Growers are learning more and more about cover crops as their prevalence grows, but with so many variables impacting which cover crops may be the most beneficial, it can be difficult to sort through the possibilities.
It’s important to note that there are four classes of cover crops: grasses (such as ryegrass or barley), legumes (such as alfalfa or clover), brassicas (such as radishes or turnips) and non-legume broadleaves (such as spinach or flax).
Specific situations should drive cover crop decisions, as each of the four types of cover crops has its own distinct benefits.
Grasses are a good choice of cover crop if there is a need to scavenge nutrients, especially nitrogen, left over from a previous crop. They also produce large amounts of residue and add organic matter to the soil. Grasses are higher in carbon than legumes, so when they break down in the soil, the residue tends to last longer, which can be beneficial for weed control and increasing soil organic matter long term. Although grasses can be helpful in scavenging excess nitrogen from the previous crop, it is less likely to be released as it breaks down, making it difficult for subsequent crops to access nitrogen from grass residue.
The most popular leguminous cover crops are used to fix atmospheric nitrogen, prevent erosion, and add organic matter to the soil. Legumes are not as effective at removing excess nitrogen as other cover crops; rather, most of their biomass and nitrogen is produced in the spring. Legumes are generally lower in carbon and higher in nitrogen than grasses, resulting in faster breakdown of any residues in the field. This also means that nitrogen and other nutrients are generally released faster from legumes than from grasses. Because of the faster breakdown of legume residue in the soil, weed control from legume residue does not last as long, nor does it increase soil organic matter as much as grasses do. The benefits of legumes are consistent, and any shortfalls are often overcome with grass cover crops. One of the most effective cover crop management systems is a combination of legumes and grasses, which combine the benefits of biomass production, nitrogen scavenging, and weed and erosion control.
While not as popular as grasses or legumes, brassicas do offer some unique benefits as a cover crop. They are known for rapid fall growth and good biomass production, which can help keep fall erosion to a minimum. They also are known as nutrient scavengers, absorbing the excess nutrients in the soil from the previous crop. However, they are particularly unique because of their natural pest management characteristics — most brassicas release chemical compounds that can be toxic to soilborne pathogens and pests. The natural pest management activity of brassicas is less effective than that of commercial pesticides, though, so brassicas should not be used as a sole source of pest control.
There are several general considerations and benefits in favor of cover crops. But with every soil type, climate pattern or cash crop selection, there is a specific cover crop available to help build soil health in a sustainable way. To learn more about the different types of cover crops, including individual species, read "Managing Cover Crops Profitably", a book available from the Sustainable Agriculture Research and Education site.