Soil Care

Principles of Tillage

W. A. Raney and A, W, Zingg.

The purposes of tillage are three: To change the structure of the soil, to kill weeds, and to manage crop residues. Modification of the soil structure often is required to facilitate the intake, storage, and transmission of water and to provide a good seedbed and environment for the roots of plants.

Eradication of weeds is necessary to eliminate competition for water, nutrients, and sometimes light.

Crop residues must be managed in order to provide suitable conditions for seeding and managing a crop.

In the process of producing a crop, the planting, cultivating, and harvesting operations, though highly necessary, usually work in the direction of destroying the structure of the soil. Practices associated with the preparation of the seedbed therefore are of paramount importance. They give opportunity to create a desirable structure.

A desirable soil structure is one in which large, stable pores extend from the surface to the water table or drains.

Such pores ensure rapid infiltration and drainage of excess or free water and enhance the aeration of the subsoil. The retention and storage of moisture are associated with small pores. The small pores should ramify the soil between large pores and hold a considerable amount of water against drainage and for the use of plant roots.

A combination of large and small pores gives both adequate moisture-holding ability and satisfactory water intake and transmission. The combination exists in soils, with granular structure, where the small pores within the granules provide moisture storage and the large voids between the granules or aggregates facilitate rapid water movement throughout the soil.

Theoretically the best sizes of aggregates range from 1 to 5 millimeters. Soil composed of the smaller size is less well drained and is desirable under dry conditions. The larger sizes are preferred for moist conditions.

The soil texture and the change desired in size of aggregates determine the procedure for preparation of the seedbed. On coarse-textured soils any increase in aggregate size depends mainly on tillage. The size of aggregates is increased by tilling the soil at a moisture content that will just permit the particles to slip over one another. This moisture corresponds to a situation in which only the small pores are full of water. If the moisture content is higher or lower than this ideal, tillage makes smaller aggregates.

Fine-textured soils tend to have clods, which need to be broken into smaller units by weathering and machines. The power requirements for cutting wet clods or shattering dry ones into smaller units by tillage alone tend to be prohibitive. A slow, drizzling rain causes such clods to develop planes of weakness, which allow them to be broken easily with implements. It is therefore wise to reduce the size of clods mechanically only after they have been moistened in such a way that planes of weakness exist throughout their mass.

DEEPENING the root zone is necessary for many soil profiles. A soil horizon or layer of high density and low porosity reduces the rate of downward movement of water, limits the amount that is stored during periods of heavy rainfall, and reduces the intake of irrigation water.

Because roots do not grow in dry soil, the zones that limit moisture storage may reduce the rooting depth of a crop. Unfavorable aeration in zones of poor internal drainage also limits the development of roots and prevents the use of water in the subsoil that plants might otherwise utilize.

Compact soil layers may occur naturally. They also may be caused by tillage, notably in loam or silt loam soils that have been continuously in row crops.

A tillage pan, if it is present, occurs just below the zone disturbed by normal tillage. It is similar in texture and chemical properties to material immediately above and below it.

This kind of layer occurs more and more frequently as mechanization is increased. It is best shattered when the soil is relatively dry, usually in the fall. Water from rainfall or irrigation after the shattering operation will supply moisture for storage in and below the shattered zone for later crops. If dry conditions prevail after shattering, water loss by evaporation will be greater than from untreated areas, and the yield of the subsequent crop may be reduced.

If aggregate size is satisfactory, preparation of the seedbed is no more than the eradication of weeds and the management of residues.

If the field is weedy, it is usually economical to plow under both crop residues and weeds in order to cut subsequent cultivation costs. This should be done far enough ahead of seeding to allow the residues to decompose partially so as to minimize the temporary reduction of plant nutrients, particularly nitrogen.

In humid sections where erosion is not a problem, crop residues (such as oats straw) that are primarily cellulose often are removed before plowing if another crop is to follow immediately. Such residues supply practically no plant nutrients when they rot, and the products of decomposition may contain little structure-stabilizing materials. On erosive sloping lands such materials should be left on the surface to maintain the infiltration rate and help the soil surface resist erosion.

THE MOISTURE CONTENT of soils and compaction from the use of heavy tillage implements are related. Moisture acts somewhat like a lubricant and allows soil particles to slide over one another. The result is a decline in the ability of soil aggregates to withstand implement traffic as the moisture increases. When the moisture in the surface soil is best for tillage, the subsoil moisture may be higher and best for compaction. Tractor tires then may compact the soil below normal tillage depths. No more tillage than necessary should be done when the subsoil is moist enough to compact easily.

Soils can bear most weight and are least susceptible to compaction at the end of the cropping season when the moisture in the soil is depleted. That is the best time to use the implements necessary for turning under the residue or for undercutting it with sweeps if mulches are to be left on the soil.

Deep-rooted crops improve the soil structure in places where a packed subsoil does not restrict growth of roots. Compact subsoils can be shattered by tillage, but often they remain shattered only temporarily, because compact layers are basically unstable. Often tillage and the wise use of deep-rooted crops together are needed to improve the structure.

The use of cropping systems that involve the least amount of tillage may have merit, because intensive tillage destroys structure a great deal. Cropping systems that offer maximum protection to the tilled surface in the form of mulches and protective cover tend to keep raindrops from breaking down the surface soil structure.

WEED CONTROL is only partly effective unless consideration is given to all the circumstances of their propagation, growth, and destruction. Because weeds have about the same requirements as crop plants for germination, emergence, and growth, their control is influenced by tillage practices Associated with preparing the seedbed.

Some control of weeds is obtained by tillage that leaves the middles between the crop rows loose and cloddy. When a good seedbed is provided only in the row or close to the place crop seeds are planted, the seeded crop can become established ahead of the weeds. Plowing with a moldboard plow buries the weed seeds, retards their sprouting, and tends to reduce the operations needed to control them.

If weed infestations are particularly bad, they may be reduced somewhat by undercutting them with subsurface sweeps a method particularly effective in fallowing. Some weeds, like pig-weed, which produce thousands of seeds that stay viable for many years, are hard to get rid of. If soil layers restrict the downward movement of water, spring rains may drain away slowly and delay cultivation and intensify the weed problem.

Chemicals for weed control, which have given some measure of success but which need further development before they are adapted to many variable situations, may reduce the number of tillage operations for a crop.

Because the amount and distribution of rainfall seldom coincide with the needs of a crop, effort must be made to store as much soil moisture as possible during periods when crops are not grown. From the standpoint of soil moisture, any tillage practice that does not control weeds and at the same time result in increased moisture intake and retention during the storage period is unnecessary or undesirable.

Excessive tillage breaks down the structure of the soil and leaves it susceptible to crusting. The small pore sizes associated with surface crusts impede water intake, increase runoff, and reduce the amount of moisture stored for crop use. Improper use of machinery often results in reduced water intake. For example, the disk harrow, improperly used, may reduce the aggregate sizes of surface soil excessively, increase susceptibility to crusting, and compact the subsoil immediately below the depth of surface working.

Intensive cropping systems, especially vegetable production in warm climates, tend to cause poor soil structure. In extreme circumstances, where three different crops may be grown on the same land each year, tillage practices may accomplish no more than to loosen the seedbed. Such cropping systems in time break down the soil into an almost single-grain structure and may bring problems of surface cementation, crusting, and poor aeration.

Regardless of the implement used for tillage, the soil mass generally is compressed, lifted, or moved in a manner that will change the structure of soil.

PLOWING, the standard method of breaking land, consists of cutting loose, granulating, and inverting a slice of earth and turning under organic litter. The furrow slice is cut loose by the edge and shin of the plowshare. The crumbling or granulating action takes place at the plow surface and at right angles to the moldboard throughout its length. The loosening and granulating actions of plowing are structure-improving practices, if the plowing is done when the soil has the correct moisture content. Lifting and inverting the furrow slice and turning under organic litter take place throughout the length of the moldboard. Inversion of the furrow slice and burial of residues are not always desirable. In many instances it is advisable to leave a trashy surface.