Green manure (otherwise known as cover crops), is a climate smart fertiliser process that involves growing plants (mainly legumes) and distributing uprooted or sown crop-parts to wither and cover soil. It provides soil coverage to enhance biological, physical and chemical properties of soil while mitigating soil erosion, supressing weed growth, adding biomass to soils, improving soil structures, promoting biological soil preparation, and reducing pests, diseases and weed growth. These functions can increase economic return, reduce the need for herbicides and pesticides, while increasing productivity and potentially the quality of crops. It can also increase soil nitrogen, improve soil fertility, conserve soil humidity and reduce fertiliser costs. Green manure also has low management costs, presents good conservation characteristics, and improves biodiversity. Green manure is a feasible and sustainable option for farmers to improve soil quality and productivity, depending on local context and availability of different leguminous plants that best fit for farmers’ cropping systems. Examples of leguminous plants that can be used in southern Africa include: Mucuna (Mucuna pruriens); Sunhemp (Crotalaria juncea), Lab-lab (Lablab purpureus); Pigeon pea (Cajanus cajan); Cowpea (Vigna unguiculata) and Butterfly pea (Clitoria ternatea). Green manure has climate smart benefits as contributes to sustainable maintenance of agricultural production without the use of chemical fertilisers and depending upon the cover crop can contribute to adaptation of agricultural practices to climate change. Furthermore, coverage of soil with additional plant material can assist with carbon sequestration in soil. Not only does growing a secondary green manure crop provide a soil amendment benefit, but the crop can also be used as fodder for livestock. As the most common green manure plants are legumes, the pods and seeds can be fed to livestock while leaving the crop residue to perform the cover crop function in in the fields.
To effectively apply a green manure approach, the following should be considered:
- Step 1: Select legumes that grow well under local conditions and in local soils. Green manure crops should be resilient and require few crop management practices. A thorough investigation should be made to ensure that green manure crops are appropriate for the local conditions in terms of rainfall, climate, soil pH and texture, and salt tolerance.
- Step 2: Identify the appropriate time for planting the green manure crop to ensure growth, but not impacting the primary crop. Especially if the secondary crop is a climber/creeper. Main crop may need to be mature before planting the manure crop, as if a creeper, it may outcompete or constraint growth of maize or sorghum plants.
- Step 3: If seeking to enrich soil properties, the farmer must allow crop residue to remain in the soil longer. This is particularly relevant with multiple uses – e.g. soil amendments and livestock fodder. In these cases, pods can be harvested for fodder, and the remaining plant residue left in the field to cover the soil.
- Step 4: Crop planting should be alley cropped between the primary crop rows, allowing management of the primary and secondary crops, also reducing the competition between the primary and the secondary crop. If the secondary crop also has pest management properties, it may be beneficial to consider boundary planting.
- Step 5: When harvesting the secondary crop, the farmer should consider leaving the residue in the ground. If it is uprooted, it should left on the soil surface. A common mistake is to remove it from the field and accumulate it in one location, missing the benefits of cover-crops, and exposing the residue to decay.
Unless local examples are available, small test plots should be used to test different cover crops to determine which is the most appropriate, and if necessary, demonstrate value to farmers and communities. As the secondary (green manure) crop is not a direct cash-crop, you may need to ensure expectations are measured. It may take several years to develop enough green manure crop to contribute to crop production; hence, crop production has to fit around existing cash/subsistence crops. Furthermore, benefits may not be realised within a single planting season., e.g. Nitrogen may only be available in the soil in the subsequent season.
- Bunch, Roland 2012. Restoring the soil: a guide for using green manure/cover crops to improve the food security for smallholder farmers. Canadian Food grains Bank, Winnipeg, Canada
- EMNZ 2015. Green Manure Crops and the Benefits. New Zealand.
- ECHO Community Staff 2017. Selecting Legumes as Green Manure/Cover Crops. ECHO Best Practice Notes BPN #7.
- Food and Agriculture Organisation of the United Nations (FAO) 2011. Green manure/cover crops and crop rotation in Conservation Agriculture on small farms. Rome.
- Food First 2016. Green Manure Crops in Africa: A Report from the Field. Policy Brief No. 20.
- Motis 2016. A “2:4:2” Maize/Legume Intercropping Pattern. ECHO Development Notes Issue 133.
- Odhiambo, J. (2011). Potential use of green manure legume cover crops in smallholder maize production systems in Limpopo province, South Africa. African Journal of Agricultural Research. 6
Benefits
- Green Manure is a non-tillage method that promotes soil fertility through enhancement of soil organic content. In doing so, it mitigates erosion, maintains soil humidity, and promotes biological activity.
- Many green manure plants can be used to feed livestock if there is an excess
- Green manure cover crops also make organic matter to apply – compost requires work and time to develop, whereas this approach sees it added immediately.
- Cover crops can reduce weed competition by shading soil.
- If using legumes, they can thrive in poor quality soils.
- Cover crops such as Cow pea can also be used for animal and human consumption.
Drawbacks
- Require access to seedbanks for legumes and other viable cover crops.
- May require the testing of crops in test plots prior to implementation.
- If so, community action may be required to test varieties and make decisions.
- Farmers may require more land to plant the same amount of the main crop, as they need to be intercropped with the cover crop. This can be unattractive to some farmers.
