Cover crops are commonly grown during fallow periods, between harvest and planting of commercial crops, hence utilizing the residual moisture in the soil. Their growth is interrupted either before or after the next crop is planted, but before competition between the two crops starts. Cover crops energize crop production, but they also present some challenges.

Different plants, have diverse rooting systems, explore different soil depths, have the ability to absorb different quantities of nutrients and with the production of distinct root exudates (organic acids) result in benefits both for the soil and for the organisms.

The presence of a mulch layer (dead soil cover) contributes to a reduction in evaporation of soil moisture and leads to a higher water infiltration capacity in the soil profile. The percentage of rainwater that infiltrates the soil depends on the amount of soil cover provided. Whether through the effects of pores left by plant roots, earthworms, insects or other soil organisms, or through an improved soil porosity and higher number of bigger aggregates, in both cases it is accumulated organic matter that creates the favourable conditions.
Crop residues left on the soil surface lead to higher soil aggregation, and higher porosity and higher number of macro pores, and thus to a higher trend in infiltration rates. 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 contribute to the protection of the soil surface and thus to the maintenance and/or improvement of the physical, chemical and biological characteristics of the soil, including adaptation of effective soil depth through their roots. Vegetative cover is essential for conservation agriculture for the protection of the raindrop impacts of raindrops and to keep the soil shaded and with the highest humidity level possible, for utilizing and thus recycling the nutrients, and for its allelopathic and physical effects on weeds leading to a reduction in agrochemical use and thus in production costs.

In conservation agriculture the terminology 'green manure' is not used often, although generally the same plant species are used for both practices. Green manure is considered as a crop in rotation with the objective of conserving or restoring the productivity of the land through incorporation of the non-decomposed vegetative matter in the soil. In conservation agriculture this is not desired for two reasons:

The dynamics of residue decomposition depends, amongst others, on the activity of micro organisms but also on soil meso and macro fauna. The macro fauna constitutes mainly of earthworms, beetles, termites, ants, millipedes, spiders, snails and slugs, etc. These organisms promote the integration of the residues into the soil and by doing so, modifying the physical properties through the creation of burrows.

The non incorporation of cover crop and weed residues from the soil surface to deeper layers in the soil is a slow process and depends heavily on the activity of these macro organisms. The activity of micro organisms is regulated by the activity of these macro organisms, because they provide them with 'food' and air through their burrows. In this way, nutrients are released slowly and can provide the following crop with nutrients. At the same time, the soil is covered for a long time and is protected against the impact of rain and sun.

In comparison, when green manure crops are mixed into the soil, a large amount of oxygen and 'food' is worked into the soil that leads to a rapid development of the micro fauna population. This big population can decompose the offered 'food' in a rather quick way but at the moment, the 'food' is finished and the microorganisms die, because they cannot sustain their way of life. At this moment a large amount of nutrients is released, that if not taken up by plant roots it can easily be lost through leaching. The whole time, after incorporation, the soil is not protected by a cover and thus is susceptible to degradation processes, resulting in erosion.

Leguminous crops enrich the soil with nitrogen and decompose rapidly because of the low C/N ratio. Later, when the system is stabilized it is possible to include cover crops with an economic function, like livestock fodder. In order to be able to successfully integrate cover crops in a production system, it is crucial to select the plants that are adapted to the different soil and climate conditions and show advantages that fit in the rotation scheme. For this, it is not only necessary to know all the agronomic details of the species, but also all specific conditions of the site where they will be sown (soil and climate) and the anticipated objectives and socio-economic conditions of the farmers.

The selection of cover crops should depend on two main criteria, i.e. the presence of high leveIs of lignin and phenolic acids, which give the residues a higher resistance to decomposition and thus results in soil protection for a longer period.

Another determining factor of the dynamics of residue composition is the biochemical composition of the residues. According to species, their chemical components and the time and way of managing them, differences exist in decomposition rates. Grain species show more resistance to decomposition than legumes. The latter has a lower C/N ratio and less lignin content and thus is subject to a rapid decomposition. Because one of the main objectives of a cover crop is the protection of the soil, their resistance to decomposition should be one of the criteria for selection. In addition, for tropical and subtropical cover crops the resistance to decomposition has been researched. Pearl millet followed by pigeon pea, white mucuna and Cratalaria paulina demonstrated higher resistance and left more soil cover for a long period than the other crops.

Therefore, these crops are better suited as a cover crop, especially during the first years of conservation agriculture than other crops.

Generally, the type of crop and the management afterwards determine the quantity of resides produced, the soil cover generated and the time the cover remains on the surface before being decomposed.

In conservation agriculture, residues should be manipulated from harvest onwards. An equal distribution of residues provides homogenous temperature and humidity conditions at time of sowing. In case residues are not distributed more or less equally on the soil surface this may cause the following problems:

It is important to choose the precise moment at which the vegetative cover is to be controlled, because most of the species used can regenerate if their growth is interrupted prematurely. Alternatively, mature seeds of the cover crop can germinate if the plants are allowed to mature.

There are, however, species and rotations where cover crops are purposely brought to maturity to establish a seed bank which would allow the cover crop to grow automatically once the cash crop has been harvested.

The best moment to control the majority of cover crop species is at full flowering when they have accumulated maximum biomass. In case of legumes, the pods from the first flowering should be already formed but not yet mature. For oats and rye, the best moment to manage is the milky stage. Forage radishes can be slashed at any growth stage, but in systems of direct sowing and minimum tillage, seeds should be green and physiologically immature to avoid the germination of new plants.

Both Crotalaria and pigeon pea need to be controlled before flowering due to high re-growth rate and excessive wood development. The most adequate form to manage the cover crops depends on the objective of the cover crop and the possibilities of the farmer. In case the dead mulch should cover the soil as long as possible, the best way to manage the biomass is by using knife roller, chain, sledge or herbicides. When the decomposition process has to start immediately in order to release the nutrients it is recommendable to slash or mow the cover crop. In some cases, it might be necessary to complete this with herbicides. Research shows that the period between slashing/management of the cover crop and seeding of the commercial crop (maize, beans, soya, etc.) defines the production level of the crop. This is related to some of the substances that are released during decomposition of the cover crops. These can harm the germination of the seeds, or sometimes even delay the development of subsequent crops. This is called allelopathy. In general, management of leguminous cover crops ten days before planting of maize gives highest yield responses.

Mixed cropping is not a new agricultural activity; for generations, farmers have grown maize and beans in association and legumes are sown into pastures with the aim of improve the grazing quality of the pasture. Associations of crops have numerous benefits in production systems. Effect on nitrogen availability in proper quantities and adequate moment for subsequent crops and providing soil cover for a longer period are some of the positive impacts of cover crop mixtures. Crop residues can be an effective source of nitrogen for grain crops, like maize, sorghum and others with a high nitrogen demand, provided that the nutrient release from the residues is synchronized with the demand from the crop.

Therefore, the ideal crop association is the one that offers enough residues to provide a 'pool' of mineral "N" from decomposition to attend to the commercial crop. The strategy for crop associations is to look for species from different families that have different C/N ratios and lignin contents, and that are able to contemplate with both the supply of nutrients and the provision of soil cover for a long time. Mixtures can be made from cereals/grasses and legumes, cereals/grasses and oil crops or even mixing 2-3 or more species that besides presenting an important impact on the improvement of soil properties produce a C/N ratio that favours a gradual nitrogen mineralization. In general, when residues of grass species are mixed with residues of legumes no immobilization of nitrogen will take place and the gradual mineralization favours the availability of nutrient s for absorption by plants.

Millet+sorghum; millet+Crotalaria juncea; pigeon pea+sorghum; cowpea+millet; sorghum+ Crotalaria juncea.
Mixed cropping is another form of crop mixtures. For instance, maize can be mixed with pigeon pea and Crotalaria juncea both planted when the maize plants have reached a height of 30cm by using the same machine as for planting maize.

In Latin America very common practices include the mixture of maize with Mucuna or Canavalia in both cases planted about 80-100 days after the maize was planted. After the maize harvest these species accelerate their development, completely overgrowing the maize residues. Subsequent crops can include beans, sorghum or sunflower.

It is important to plant cover crops at the right time if better result is to be realized. Below are explanations under common practices in the planting of cover crops;

This involves planting legume cover crop at the same time as the main crop. This is easy because you can plant both crops at the same time. It is suitable for sub-humid and humid areas. A possible problem is that the cover crop might grow so quickly that it smothers the main crop. Also, you cannot plant a cover crop this way if you are already intercropping a cereal (e.g., maize) with beans.

This involves planting the legume cover crop when weeding the main crop. This is suitable for sub-humid and humid areas. You can plant the cover crop when you do the first weeding (usually about 4 weeks after planting the main crop). Or if you are intercropping maize and beans, you can plant the cover crop when you harvest the beans.

This involves planting the legume cover crop after you harvest the main crop. In dry areas, you can plant the cover crop when you harvest the main crop. This prevents the cover crop from competing for moisture with the main crop. You can use the cover crop to provide extra mulch and to produce livestock feed. Harvest water to make sure there is enough moisture to grow the cover crop. If you live in a semi-arid area with only one rainy season, you can plant a drought-tolerant cover crop such as lablab between your rows of maize. Leave the cover crop in the field after the maize harvest so it covers the soil and suppresses weeds. Before the next season's rains, slash it or roll it, and leave it on the ground as mulch.

Varieties of cover crop seeds are available within the supply chain of local agro-stockist, research centres such as Kenya Agricultural Research Institute (KARI), Selian Agricultural Research Institute (SARI), World agro forestry Centre ICRAF and many other similar outlets. The table below provide guidance on the contacts

Yes! You will need to weed cover crops at least once while they are becoming established. Once they have covered the soil well, they will prevent most weeds from germinating. If you are intercropping or relay cropping your cover crop with maize or sorghum, plan to weed according to the requirements of the cereal crop. Make sure the cover crop does not tangle with the cereal crop.

In parts of Kenya and Tanzania, farmers plant lablab as the only cover crop. In some villages, quite large areas are covered with the same crop. Farmers in these villages risk a pest outbreak that may ruin their cover crop. How to reduce the danger of pests and diseases:

Harvest the seeds before you slash a cover crop to make mulch. You may need to harvest the seeds for several reasons: so you can plant the cover crop next season, if you want to use the seeds as food or fodder, or if you want to sell them to other farmers.
Store cover crop seeds well. Here are a few tips on how to handle them:

At the beginning of the next season, your field may have a cover crop, stalks still standing from the previous main crop, and of course, weeds. You need to prepare the field so you can plant the next main crop. You can do this in several ways: by slashing the cover crop and weeds with a machete or hoe, using a knife roller to bend over and crush the standing vegetation, or by using a herbicide. It may be better to use a knife roller to crush the vegetation rather than slashing it, because equipment such as rippers and planters can easily drag pieces of vegetation along with them.

Mulch is dry, vegetative material used to cover the soil. It helps reduce evaporation and retain moisture, reduce soil erosion, suppress weed growth and provide plant nutrients as the material decomposes.

Mulch can be used in fields before and after planting, as well as around young crop plants. It is especially useful for high-valuable vegetable crops, and for growing crops in dry areas, during dry-season cropping, and in places where the soil is easily eroded by heavy rains. Where soil erosion is a problem, slowly decomposing mulch material (low nitrogen content, high omC/N) will provide a long-term protection compared to quickly decomposing material.