11.3: Rotations and Soil Organic Matter Levels

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.

Figure 11.2. Organic matter changes in the plow layer during long-term cultivation followed by hay crop establishment (generalized drawing based on research).

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 aboveground and belowground. Therefore, when soils have become depleted of organic matter due to simple rotations and nutrients being supplied only through inorganic fertilizers, stopping the application of those fertilizers is not the solution. Fertility levels still need to be maintained while also changing the rotation to improve soil health.

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 10 years, but, eventually, a low level 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 25 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 because rotations that include perennials reduce the total amount of tillage and the associated soil organic matter losses.

A long-term cropping experiment in Missouri compared continuous corn to continuous sod and various rotations. More than 9 inches of topsoil were lost during 60 years of continuous corn. The amount of soil lost each year from the continuous corn plots was equivalent to 21 tons per acre. After 60 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 was 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. Also, as soil organic matter increases to such an extent that mineral surfaces are fully saturated with organic matter and the soil is already highly aggregated, organic matter content increases plateau no matter how much is added in crop residue, manures and composts.

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 non-sod crops such as corn.) Second, grass and legume sods develop extensive root systems, part of which will naturally die each year and add 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 do 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 (to provide food for soil life), high levels of organic matter inside aggregates (to help form and stabilize them), and well-decomposed soil organic matter, or humus (to provide more cation exchange capacity). 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 and other soil organisms. More complex rotations increase soil biological diversity. Including perennial forages in the rotation enhances this effect.

Rotations And Energy Use, Climate Change Impacts And Potential Human Health Impacts

An experiment comparing a typical corn-soybean crop alternation with a rotation that adds a year of oats and red clover (harvesting oats and straw) or a two-year alfalfa crop found many improvements with the more complex rotations: they resulted in less energy use, lower greenhouse gas emissions and better air quality without “compromising economic or agronomic performance.” We know from many other experiments that complex rotations improved soil health in many ways: biologically, physically and chemically.

—Hunt et al. (2020)