Residual Effects of Applied Phosphorus Fertilizer on Maize Grain Yield and Phosphorus Recovery from a Long-Term Trial in Western Kenya

Abstract

Phosphorus (P) application is essential for crop production in the weathered, P-fixing soils of Western Kenya. It is hypothesized that a single large application of phosphorus fertilizer can shift soil-available P levels above a critical threshold, while further seasonal applications are requisite for sustaining yields. A field study was conducted in Siaya district to evaluate maize yield, P uptake, soil P balance and economic returns from P applied at different initial P rates and further seasonal P additions. In the first season, triple superphosphate (TSP) was added at rates of 0, 15, 30, 50, 100, 150 and 250 kg P ha^–1, and maize yield and P uptake were assessed during 10 consecutive seasons. Additional treatments were included where an initial application of 100 kg P ha^–1 was supplemented with seasonal additions of 7 kg P ha^–1, supplied as TSP, manure or Tithonia diversifolia. Residual benefits of maize in terms of increased grain yields were high with cumulative yields ranging from 17.4 to 54.8 t ha^–1 when P was applied at rates above 100 kg P ha^–1. Resin-extractable P increased significantly with initial P addition but decreased rapidly with time, particularly for treatments with one-time high dose of P application. Economic evaluation of these technologies revealed that application of initial P as 100 kg P ha^–1 with seasonal additions of 7 kg P ha^–1 as TSP would give the best marginal returns to investment.