Integrated nutrient management

The next challenge is to extend the principles of integration established in integrated pest management to other subsystems of agriculture: to nutrient conservation, and to the management of soil, water, and other natural resources, such as rangeland.

African soils are eroding and losing nutrients fast. The losses of nitrogen per hectare often exceed the amounts a prosperous western farmer would put on his land each year. The losses far exceed the replenishment African farmers can afford. They pay some of the highest fertilizer prices in the world—whether in US dollars or grain equivalents (Mwangi, 1997). Prices in western Kenya are $400/ton of urea, in contrast to $90/ton in Europe (Sanchez, 2002). On average— and many use none at all—African farmers use fertilizer at only 10 kg/ha, whereas European farmers use over 200 kg/ha. This means that Africans must make as much use as possible of organic sources of nutrients, and apply them in an integrated fashion with inorganic fertilizers.

One route is through highly integrated crop/ livestock systems, where soil structure and nutrients benefit both from livestock manure and the nitrogen-fixing capacity of forage crops. Careful ecological management of crop/livestock systems can create virtuous circles: 'Cowpea thus feeds people and animals directly while also yielding more milk and meat, better soils through nitrogen fixation, high quality manure, which, used as fertilizer, further improves soil fertility and increase yields' (International Livestock Research Institute, 1999a). Forages identified by the International Livestock Research Institute for intercropping have led to wheat-yield increases of 30-100% and up to 300% increases in fodder protein while fixing 55155 kg N/ha (International Livestock Research Institute, 1999b).

Often such forages are legumes, whose nitrogen-fixing capacity is the key to improving soil fertility (the Romans used lupins for this purpose). There are numerous examples of mixed cropping systems, sometimes based on tree legumes, sometimes on legumes grown as cash crops. A recent, highly productive system involves growing groundnuts and maize, alternating two rows of each. Yields of the maize can be over 5t/ha whereas ground nuts achieve 1 t/ha or more (Langat et al., 2000). Sometimes, as in the case of the legume Desmodium intercropped with maize, there can be a double benefit since the Desmodium helps to destroy Striga (Hassanali et al., 2006).

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