Farm-level assessment of the nutrient balance in northern Nigeria.
The farming system of the Kano close-settled zone, northern Nigeria, is known for its longevity and continued productivity in spite of population increases in the area. Mounting circumstantial evidence has led to its reputation of being `sustainable'. Rural population density in this region now exceeds 300 km−2, and almost all land (86.4%) is under annual cultivation. It is generally believed that such an intensive farming system is only possible under high input levels. This study set out to determine the nutrient balance to see if the system really was sustainable. Agronomic and soil fertility management practices were monitored on three farms over a two year period. Inputs and outputs measured at the field level included fertilizer use (manure and inorganic), dry deposition, biological N fixation, and the harvest of crops and hedgerow products. The results indicate great variability among fields, farmers, and years, and showed that the N balance was strongly negative, whereas P, K, and Mg balances were close to zero. The Ca balance was quite positive as a result of dry deposition. The study also quantified nutrient dynamics within the farming system, highlighting, (1) the role of leguminous crops in bringing in N through fixation, and as a source of fodder for small ruminants, (2) the role of small ruminants in converting cop residues into manure, and (3) the input of nutrients through dry deposition.