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Longtern fertilizer trials on alfisols in West Africa
Results of longterm fertility trials have been useful in identifying long-term nutrient needs and problems arising with continuous and intensive cropping on Alfisols. Available data have shown that high and sustained crop yields can be obtained with judicious and balanced NPK fertilization, combined with organic material amendments. Although the need for secondary and micro nutrients has not been clearly identified, increasing problems can be expected with more intensive and unbalanced fertilizer use, as currently practiced in the region. Continuous use of N fertilizers can give rise to serious soil acidity problems, particularly in soil with a low soil organic matter content. Liming of acid soils has not given consistent results. Although application of high rates of organic materials can lessen the acidity problems, low availability and high transport costs of bulky residues will constrain their use. Thus, an integrated approach to soil fertilitymanagement is urgently needed. This approach should maximize the complementary benefits of the combined use of fertilizers, biologically fixed N, and organic residues/wastes, and minimize losses of nutrients from the soil-plant system. Inclusion of suitable agroforestry systems (e.g. alley cropping, planted fallow) for the in situ production of N-rich plant biomass for soil amelioration deserve serious attention.