Soil degradation is one of the most serious impacts of population growth in tropical agroecosystems. We investigated long-term dynamics of emerging phosphorus (P) constraints in degrading soil along a 100-year chronosequence in the agricultural highlands of western Kenya. Treatments included five P rates (0, 25, 50, 75, and 100 kg P ha^-1). Strongly bound P and plant available P decreased by 31 and 36% and P adsorption capacity increased by 51% after 100 years of continuous cultivation. Total P (sum of all P pools) and grain yield were significantly (p < 0.05) affected by the cultivation time. Control grain yield declined rapidly in the first 25 years of cultivation and then more gradually approached a minimum yield of 1.6 Mg ha^-1. All three categories of conversions (old, medium and young) responded positively to the applied P rates at the highest rate of applied N (120 kg N ha^-1). Grain yield predicted by regressions models when 120 kg N ha^–1 was applied but with no added P was 6.6, 4.3 and 3.2 Mg ha^-1 for the young, medium and old conversions respectively. The young conversion category had a lower rate of increase in grain yield in response to added P than the medium and old conversions, which were similar. At the highest rate of applied P (100 kg P ha^-1), the medium and young conversion grain yield production was similar. The P fertilizer recovery efficiency (RE_P) decreased similarly with increasing P rates in the medium and old conversions. The young conversion had lower RE_P and lower rate of REp decline than the old and medium conversions. The inorganic Resin-P was correlated with soil organic matter content, suggesting, as in other highly weathered soils, organic matter accumulation and turnover are important in maintaining labile P pools.The findings of this study have important implications for the restoration of soil fertility and alleviation of environmental degradation in tropical ecosystems of developing economies