailing soil conditions to those that are more suitable, or

ii. to use soil for support, and locomotion of vehicles.

The scope of soil dynamics thus includes soil-machine relations in both tillage and traction. It is not restricted to agricultural soils and problems since information on basic soil behavior is universally applicable.

The tremendous amount of earth construction and land forming throughout the world has made machine handling of soil increasingly important in construction, military, and mining operations. The final applications of soil dynamics knowledge may differ, but the principles are independent of application.


The machine, which can be a tillage implement, traction device, planter, fertilizer applicator, excavator etc., is used to change the soil condition by breaking it up or compacting it. In reaction, the soil offers some resistance to the machine resulting in wear and tear, high energy demand and hence high production cost. By analyzing the soil-machine interaction, it is possible to optimize this process. On the other hand, the soil provides the environment which the crop requires for growth. In so doing, the crop also replenishes the soil with organic matter or depletes nutrients in the soil.

Analysis of the soil-crop system ensures optimization of crop yield and at the same time ensuring sustainability in terms of soil and water conservation, and energy conservation. The field of soil dynamics has been developed to identify various subsystems namely, machine, soil and crop, identify various parameters for each subsystem and to explain the interactions among the machine, soil and crop.

The machine is usually characterized by a number of parameters namely,

  i. draught required,

 ii. energy required,

iii. speed of operation,

iv. width of operation,

 v. depth of operation, and

vi. rake angle.

The soil is characterized by,

  i. cohesion,

 ii. structure,

iii. angle of internal friction,

iv. cone index,

 v. dry density, and

vi. soil nutrients.

The soil-metal interface is characterized by,

  i. angle of soil-metal friction,

 ii. adhesion,

iii. wear factors etc.

The crop is characterized by,

  i. germination,

 ii. weed infestation,

iii. leaf area index, and

iv. yield.

Analysis of the entire system and the subsystems provides the tool to predict the performance of the machine, crop and soil. The main aim of soil dynamics analysis is to conserve energy, soil, and water, then ensuring high yields at low costs and sustainability of production.

2.2.1. Tillage

Tillage refers to the mechanical manipulation of the soil in order to provide the conditions necessary for crop growth. In conventional crop production systems, tillage accounts for over 50% of the energy expended from land clearing to harvesting. Therefore, in trying to improve productivity of crop production, more efforts should be devoted to improving the productivity of tillage operations. Since these conditions vary with crop and soil conditions, it is important to study the different aspects of soil-machine-plant system in order to minimize the deleterious effects of the interaction among the three main components of the system (soil, machine and crop).

In a classical modern production system, a machine is used to manipulate the soil in order to provide the conditions required for the crop to grow.

The conditions required for favourable crop growth may include:

  • weed control,

  • providing desired soil structure,

  • incorporation of residues,

  • preparation of land for irrigation,

  • mixing fertilizer and other soil amendments into the soil and

  • destroying insects, pest etc.

Because of the various requirements, soil type and condition, there are various tillage methods covering a wide spectrum from zero tillage to conventional tillage. Whatever the type of tillage adopted, the interactions can be understood by looking at the soil – machine – crop subsystems as a complete system in itself. This is illustrated in Fig. 1.1.

Phases in the development of a soil-tillage tool mechanics

Any tillage operation is basically a dynamic process. Movement of soil particles during a tillage operation is the result of the application of force by a tillage tool. The soil fails due to the action of the applied force, and soil particles move in various directions. The tool geometry, operating speed, and soil physical properties are important factors influencing the soil movement.

2.2.2. Traction

Traction is the force derived from the soil to pull a load. This force is exerted against the soil by a traction device such as a wheel, track, winch sprag, or spade. The dynamic resistance of the soil to provide traction is supplied through an interaction between the traction device and the soil. This interaction is very complex and little headway has been made in solving some of the problems that result from the interaction.

2.2.3. Tillage tools

Tillage tools are mechanical devices that are used to apply forces to the soil to cause some desired effect. The desired effects that can be produced by a tillage tool are:

  i. pulverization of the soil,

 ii. cutting of the soil,

iii. inversion of the soil, and

iv. movement of the soil.

Tillage tools usually produce several effects simultaneously. The ultimate aim of tillage is to manipulate a soil from a known condition into a different desired condition by mechanical means.

Last modified: Wednesday, 12 February 2014, 8:39 AM