Several types of tillage have been classified as minimum tillage.

Seeding small grains in sods that have had little or no prior disturbance is one type. It consists of cutting narrow slits in the sod and placing seed and fertilizer in the breaks formed in the sod. Soil normally subject to erosion thus can be planted to small grains while maintaining the beneficial effects of the sod. Seeding in sod has been used successfully in preparing winter grazing for livestock in the Southeast.

Breaking the land and planting without any other preparation of the seedbed is another type of minimum tillage. One way is to break land and plant seeds in the tractor tracks the weight of the tractor crushes the clods and permits plantings in a firm seedbed.

Another method consists of mounting a planter behind the plow and planting without further traffic. That leaves a rather loose seedbed and is satisfactory in places in the Northeast, where rainfall after planting may be heavy.

In the Pacific Northwest, where winter rain often comes soon after wheat is drilled, a rotation of wheat-peas has worked well. The field is rough-plowed after canning peas are harvested. Fall wheat then is drilled directly in the rough-plowed land.

All these methods require a minimum of expense and land preparation before a crop is seeded. They tend to leave the soil in a rough condition so that water intake is increased and runoff and erosion are reduced.

Mulch tillage was developed primarily to protect the soil surface from erosion. Crop residues are left on the soil surface and subsurface tillage is employed so as not to disturb the mulch. In dryland areas of the Great Plains as much mulch material as possible is left on the surface. It is usually better in more humid sections to bury some of the residue. Planters with disk openers, which go through several inches of mulch without clogging, are used for seeding.

If one is to make the most of the benefits of sod or mulch protection in subhumid and humid areas, one should apply extra plant nutrients, particularly nitrogen; otherwise, the microbes that decompose the residues may deprive crops of nutrients. The mulch lasts longer if fertilizer is placed below the mulch rather than on the surface with the mulch.

Intercrops are used to extend the protection that surface mulches give against erosion. They are usually small grains or the sod crops that are grown between the rows of the field crop. Crimson clover is excellent in the Southeast for such a crop between rows of a field crop. It reaches maturity shortly after the field crop has been established and furnishes a mulch cover for an extended period. Alfalfa has been used successfully in the Midwest for this purpose. Because of the competition for water, the use of an intercrop is limited to rainy sections.

Planting furrows usually are prepared 2 to 4 weeks before seeding time when intercrops are used. The furrows usually are made by using a small subsurface sweep or other implement to undercut a section of the intercrop. Disk killers may be used to till the fur- row further before planting with conventional equipment. If growth of the intercrop tends to compete with the main crop for moisture and nutrients, that growth may be killed at seeding time or shortly thereafter by undercutting it with sweeps or blades.

In areas of low rainfall or where much of the rain comes in intense storms, every effort must be made to conserve rain for crop production. In southern districts, a lister or middle-buster has been used to ridge land to impound water and insure its infiltration into the soil. A general practice is to bed the land and then to rebed with the lister just before planting time. It takes less time than a number of other methods of seedbed preparation, but the depth of tillage is shallow and seldom varied. In humid regions, where tractor tires may compact the subsoil, shallow tillage might cause hardpans.

Crops such as tobacco, which are sensitive to soil aeration, are grown occasionally on poorly drained soils. To provide better drainage, the crop is planted on rather high ridge rows or beds, which allow excess water to drain from the zone where sensitive roots are.

Subsoiling (breaking of compact subsoils without inverting them), chiseling (one method of subsoiling), and deep breaking (breaking of compact subsoils with inversion of the soil) have been praised and condemned. Limited experiments years ago showed a favorable crop response from breaking the soil deeper than normal, and the practice was applied widely with little regard for the reasons for the response. It was soon discredited by many.

Now we know that deep breaking may be effective on soils with compact layers that restrict the downward movement of water and roots. The layers must be shattered when the soil is relatively dry, and the tillage operations must give a continuous shatter to the layer for maximum effect. If deep breaking is done when the soil is moist, the compact layer will not be shattered, and the problem of compaction may be made worse. Breaking when dry requires a tremendous amount of power when pans are near the surface.

Deep tillage may not increase crop yields if there are enough showers and cloudy days during the succeeding growing season to prevent deficiencies of soil moisture in untreated areas. It is impractical in dry localities to add irrigation water often enough to prevent deficiencies in soil moisture if roots are restricted to the surface 6 inches of soil, where they would be if the soil contained a plowpan.

Deep chiseling on the contour increases the rate of recharge of soil moisture reserves in parts of the western Corn Belt, where rainfall is low in amount but high in intensity. Sub-soiling also reduces runoff from spring snowmelt and increases subsoil moisture in the Northwest. There the total capacity of the soil to hold water is not affected only the rate of recharge.

Deep plowing may be used to improve soil structure in alluvial soils where clay or sand was deposited on top of the soil during overflow periods.