Chapter 5

Soil Tillage in Organic Farming Systems

Soil Tillage Systems

There are two main types of tillage systems: conventional tillage and conservation tillage. Conventional tillage usually consists of primary tillage, or plowing, and secondary tillage, which is normally done by disking with harrows, and a third step known as cultivation with spring-tooth harrows or some other implement. Conservation tillage is any method of soil cultivation that leaves the previous year's crop residue (such as corn stalks or wheat stubble) on fields before and after planting the next crop, to reduce soil erosion and runoff.

Conventional Tillage

Conventional tillage entails turning under and thoroughly mixing crop residues or cover crops into the top 6 to 10 inches (15 to 25 cm) of soil. The goal of conventional tillage is to leave a residue and vegetation-free soil surface, with a uniformly mixed soil horizon to the plow depth. Conventional tillage involves a sequence of operations (e.g., plowing, harrowing, etc.) to produce a given crop.

Primary Tillage

Primary tillage is the first soil tillage after the last harvest, which involve loosening the soil, inverting the soil, and uprooting weeds and crop stubble. Primary tillage techniques are always aggressive and usually carried out at considerable depth, leaving an uneven soil surface. For weed species that are propagated by seeds, primary tillage can contribute to control by burying a portion of the seeds at depths from which they are unable to emerge.

Secondary Tillage

In secondary tillage, the soil is not worked as aggressively or as deeply as in primary tillage. The purpose of secondary tillage is to further pulverize the soil, mix various materials such as fertilizer, lime, and manure into the soil, level and firm the soil, close air pockets, and control weeds.

Advantages and Disadvantage of Conventional Tillage

Advantages. One advantage of conventional tillage is that the needed machinery is widely available and the techniques are well known to farmers. Conservation tillage methods may require the purchase of new equipment or attachments and often a learning effort on the part of the farmer.

Disadvantages. The limited amount of residue left on fields from conventional tillage and to a lesser extent conventional tillage leaves soils more vulnerable to wind and water erosion. The lack of surface residue causes sealing at the surface, which generates runoff and erosion and creates hard crusts after drying.

Conservation Tillage

The objective of conservation tillage is to provide a means of profitable crop production while minimizing soil erosion due to wind and/or water. The emphasis is on soil conservation, but conserving soil moisture, energy, labor, and even equipment provides additional benefits. To be considered conservation tillage, the system must provide conditions that resist erosion by wind, rain, and flowing water. Such resistance is achieved either by protecting the soil surface with crop residues or growing plants or by maintaining sufficient surface roughness or soil permeability to increase water filtration and thus reduce soil erosion.

Advantages and Disadvantage of Conservation Tillage

Advantages. The principal benefits of conservation tillage are improved water conservation and the reduction of soil erosion. As the soil improves, the increase in infiltration rate, permeability, and porosity allows more rain to get in the soil before it runs off the surface. Crop residues on the surface create an effective barrier that slows water as it runs off the surface and allow more infiltration. Those same residues shade and insulate the surface from wind and sun, reducing evaporation from the soil, which is the major loss during fallow periods.

Disadvantages. Reduced tillage results more crop residues, which can delay crop planting due to lower soil temperatures thereby potentially reducing crop yields. On poorly drained soils, additional water retained by using conservation tillage, especially no-till, aggravates excess soil water problems, thus causing reduced crop yields.

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