|CERES No. 109 (FAO Ceres, 1986, 50 p.)|
For centuries, African farmers have relied on shifting cultivation and bush fallow systems to regenerate soil fertility. Although these provide only a subsistence living, they are ecologically stable and therefore suited to the tropical environment. They are, however, wasteful, especially where urbanization and the introduction of large-scale mechanised farming make the practice of idling land for several years unrealistic.
Many programmes for improving tropical agriculture have tried to remove components of the bush fallow system, but the replacements, usually based on temperate-climate farming methods, have often had a destabilizing effect on tropical environments. Thus the search for a stable and suitable alternative to shifting cultivation has engaged the attention of researchers for many years. Now, after a series of studies and experiments at the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria, scientists have come up with a method called "alley cropping" which holds promise for making tropical soils more productive
Alley cropping is a low-input soil management technology that can sustain crop production where farmers cannot afford expensive inputs and where increasing pressure on land does not favour traditional shifting cultivation. While introducing important and more reliable improvements, it retains the basic features of bush fallow and can easily be adopted by resource-poor farmers. As a biological low-input production system, alley cropping represents a useful production technique in developing countries where shortage of foreign exchange prohibits the importation or use of larger quantities of inputs such as fertilizers and herbicides.
Alley cropping consists essentially of planting food crops between rows of leguminous trees or shrubs whose leaves are periodically slashed to supply nutrients to the soil. A major advantage of alley cropping over the traditional shifting cultivation and bush fallow is that the cropping and fallow phases can take place concurrently on the same land, thus allowing the farmer to crop for an extended period without returning the land to bush fallow.
Alley cropping is another arm of the no-tillage or zero-tillage package being emphasised by IITA scientists as part of a worldwide campaign to find an alternative to the conventional system of land preparation in the tropics. Scientists discourage use of heavy equipment for land clearing and preparation because tropical soils are prone to rapid degradation as a result of high humidity and temperature. With alley cropping, conventional ploughing and harrowing are not required. As long as there is adequate mulch from prunings, tillage makes little difference in crop yields. Besides, like bush fallow, alley cropping helps to suppress obnoxious weeds, a big advantage especially in small-scale farming, in which weeding constitutes more than 30 per cent of the labour in crop production.
Perhaps the greatest gain in alley cropping is the supply of nutrients to food crops by the leguminous trees and shrubs. According to three IITA scientists, Drs B.T. Kang, a soil scientist; G.F. Wilson, an agronomist, and T. L. Lawson, an agroclimatologist, coauthors of a publication called "Alley Cropping", trees and shrubs in alley farming provide green manure or mulch for companion food crops. Eight years of continuous experimenting in Nigeria have shown that a wellmanaged row of leucaena used in the alley system produced between 15 and 20 tons of fresh prunings (5 to 6.5 tons dry matter) per hectare with five prunings per year. These prunings, excluding stakes, yielded over 160 kg N. 15 kg P. 150 kg K, 40 kg Ca, and 15 kg Mg per hectare per year. A similar study carried out in Hawaii showed that nitrogen fixation was as high as 500 to 600 kg N per hectare per year. The study also showed nitrogen yield of 127 kg per hectare from four-month-old leucaena plants grown in the Cauca Valley of Colombia. This is a reaffirmation of the high nitrogen fixation ability of leucaena. The recycling of nutrients to the surface soil is a major benefit of alley cropping. Where nitrogen fixing leguminous trees or shrubs are used, some of the nitrogen fixed is eventually released to the companion crops through decomposition of prunings of leaves and twigs. Reports show that yields of open pollinated maize, when alley cropped with leucaena continuously for six years without additional chemical fertilizer on a sandy entisol near Ibadan, was never less than two tons per hectare.
The same system of alley cropping on an alfisol alternating maize intercropped with cowpea produced even more encouraging results. There, with leucaena prunings serving as mulch and without additional use of fertilizer, the maize yield was not less than four tons per hectare. Very promising results were obtained from alley cropping of cassava with gliricidia and also from alley cropping of rice and yams with leucaena. To further expand and select suitable tree and shrub species for alley cropping, field testing is being conducted around Ibadan and the high-density rainfall area of Port Harcourt.
A peasant farmer who has practised alley cropping for two years at Ijaiye village near Ibadan said he received the "gospel" with mixed feelings when it was introduced to him. After one year of trying the package, he added, he began to see the benefits, especially in the area of mulching and weed control. Many farmers in the area are now buying the idea through him.
In the savannah region of Benue State, noted for yam production, a major problem is the supply of stakes needed for this crop. Vegetation there is mainly grass and scattered stunted shrubs. To reduce the farmers' problems, IITA scientists introduced alley cropping in some locations about five years ago. Farmers planted fast-growing Leucaena leucocephala using the stems for yam stakes and the leaves as fodder for their ruminants during the dry season.