Tillage systems are often classified by the amount of surface residue left on the soil surface. Conservation tillage systems leave more than 30% of the soil surface covered with crop residue. This amount of surface residue cover is considered to be at a level where erosion is significantly reduced (see figure 16.2). Of course, this residue cover partially depends on the amount and quality of residue left after harvest, which may vary greatly among crops and harvest method (corn harvested for grain or silage is one example). Although residue cover greatly influences erosion potential, it also is affected by factors such as surface roughness and soil loosening.
Another distinction of tillage systems is whether they are full-field systems or restricted systems. The benefits and limitations of various tillage systems are compared in table 16.1.
A full-field system manages the soil uniformly across the entire field surface. Such conventional tillage systems typically involve a primary pass with a heavy tillage tool to loosen the soil and incorporate materials at the surface (fertilizers, amendments, weeds, etc.), followed by one or more secondary passes to create a suitable seedbed. Primary tillage tools are generally moldboard plows (see figure 16.3, left), chisels (figure 16.3, right), and heavy disks (figure 16.4, left), while secondary tillage is accomplished with finishing disks (figure 16.4, right), tine or tooth harrows, rollers, packers, drags, etc. These tillage systems create a uniform and often finely aggregated seedbed over the entire surface of the field. Such systems appear to perform well because they create near ideal conditions for seed germination and crop establishment.
But moldboard plowing is also energy intensive, leaves very little residue on the surface, and often requires multiple secondary tillage passes. It tends to create dense pans below the depth of plowing (typically 6 to 8 inches deep). However, moldboard plowing has traditionally been a reliable practice and almost always results in reasonable crop growth. Chisel implements generally provide results similar to those of the moldboard plow but require less energy and leave significantly more residue on the surface. Chisels also allow for more flexibility in the depth of tillage, generally from 5 to 12 inches, with some tools specifically designed to go deeper.