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Green manure
Local names:
Mboleya kijani (Swahili)
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| Introduction | Green manure vs. cover crops | |||
| Benefits of green manure and factors to consider | Information Source Links | |||
| How to use green manures |
Introduction
In smallholder agriculture, green manure can play a major role in conservation agriculture and improve farm productivity. They are fast growing, hence accumulates high biomass within a short period. They can increase plant nutrient supply in the soil especially nitrogen through biological nitrogen fixation and improve crop yields. They are deep rooting and thus lead to improved soil structure and recycling of nutrients. They provide good ground cover, which minimises soil erosion through reduction of raindrop impact and runoff and improves soil physical properties. They smoother weeds, even the obnoxious ones like Striga hermonitica, thus reducing labour for weed control. They are an important source of human food and livestock feed. Green manures are plants grown to accumulate nutrients for the main crop. When they have built up the maximum biomass, they are cut and incorporated into the soil. This fresh plant material will decompose within a short period of time and release nutrients quickly.
What is green manuring?
Green manures are plants grown to accumulate nutrients for the main crop. They penetrate the soil with their roots, deliver nutrients and support the infiltration of water into the soil. The contribution of organic matter to the soil from a green manure crop is comparable to the addition of 9 to 13 tonnes per acre (0.4 ha) of farmyard manure. Leguminous plants fix nitrogen from the air into the soil. This nitrogen enriches the soil and feeds all plants in the area. The portion of green manure-nitrogen available to a crop planted later is usually about 40 to 60 % of the total amount of nitrogen contained in the legume. For example, a purple vetch crop that accumulated 90 kg of nitrogen per acre prior to ploughing down will contribute approximately 45 kg of nitrogen
per acre to the succeeding grain or vegetable crop. Lesser amounts are available for the second or third crop following a legume, but increased yields are apparent for two to three growing seasons.
When they have built up maximum biomass, they are worked into the surface soil. As they are usually cut before flowering, growing a green manure is thus different from growing a legume crop in the rotation. Once worked into the soil the fresh plant material releases nutrients quickly and will be fully decomposed within a short period of time. Old or coarse material (e.g. straw, twigs) will decompose at a slower rate than fine material and will therefore contribute to the build up of soil organic matter. An alternative to sowing a green manure crop in the field is to collect fresh plant material from elsewhere and work it into the soil. For example, trees and/or shrubs growing alongside crops in an agro-forestry system may provide a large quantities of green material which can be used as green manure or for mulching.
Benefits of green manure and factors to consider
- They penetrate the soil with their roots, make it more friable and bind nutrients, which would otherwise be washed away.
- They suppress weeds and protect the soil from erosion and direct sunlight.
- If legume plants are used, nitrogen is fixed from the air into the soil.
- Some green manures can be used as fodder plants or even to provide food for human consumption (e.g. beans and peas)
- When green manures decompose, they release all kinds of nutrients in the correct mixture for the main crops to utilise, thus improving their yield.
- The incorporated plant material encourages the activity of soil organisms and builds up organic matter in the soil. This improves soil structure and water holding capacity.
The following aspects must be considered before growing green manures:
- Labour is required for tillage, sowing, cutting and incorporation of plants into the soil.
- If green manures are intercropped with the main crops, they compete for nutrients, water and light.
- When old or coarse plant material is incorporated into the soil, nitrogen may be temporarily unavailable for plant growth (see nitrogen immobilisation above).
- If food and space are in short supply it may be more appropriate to grow afood crop rather than a green manure and recycle the crop residues, or to intercrop a green manure crop with the main crop.
- Benefits of green manure occur over the long term and are not always visible immediately.
How to use green manures
- If grown within a crop rotation, the time of sowing must be chosen such that the green manure can be cut down and worked into the soil before the next crop is sown.
- Green manures need water for germination and growth.
- In general no additional fertilisation is necessary. If legumes are grown in a field for the first time, inoculation of the seeds with the specific nitrogen-fixing rhizobia bacteria may be necessary to profit from nitrogen fixation of the legume.
- If under sown, the green manure is sown at the same time as the main crop. If it grows faster than the main crop and competition is too high, it can also be sown later when the crop has established. Later sowing may be combined with a weeding passage.
Intercropping green manure legumes with maize
Intercropping of green manure legumes with maize is feasible for the regions receiving bimodal rainfall where farmers plant 2 maize crops in a year. In this system green manure legumes are either planted at the same time with maize or planting is delayed by 1 to 2 weeks to reduce competition. The legume is planted in between maize rows. After the maize is harvested, the legume is left to continue growing during the short fallow period preceding land preparation for the following crop. As the land is prepared, the legume biomass is incorporated into the soil.
In the regions of Kenya receiving unimodal rainfall, the maize crop of the year is usually planted in April. The legume green manure system for such a region requires that legumes are planted in August between maize rows and that lower maize leaves are pruned to reduce competition for light. The maize is harvested in November/December and the legume is left growing in the field as short-term fallow until land preparation for the long rainy season maize crop. This system suits sites such as Kitale (Kenya).
Intercropped with vegetables
Some farmers indicated that legumes could be intercropped with vegetables like kales. Crotalaria was most preferred for this purpose because it grows upright and does not coil on the kale plants. Rotational short-term fallow with green manure legumes
Regions that receive bimodal rainfall but have an unreliable second season or where farmers do not grow maize during the second season are suited for a rotational system. The maize crop is planted as a pure stand during the long rains in February/March and harvested in July/August. Green manure legumes are planted in September as a short-term fallow crop and incorporated during land preparation in January/February of the following year. This system is practiced in areas like Kakamega (Kenya).
Cover crop for conservation agriculture in dry areas
Lablab purpureus var. rongai has been tested successfully as a cover crop for conservation agriculture in the dry Machakos district Kenya. Lablab established well and provides complete ground cover in 3 months. The legume is left on the surface as mulch. During planting of maize a furrow is created in the mulch for direct planting of the seed. Cover crops for plantation
Green manure legumes are planted in plantation crops to provide ground cover for control of soil erosion and weeds.
Cover crop as a nurse crop for establishing plantation crops When establishing plantation crops (e.g. coffee, tea, coconut, citrus, sugarcane etc.) cover crops can provide excellent ground cover for weed suppression and control of soil erosion. For example lablab was used as nurse crop in establishment of sugar cane in western Kenya.
Working the green manures into the soil:
- Timing: The time gap between digging in the green manure and planting the next crop should not be longer than 2 to 3 weeks so as to prevent nutrient losses from the decomposing green manure.
- Crushing: Green manures are worked in most easily when the plants are still young and fresh. If the green manure plants are tall or contain bulky and hard plant parts, it is preferable to chop the plants into pieces to allow easier decomposition. The older the plants, the longer decomposition will take. The best time to dig in green manure plants is just before flowering.
- Depth of incorporation: Green manures should not be ploughed deeply into the soil. Instead they should only be worked in to the surface soil (in heavy soils only 5 to 15 cm deep in light soils 10 to maximum 20 cm deep). In warm and humid climates the material can also be left on the soil surface as a mulch layer.
Green manure vs. cover crops
1) In conservation agriculture systems the soil is not disturbed, or as little as possible and
2) more important when biomass is incorporated in the soil a high microbial activity starts in order to decompose the material, resulting in a sudden release of a high quantity of nutrients that can not be captured by the seedlings of the following crop and thus disappear from the system.
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 the micro-organisms 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.
Information Source Links
- Eyhorn, Frank; Heeb, Marlene, Weidmann, Gilles (2002): IFOAM Training Manual for Organic Agriculture in the Tropics, Compiled by FiBL.
