The magic of mucuna bean

credit: Professor Jules Pretty

credit: Professor Jules Pretty

Scientific wisdom has it that soils are created only over geological time periods. But farmers with degraded soils in Guatemala and Honduras who can't wait that long, have discovered that the mucuna or velvetbean, Mucuna pruriens, used as a green manure, can help rebuild soils quickly.

Within one year, a crop of mucuna beans is able to 'fix' 150 kg of nitrogen per hectare. Other plants without root nodules, such as maize, can then benefit as the fixed nitrogen 'fertilizer' is released into the soil. In addition, the mucuna also produces up to 35 tonnes of organic matter per hectare. By cutting the so-called 'cover crop' or 'green manure', and allowing the leaf material to compost naturally in the fields, soils are being recreated. Professor Jules Pretty of the University of Exeter, UK, first saw the mucuna bean in use during a visit to the ancient Mayan city of Tikal in Guatemala. "Throughout Central America, various NGOs have promoted the use of grain legumes, especially the velvetbean, to be used as a green manure, an inexpensive source of organic fertilizer to build up organic matter," he explains.

It seems that about fifty year ago farmers in the Polochic Valley of Guatemala discovered that they could use mucuna, which had been imported to feed the United Fruit Company's mules to maintain their soils' fertility without using chemicals. It is generally used by intercropping it with maize, and is especially adapted to the high rainfall areas of the Caribbean side of the Mesoamerica isthmus," And says Roland Bunch of COSECHA, a local NGO. (Cosecha means 'harvest' in Spanish.) "Its spread throughout the region has been nothing short of fantastic."

Now, across Guatemala and Honduras, some 47,000 farm-families are benefiting from the adoption of sustainable agriculture. Alongside the use of simple technologies such as sowing grass strips along contours, building rock bunds and in-row tillage, the mucuna bean has helped regenerate local economies. In many cases, maize yields have at least doubled or tripled, and there are reports of increases from a base of around 500 kg/ha to 2000 kg/ha or more. In fact, some projects have been so successful that land prices and labour rates have increased and families are moving back from the cities. Farmers also say that their land is now producing adequate quantities of food on a sustainable basis, such that they no longer need to cut down virgin forest.

Brazil and Benin

The mucuna bean has also been introduced into Brazil. In the southern region of Santa Catarina, state extension workers have promoted soil and water conservation practices at the 'microbacias' or watershed level. As well as using contour grass barriers and contour ploughing, farmers have been experimenting with green manures. "Farmers use some inorganic fertilizers and herbicides, but there has been particular success with green manures and cover crops," says Prof. Pretty. "Some sixty species have been tested with farmers, including leguminous plants such as mucuna, jackbean and cowpeas, and non-legumes such as oats and turnips. For farmers, these involve no cash costs except for the purchase of seed."

The green manures are either intercropped or planted during fallow periods and are used in cropping systems which include maize, onions, cassava, wheat, grapes, tomatoes, soybeans, tobacco and orchards. Animal-drawn tools are used to knock down the cover crop or green manure and another instrument, designed by the farmers themselves, clears a narrow furrow in the resulting mulch into which the next crop is planted. In most cases there is no need to plough the land and the cover crops keep down weeds, so reducing the labour needed to produce the crop.

As in Mesoamerica, once the mucuna bean had been introduced into the cropping system with a range of other soil conservation strategies, yield increases in Brazil have been spectacular. In eight years up to 1999, yields of maize increased by 47%, soya by 83% and wheat by 82%. EPAGRI, the Santa Catarina state agricultural extension service responsible for introducing the technology, has also recorded improvements in water quality, soil health and water retention.

The 'magic' of the mucuna bean is also being appreciated in Africa. As in Latin America, its spread throughout Benin has been nothing short of impressive. In 1987, it was introduced to just 15 farmers. By 2000 over 10,000 farmers were using it, not only to improve their soils, but also as a way to smother weeds, which are otherwise difficult to control. Where it is used, there are again reports of maize yields more than doubling. Even so, extension workers in Africa believe the impact of mucuna could be even more impressive if it wasn't for one drawback. Unfortunately, the bean isn't ideally suited for human consumption. However, breeding efforts are underway to try and further improve digestibility of mucuna.

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