The use of nutrient audits to determine nutrient balances in Africa

A model, designed to carry out soil nutrient audits, has been used to calculate nutrient balances for Africa for the period 1961-1998 for land used for arable and permanent crops. The model indicates that in Africa and most African countries, nutrient depletion has been increasing and in 1998 was 3.5 million tonnes nitrogen (17.4 kg N ha(-1) year(-1)). The corresponding data for phosphorus were 0.7 million tonnes (3.3 kg P ha(-1) year(-1)), and for potassium, 4.1 million tonnes (20.0 kg K ha(-1) year(-1)). Average total nutrients output (embodied in crop output) increased by 2.5% year(-1) between 1961 and 1998, but in the same period, output capita(-1) per year fell by about 10%. Nutrient audits for six countries in SSA (Kenya, Malawi, Nigeria, Uganda, Zambia and Zimbabwe), for the period 1961-1998, show large differences in input and output rates and balances. Ill 1996-1998, total nutrient inputs (N + P + K) ranged from 23 kg ha(-1) year I (Zambia) to 81 kg ha(-1) year(-1) (Zimbabwe). Total nutrient output (crop production) for the corresponding period was in the range 21 kg ha(-1) year(-1) (Zambia) to 97 kg ha(-1) year' (Nigeria). Nutrient balance time series show that for most countries, N, P and K nutrient depletion rates have increased between 1961 and 1998 but have fluctuated considerably during this period. The depletion rate in Nigeria has increased threefold since 1980 and in 1998 was 73 kg ha(-1) year(-1). Because both inputs and outputs were low, Zambia had the lowest total nutrient depletion rate of 5 kg ha(-1) year(-1). In all cases, the highest depletion rates are for K. To achieve a sustainable nutrient output per capita will require a large increase in fertilizer use. It is unlikely that these consumption rates will be achieved and hence future food security could be impaired. Policy implications are discussed. (C) 2004 Elsevier Ltd. All rights reserved.