Soil Phosphorus (P) Fractions and Lability in Western Kenya Oxisols after a Decade of P Inputs of Different Quality, and Response to Simulated Liming: Implications for Cost-Effective P Management.

Phosphorus (P) deficiency is a common constraint to agricultural productivity in weathered soils prevalent in East Africa. Alleviating soil P constraints can be achieved with P inputs, which in western Kenya are commonly organic amendments like farmyard manure (FYM), as well as mineral fertilizers like triple super phosphate (TSP) and Mijingu rock phosphate (MRP). However, soil acidity and high cost of P inputs to smallholders in this region offer pH management as a potential strategy to improve P availability. To this end, the effects of different P additions on fractions and lability of soil P were determined for two long-term trials in western Kenya. Trial I (13 yrs) compared different mineral P inputs (TSP vs. MRP), and Trial II (11 yrs) compared organic (FYM), mineral (TSP), and mixed (TSP+FYM) inputs. Each trial also included an unfertilized control. Soil P lability and saturation was determined by Hedley fractionation and sorption isotherms for 0-15, 15-30, and 30-60 cm depths. The potential of pH management to improve P availability was evaluated with liming mesocosms using CaCO₃. Results were used to evaluate the cumulative effects of different input strategies on soil P stocks and availability. Additionally, local prices of amendments (i.e. P inputs, liming agents) were used to estimate economic feasibility and cost efficiency of P management strategies for smallholders in western Kenya.