Organic Matter Increase

Cover Crops

PURPOSES OF COVER CROPS

Organic Matter Increase

Organic matter is the star in the firmament of soil quality. Cover crops help to increase soil organic matter content by adding crop residue above and below the ground. The total quantity of biomass produced by the cover crop, its C:N ratio, and how it is managed will determine how much soil organic matter is likely to increase. Amounts of dry matter produced by cover crops depend on species (Table 1.10-1), time of establishment, growing conditions, and when the cover crop is killed (Figure 1.10-1). Cover crop residue with a low C:N ratio will decompose much faster than residue with a high C:N ratio. The C:N ratio is influenced by the crop type (Table 1.10-2), but also by when the cover crop is killed (Figure 1.10-2). If the C:N ratio of the cover crop is higher than 30, and the cover crop is incorporated, it is likely to rob the crop of nitrogen, so extra nitrogen should be supplied. As Figure 1.10-2 shows, growing different cover crops together can be a way to lower the C:N ratio of the total biomass produced. The type of biomass produced by the cover crop is also important for other reasons, such as stimulation of soil ecological diversity and soil structure improvement. Cover crops can be managed to supply different types of crop residue to the soil at different times of the year. The type of cover crop, time of establishment, and time of kill will determine what kind of crop residue will be added to the soil. A cover crop should preferably be of a different type than the preceding or succeeding crop to increase ecological diversity in the cropping system. Grand groups can be identified, such as brassicas, nitrogen-fixing legumes, and grassy species. Planting a mixture of cover crops also helps stimulate soil ecological diversity.