Experiments were conducted in both the greenhouse and the field on highly weathered (Orthic Ferralsol) soils to evaluate the effect of combining on-farm organic residues (chicken manure, farmyard manure, sugar bagasse) with Minjingu phosphate rock (MPR) on soil pH, extractable (available) and sorbed phosphorus (P), and to assess the cost benefit for maize production. The greenhouse study results indicated that, in the first eight weeks of incubation, soil pH increased linearly with increase in MPR rates and decreased thereafter. The available P also increased linearly. Chicken manure and sugar bagasse were most and least effective respectively in reducing P sorption, while there was a significant negative relationship between P adsorption maxima and extractable P. The field experiment data showed that the available P values increased significantly above the control in all the treatments where MPR and organic materials were applied separately or combined. Treatment effects on Langmuir sorption maxima (S max ) in the field were variable. An increase in S max with an increase in MPR rates was noted. However, there was a positive relationship between S max and available P (r = 0.52 to 0.69), suggesting the diversity of factors affecting the complex nature of P dynamics under field conditions. MPR applied alone in the first season gave a significant residual positive effect on maize grain yield for two additional seasons. The economic analysis revealed that a single application of chicken manure (2 t ha −1 ) combined with MPR at 60 kg P ha −1 gave the highest incremental net benefit equivalent to US$ 657 ha −1 during the three maize cropping seasons. I N T RO D U C T I O NPhosphorus (P) is an essential element for plant growth and development. However, among the nutrient elements, P is the most susceptible to fixation (sorption), particularly in acid or calcareous soils so that crops usually recover less P in the short term than other nutrients. Fixation occurs by sorption or precipitation reactions with Fe and or Al oxides in acid soils or Ca in calcareous soils (Chien and Hammond, 1988). Liming acid soils to reduce acidity is one way of overcoming P fixation; alternatively direct application of reactive phosphate rock (PR) or manure can reduce P fixation, or large quantities of P fertilizer can satisfy P fixation sites and thus increase

In western Kenya, severe nutrient depletion, especially that of nitrogen and phosphorus, has drastically reduced crop yields over the past two to three decades. The potential effects of P application, with a single direct application of Minjingu phosphate rock (MPR) at 20, 40 and 60 kg P ha −1 , and fallow biomass in terms of nutrient replenishment on maize and bean yields were investigated on N and P deficient soils of western Kenya for six cropping seasons (three years). The agro-forestry shrubs tested were one season (six months) Crotalaria grahamiana and Tephrosia vogelii followed by maize monocrop. These fallow species were intercropped with beans in the 2000 long rains season to provide a food benefit to the farmer. Soil samples were taken at the end of each season and analyzed for soil pH and extractable P. The results showed that soil available P increased significantly following application of MPR. Significant increases in bean yields were obtained when 60 kg P ha −1 as MPR were applied, and contributed to a 260 % increase above the control. Significant maize grain yield increases were obtained when MPR was applied alone or in combination with fallow biomass as compared with treatments with either no external nutrient addition (control) or with fallow biomass alone in all seasons. The 60 kg P ha −1 MPR rate gave the highest cumulative maize grain yields (9.6 t ha −1 ) over the five consecutive maize growing seasons, followed by 40 kg P ha −1 (8.8 t ha −1 ). The residual benefits of MPR at modest rates of application (60 kg P ha −1 ) were found to persist in the soil for only three cropping seasons. Thereafter, there was a steady decline in soil chemical properties (pH and available P), grain yields and net benefits. The study demonstrated the need for frequent additions of P especially in the fourth consecutive season to ensure sustained availability of P, favorable pH, and increased crop yields and net benefits on the nutrient-depleted soils of western Kenya. I N T RO D U C T I O NIn the highly populated areas of western Kenya, where continuous cropping with minimal external nutrient addition has been practiced for decades, nutrient depletion of soil is widespread and seasonal grain yields of the staple maize crop rarely exceed 1 t ha −1 . Phosphorus deficiency is of primary concern: more than 90 % of farm soils tested had less than 5 mg P kg −1 of bicarbonate-extractable P (Lijzenga, 1998). As a