Farm-level assessment of the nutrient balance in northern Nigeria

The farming system of the Kano close-settled zone, northern Nigeria, is kno wn for its longevity and continued productivity in spite of population incr eases in the area. Mounting circumstantial evidence has led to its reputati on of being 'sustainable'. Rural population density in this region now exce eds 300 km(-2), and almost all land (86.4%) is under annual cultivation. It is generally believed that such an intensive fanning system is only possib le under high input levels. This study set out to determine the nutrient ba lance to see if the system really was sustainable. Agronomic and soil ferti lity management practices were monitored on three farms over a two year per iod. Inputs and outputs measured at the field level included fertilizer use (manure and inorganic), dry deposition, biological N fixation, and the har vest of crops and hedgerow products. The results indicate great variability among fields, farmers, and years, and showed that the N balance was strong ly negative, whereas P, K, and Mg balances were close to zero. The Ca balan ce was quite positive as a result of dry deposition. The study also quantif ied 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 c op residues into manure, and (3) the input of nutrients through dry deposit ion. (C) 1998 Elsevier Science B.V. All rights reserved.