Farmers Review Africa Nov-Dec 2018 Farmers Review November-December 2018-6 | Page 40

FEATURE improve the soil structure and break compacted layers and hard pans. The cover crops also permit a rotation in a monoculture and can be used to control weeds and pests. Cover crops are grown during fallow periods, between harvest and planting of commercial crops, utilizing the residual soil moisture. Their growth is interrupted either before the next crop is sown, or after sowing the next crop, but before competition between the two crops starts. Cover crops energize crop production, but they also present some challenges. The presence of a mulch layer (of dead vegetation) in conservation agriculture inhibits the evaporation of soil moisture, yet leads to greater water infiltration into the soil profile. The percentage of rainwater that infiltrates the soil depends on the amount of soil cover provided. As different cover crops produce different amount of biomass, the density of the residues varies with different crops and thus the ability to increase water infiltration. Cover crops are useful for protecting the soil, when it does not have a crop. They also act as an additional source of organic matter to improve soil structure. While recycling nutrients such as potassium and phosphorus, these crops make the nutrients readily available to the crops that follow. Vegetative cover is important in CA for the protection of the soil against the impacts of raindrops; to keep the soil shaded; and maintain the highest possible moisture content. We have seen their importance for nutrient recycling; but they also have a physical and perhaps allopathic effect on weeds such that they reduce their incidence leading to a reduction in agrochemical use. Different plants, with diverse rooting systems, explore different soil depths within the profile. They may also have the ability to absorb different quantities of nutrients and produce distinct root exudates (organic acids) resulting in benefits both for the soil and for the organisms. Species diversification Crop rotation The rotation of crops is not only necessary to offer a diverse “diet” to the soil micro organisms, but as they root at different soil depths, they are capable of exploring different soil layers 38 |November - December 2018 for nutrients. Nutrients that have been leached to deeper layers and that are no longer available for the commercial crop, can be “recycled” by the crops in rotation. This way the rotation crops function as biological pumps. Furthermore, a diversity of crops in rotation leads to a diverse soil flora and fauna, as the roots excrete different organic substances that attract different types of bacteria and fungi, which in turn, play an important role in the transformation of these substances into plant available nutrients. Crop rotation also has an important phytosanitary function as it prevents the carry over of crop-specific pests and diseases from one crop to the next via crop residues. Crop rotation results in a higher diversity in plant production and thus in human and livestock nutrition. It also causes a reduction in risk of pest and weed infestations. With crop rotation there is a greater distribution of channels created by diverse roots. This then ensures better distribution of water and nutrients through the soil profile.