Soil Structure Improvement
PURPOSES OF COVER CROPS
Soil Structure Improvement
Cover crops can help to maintain or improve soil structure because they protect soil from rainfall impact and extreme temperature fluctuations, their roots hold soil particles together and create biopores in soil, and organic molecules are released and fungal networks are produced when the cover crop grows and decomposes. The recent discovery of the glycoprotein glomalin has improved our understanding of soil management practices on soil quality. Glomalin seems to be produced only by mycorrhizal fungi. These fungi live in a symbiotic relationship with most cultivated plant species except for the brassicas (forage radish, rape, cabbages, etc.). The plants provide energy to these fungi, and the fungi extend the reach of the root system—they can help plants take up water and nutrients such as phosphorus. Glomalin is found on the outside of the fungal hyphae of these fungi. The mycorrhizae invade the cells of root tips. As the root grows, mycorrhizae higher up die off and release glomalin. It is this sloughed-off glomalin that contributes to good soil “tilth.” Practices that stimulate mycorrhizae and hence glomalin production include continuous use of no-tillage, continuous occupation of the soil by living root systems that can be colonized with mycorrhizae, and avoidance of overfertilization with phosphorus. Cover crops help fill the gap between main crops, guaranteeing a large population of mycorrhizae.
Overall, the best cover crops for soil structure improvement are those that produce a fibrous, extensive root system such as gramineous (grassy) species. The roots of these species hold soil together and also find ways to penetrate dense soil. Tap-rooted species that penetrate subsoil create gateways for the roots of the succeeding crop. This way, a soybean crop following a cover crop such as ‘Daikon’ radish extracted moisture from deeper depths than if the soybeans did not follow a cover crop. Plant species that produce large amounts of root biomass can also help alleviate soil compaction. That is probably why crops such as corn, sorghum, sudangrass, pearl millet, ryegrass, and rye have been observed to relieve the effects of compaction. Cover crops that grow during the winter are well suited to loosen hard layers in the soil because they can grow into those layers when they are softened by plentiful water (this is less likely in the summer).
Cover crops with a low C:N ratio will, when decomposing, release large quantities of organic molecules such as polysaccharides that improve soil structure. This is the reason why soil is mellow in the spring following a soybean crop. This effect is expected to last only as long at there is decomposable residue. Cover crops with a high C:N ratio will have a slower release of polysaccharides, which improves soil structure more slowly but for a longer time than cover crops with a low C:N ratio (Figure 1.10-3).