You might think you’re doing pretty well if soil organic matter remains the same under a particular cropping system. However, if you are working soils with depleted organic matter, you need to build up levels to counter the effects of previous practices. Maintaining an inadequate level of organic matter won’t do.
The types of crops you grow, their yields, the amount of roots produced, the portion of the crop harvested, and how you manage crop residues will all affect soil organic matter. Soil fertility itself influences the amount of organic residues returned, because more fertile soils grow higher-yielding crops, with more residues.
The decrease in organic matter levels when row crops are planted on a virgin forest or prairie soil is very rapid for the first five to ten years, but, eventually, a plateau or equilibrium is reached. After that, soil organic matter levels remain stable, as long as production practices aren’t changed. An example of what can occur during twenty-five years of continuously grown corn is given in figure 11.2. Soil organic matter levels increase when the cropping system is changed from a cultivated crop to a grass or mixed grass–legume sod. However, the increase is usually much slower than the decrease that occurred under continuous tillage.
A long-term cropping experiment in Missouri compared continuous corn to continuous sod and various rotations. More than 9 inches of topsoil was lost during sixty years of continuous corn. The amount of soil lost each year from the continuous corn plots was equivalent to 21 tons per acre. After sixty years, soil under continuous corn had only 44% as much topsoil as that under continuous timothy sod. A six-year rotation consisting of corn, oats, wheat, clover, and two years of timothy resulted in about 70% as much topsoil as found in the timothy soil, a much better result than with continuous corn. Differences in erosion and organic matter decomposition resulted in soil organic matter levels of 2.2% for the unfertilized timothy and 1.2% for the continuous corn plots.
In an experiment in eastern Canada, continuous corn led to annual increases in organic matter of about 100 pounds per acre, while two years of corn followed by two years of alfalfa increased organic matter by about 500 pounds per acre per year and four years of alfalfa increased organic matter by 800 pounds per acre per year. (Keep in mind that these amounts are small compared to the amounts of organic matter in most soils—3% organic matter represents about 60,000 pounds per acre to a depth of 6 inches.)
Two things happen when perennial forages are part of the rotation and remain in place for some years during a rotation. First, the rate of decomposition of soil organic matter decreases, because the soil is not continually being disturbed. (This also happens when using no-till planting, even for nonsod-type crops, such as corn.) Second, grass and legume sods develop extensive root systems, part of which will naturally die each year, adding new organic matter to the soil. Crops with extensive root systems stimulate high levels of soil biological activity and soil aggregation. The roots of a healthy grass or legume-grass sod return more organic matter to the soil than roots of most other crops. Older roots of grasses die, even during the growing season, and provide sources of fresh, active organic matter. Rotations that included three years of perennial forage crops have been found to produce a very high-quality soil in the corn and soybean belt of the Midwest.
We are not only interested in total soil organic matter—we want a wide variety of different types of organisms living in the soil. We also want to have a good amount of active organic matter and high levels of well-decomposed soil organic matter, or humus, in the soil. Although most experiments have compared soil organic matter changes under different cropping systems, few experiments have looked at the effects of rotations on soil ecology. The more residues your crops leave in the field, the greater the populations of soil microorganisms. Experiments in a semiarid region in Oregon found that the total amount of microorganisms in a two-year wheat-fallow system was only about 25% of the amount found under pasture. Conventional moldboard plow tillage systems are known to decrease the populations of earthworms, as well as other soil organisms. More complex rotations increase soil biological diversity. Including perennial forages in the rotation enhances this effect.