Agronomic and financial benefits of phosphorus and nitrogen sources in Western Kenya.

A better understanding of the agronomic and economic benefits of integrating organic and inorganic fertilizers is essential for their wide-scale dissemination and adoption in smallholder farming systems in developing countries. Field studies were conducted for 10 cropping seasons (1996–2000) on a Kandiudalfic Eutrudox soil to compare the effects of Minjingu phosphate rock (MPR) and triple superphosphate (TSP) on the yield of maize (Zea mays), in combination with three nitrogen (N) sources: urea, Tithonia diversifolia and Sesbania sesban leafy biomass. Urea and tithonia were applied to provide 60 kg N ha−1. Phosphorus (P) from either MPR or TSP was added either once at 250 kg P ha−1 at the beginning of the experiment or annually at 50 kg P ha−1 for five years, the two rates representing two P recapitalization strategies. From the second year, potassium (K) was added to half of each plot to correct for the deficiency that emerged. Over the 10 cropping seasons, the agronomic and economic benefits of the two P recapitalization strategies were similar and were not influenced by the P sources used. With N and K application, both P sources resulted in at least a two-fold maize grain yield increase over the control with no P application that averaged 1.1 tons ha−1. Comparing the N sources, although urea and tithonia had similar net benefits when P was applied, the total cost associated with tithonia was considerably higher. Without P application, sesbania fallow was the most financially attractive option. Sesbania fallows as an N source were also least sensitive to increases in the price of fertilizers and the cost of labour although the financial benefits of this system remained low in the absence of P and K application.