Evaluating NUTMON nutrient balancing in Sub-Saharan Africa

Abstract

Nutrient balances for the last two decades in Sub-Saharan Africa (SSA) reveal, almost unequivocally, alarming nutrient deficiencies. The nutrient balancing in SSA was initiated by Stoorvogel and Smaling [Report 28, Winand Staring Centre, 1990] and Smaling et al. [Fert. Res. 35 (2003) 237]. Their nutrient monitoring (NUTMON) concept is conducted as input–output analysis. Inputs are fertilizers, mineral and organic, wet and dry deposition, nitrogen fixation and sedimentation. Outputs are harvested crops and residues, leaching, denitrification and erosion. Nutrient flows, as fertilizers and harvested crops, are in general measured or estimated by interviews, whereas flows that are difficult to quantify, such as leaching, denitrification and erosion, are modeled by means of transfer functions mainly elaborated by the NUTMON initiators. Generally, for the NUTMON studies the average losses worked out by means of transfer functions seem high in comparison to similar rates in high input agriculture in temperate regions. Therefore, we decided to review some of the original transfer functions and their impact on the NUTMON calculations. The review revealed that the transfer functions have a strong tendency to overestimate losses. No attempts have been made to validate whether the modeled losses are consistent to empirical measurements for the actual locations for which the nutrient balances have been applied. Contemporary NUTMON studies rely on the original transfer functions, as well as the original assumptions employed in the parameterisation. Therefore, NUTMON balances are bound to reveal alarming nutrient deficiencies, no matter what the SSA farmers do.