Green manure - Slow on the uptake?
The benefits of increased organic matter for soil health and crop production are widely recognised but extension programmes designed to introduce green manures have had very mixed success. In some areas farmers have adopted and continue to grow them, while in other areas they are tried and quickly abandoned. Why the variation in success, and what lessons can be learned?
A workshop run by the International Institute for Rural Reconstruction (IIRR) in partnership with the International Centre for Research in Agro-Forestry (ICRAF) has suggested some answers. They suggest that to be attractive, farmers must get some direct benefit from their green manure crop other than just the prospect of soil improvement. Examples of green manure species can offer such direct benefits are an additional food crop, such as pigeonpeas or mungbeans, an animal food crop like velvetbean (Mucuna sp.), or an insecticide like Sunnhemp, used to protect stored grain. For poor farmers, spending money and labour on a crop that will only offer soil improvement is unaffordable, since such improvements normally takes a couple of seasons to have an impact on main crop yields. But, getting an extra food or feed crop provides a more rapid and obvious return on their investment.
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| credit: Professor Jules Pretty |
Green manure crops also stand a better chance of being adopted if they can fill an otherwise empty space in a farming system, and do not occupy land that could otherwise be used for a food or cash crop. Intercropping among food crops offers the most readily available space and, in some cases, by intercropping, farmers can reduce their overall workload. Planting and later cutting a combined manure and cover crop can be an effective way of stopping weed growth, and it generally requires much less time than weeding. Land, which has become exhausted and needs to be left fallow, is another space where a manure crop can be of benefit. For example in Vietnam the shifting cultivators of Son La Province have been able to restore exhausted soils to fertility, in one or two seasons, as opposed to five seasons or more, by broadcasting Tephrosia seeds. Other Vietnamese farmers have used Jackbeans and Indigofera spp. trees to reclaim land, which is now yielding good harvests of coffee and maize.
A further factor supporting the adoption of green manures and cover crops, is that there must be need for little labour or cash. In terms of cash, farmers need to be able to harvest their own seed year after year, and the green manure crops must be free from disease or insect problems that slow down growth. In terms of labour, most poor farmers need crops that do not need to be ploughed into the soil in order to be beneficial. Crops that enable zero tillage systems of cultivation are understandably popular, since these offer a quicker method of land preparation and are less costly.
Research and extension workers also need to address some other common problems associated with green manures. While some manure crops can tolerate very dry conditions, and thus can fill that niche, many others suffer during these months and are easily destroyed by grazing animals, termites or fires. Also, under intense solar radiation, much of the nitrogen and organic matter is oxidised and lost to the soil, long before the next planting season. And, while manure crops may flourish on deep, rich soils, in general they are less productive on the shallow, nutrient-depleted soils often farmed by poorer communities. A further difficulty that research work and extension need to address relates to the timing of the green manure's output against the main crop's nutrient requirements. If these cannot be made to coincide, farmers will remain under pressure to continue using chemical fertilizers.
Interestingly, it is farmers themselves who have been responsible for much of the recent development in green manuring and an international survey of green manure/cover crop systems found that 60% had been devised by farmers. This indicates both of the enthusiasm felt by many smallholders for these systems, and of the need for extension and research staff to base their work on partnerships with farming communities, not top-down approaches.
