Keeping the soil covered is a fundamental principle of CA. Crop residues are left on the soil surface, but cover crops may be needed if the gap is too long between harvesting one crop and establishing the next. Cover crops improve the stability of the CA system, not only on the improvement of soil properties but also for their capacity to promote an increased biodiversity in the agro-ecosystem.

While commercial crops have a market value, cover crops are mainly grown for their effect on soil fertility or as livestock fodder. In regions where smaller amounts of biomass are produced, such as semi-arid regions or areas of eroded and degraded soils, cover crops are beneficial as they:

• Protect the soil during fallow periods.
• Mobilize and recycle nutrients.
• Improve the soil structure and break compacted layers and hard pans.
• Permit a rotation in a monoculture.
• 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.

TABLE 1 Opportunities and challenges of cover crops

Cover crops are useful for:

• Protecting the soil, when it does not have a crop.
• Providing an additional source of organic matter to improve soil structure.
• Recycling nutrients (especially P and K) and mobilizing them in the soil profile in order to make them more readily available to the following crops.
• Provide "biological tillage" of the soil; the roots of some crops, especially cruciferous crops, like oil radish are pivotal and able to penetrate compacted or very dense layers, increasing water percolation capacity of the soil.
• Utilizing easily leached nutrients (especially N).

The roots of some cover crops are able to break hardpans or compacted layers in the soil. (A. Calegari).

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.

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.

A good residue cover (4 t/ha) will suppress weed emergence, increase water infiltration and reduce erosion. Here a crop of onions transplanted into a good maize stove cover. (V.H. de Freitas).

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.

A dense cover crop (hairy vetch - Vicia villosa) will provide good soil cover. This is one important criterion to be considered in cover crop selection. (S. Vaneph).

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, an allelopathic effect on weeds, depressing their incidence and leading to a reduction in agrochemical use and thus in production costs.

Straw residues function as a cushion that reduces the pressure on the soil under wheels and hooves and so they play an important role in reducing soil compaction.