Cover crops are planted to cover the soil rather than for the sole purpose of being harvested. Commercial cropping and removal of crop residue leaves the soil surface bare until the next crop is planted. Bare soils are very fragile and prone to erosion, capping, heating, and degeneration. Bare soils cause floods and dust storms.
Planting cover crops to protect and improve the soils provides multiple benefits for our agroecosystems. Cover crops are the way forward for climate-smart regenerative agriculture.
Cover crop canopies reduce the impact of raindrops and decrease the breakdown of the soil structure and soil aggregates, which greatly reduces soil erosion and runoff and improves water infiltration. Holding the topsoil in place reduces the risk of environmental pollution and contamination of water sources by nutrients, pesticides, and pathogens. Cover crops protect against wind erosion of the topsoil. Topsoil loss is a huge driver of soil fertility collapse in Sub-Saharan Africa.
Cover crops can be grown for nitrogen production and to prevent nitrogen loss. Reducing the cost and environmental impact of commercial nitrogen fertilisers. There is more incentive for this due to the high costs and low availability of fertilisers.
Cover crops can be grown to release locked-up phosphorus. For example in very alkaline or calcitic soils, sunflowers, a highly mycorrhizal crop, will take up phosphorus and release it as available organic phosphorus to the following crop. Some cover crops are deep-rooted and bring nutrients up, making them more available for shallower rooted crops.
Cover crops promote microbial diversity and activity. They keep soils cooler protecting the microbes in the soil and reducing organic matter burn-off or nitrogen volatilization. The root exudes from cover crops feed the microbes and encourage soil aggregation. They produce different plant wastes that stimulate different microbes.
Cover crops can provide high-quality animal or wildlife fodder, and food for beneficial insects and pollinators.
Vetiver grass is drought and goat resistant, excellent for contouring. It’s very deep-rooted and can be used as a “cut and carry” fodder. The roots have a delicious scented insecticidal essential oil that can bring in extra money!
Cover crops provide a natural means of suppressing soil diseases, pests, and nematodes. Specific problems can be dealt with by selecting the right cover crop.
Cover crops help control weeds in different ways. They can suppress weeds by direct competition for nutrients, moisture, and light. Some can have an allelopathic effect on weeds. Crop waste can be used as a mulch. Cover crops can be planned so that different herbicides can be used on the field to target specific crops. (e.g., broadleaf cover crops allow the use of graminicides).
Cover crops can reduce soil compaction. In very wet seasons, cover crops reduce the deep soil moisture content, allowing machinery on the fields earlier in the season and reducing compaction of wet soils. Cover crops, with deep tap roots, planted when the soil is moist, and the compaction zones are softest can penetrate hard pans. The increased organic matter, structure, and aggregate stability that cover crops bring to soils improve the soil’s ability to withstand heavy equipment, resulting in less sub-surface compaction.
Cover crops can be planted to bioremediate specific soil problems. They can be used to decrease the pesticide residues or heavy metals in a field. Halophytes (salt lovers) can be used for land reclamation in salt-affected soils.
Over time a well-planned cover crop regime will increase your soil organic matter levels, help sequester carbon, alleviate climate change, and improve the soil fertility, reducing reliance on chemical interventions and bringing good returns to the farmer. Improved water infiltration and moisture storage in the soil is a big plus for rain-fed crops.
Fast-growing leguminous trees can bring huge amounts of biomass, have multiple uses and be more resilient to dry weather
Green manure crops are grown to be incorporated into the soil when they are still green. The primary aim is to add organic matter. Fresh green material increases microbial abundance, can reduce soil pH and releases plant nutrients for subsequent crops. Leguminous green manure crops fix nitrogen, the amount of nitrogen available for the subsequent crop can be as high as 40-60% of the total nitrogen that was in the green manure crop.
A catch crop is a quick-growing crop sown between seasons to make use of temporary idleness of the soil or to compensate for the failure of the main crop. The catch crop scavenges the available nutrients in the soil, reducing nutrient loss and making them available for the following crop. It can be rapid growing vegetables such as radishes, onions, spinach, or quick grain crops cereals such as rye, millet, buckwheat, or an annual legume such as soya bean. These crops can be ploughed back into the soil to increase the soil’s fertility.
Trap crops are planted to attract insect pests from a commercial crop. The pests may fail to survive or reproduce in the trap crop. Or it allows the farmer to spray different pesticides or lower volumes of pesticides in the trap crop.
Break crops are secondary crops grown to interrupt the repeated growing of cereals or maize. Canola is a beneficial break crop that brings financial returns.
Fodder crops are planted to break cropping cycles. Common fodder crops are lucerne, Rhodes Grass, millet, Brachiara, Napier Grass, and sorghum.
Brachiaria grass – hailed as a climate smart wonder grass – makes a good cover crop and a highly nutritious “cut and carry” fodder crop. It is also indigenous to Kenya
Intercropping cover crops are grown inside the main crop planted at the same time or a short while after. A downside is that cover crops can compete with the main crop for light, moisture, and nutrients and can reduce the yield of the main crop. Intercropping is often practiced in regions with bimodal rainfall that have two maize crops a year.
With relay cropping, the cover crop is planted inside the main crop but much later. The relay crop is left in the field when the main crop is harvested and is only incorporated into the soil during land preparation in the next planting season. This is mainly practiced in unimodal rainfall areas.
Rotational cover crops are grown in rotation with the main crop. This is often practiced in bimodal rainfall areas where the second rainy season is unreliable or too short for the commercial crop. Quote from David Jones, the Agventure Agronomist, “like it or not we have two growing seasons, one grows a crop and the other one grows good weeds.”
Long-term cover cropping involves growing perennial fodder or agroforestry crops for several years before rotating to a new piece of land. The crop can be used in a cut and carry system and the manure produced is returned to the soil. Long-term fodder crops can be very beneficial to the soil structure.
There are so many benefits to be gained from properly planned cover cropping! But cover cropping can have its drawbacks and limitations. A badly planned cover crop can have disastrous effects. In the next issue, we will explore the Do’s and Don’ts of cover cropping, and investigate different local cover crops and their uses, including their Nitrogen and Carbon Contribution.
If you have any questions, please feel to contact us on [email protected].
Technical Advisory Services
Ruth Vaughan is the Technical Advisory Services Manager at Crop Nutrition Laboratory Services Ltd. (CROPNUTS). Ruth is also a contributing author to Kenya’s leading horticulture magazines. Ruth is a great believer in soil health, organic matter, biochar and carbon sequestration as a way to alleviate climate change and increase food security. She loves visiting farmers and seeing all the different farming methods. Follow her on Twitter
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