Residual Effectiveness of Minjingu Phosphate Rock and Fallow Biomass on Crop Yields and Financial Returns in Western Kenya

Abstract

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

Comparative Effects of Soil Amendments on Phosphorus Use and Agronomic Efficiencies of Two Maize Hybrids in Acidic Soils of Molo County, Kenya

“…Sholly et al [66] recorded about 29% yield increase with manure application in wheat. Application of 60 kg P ha −1 as MPR has also been found to significantly increase bean yields by over 260 % above the control [67]. They [67] also observed significant maize grain yield increases with sole application of MPR or in combination with fallow biomass as compared with treatments without external nutrient addition (control) or with fallow biomass alone in all seasons.…”

“…Application of 60 kg P ha −1 as MPR has also been found to significantly increase bean yields by over 260 % above the control [67]. They [67] also observed significant maize grain yield increases with sole application of MPR or in combination with fallow biomass as compared with treatments without external nutrient addition (control) or with fallow biomass alone in all seasons. Zafar et al [68] further observed that the ability of an organic material to lower the exchangeable Al is more important in increasing maize yields than its ability to increase P availability.…”

Reflections on Efforts Geared toward Improved Soil Fertility and Crop Yields in Kenya

“…PREP-PAC consists of 2 kg Minjingu Rock Phosphate (MPR), 0.2 kg Urea, 120 g legume seed, rhizobial innoculant (Biofix) packed with lime pellets to raise the pH of the inoculated seed environment and gum Arabic sticker to hold the innoculant onto the surface of the seed. Several other works citing the promising effect of MPR on soil fertility replenishment [14,15] has been documented. However, adoption by farmers is negligible due to the unavailability of rock phosphate locally and its extra cost incurred when imported from the neighboring Uganda and Tanzania.…”

Testing of Commercial Inoculants to Enhance P Uptake and Grain Yield of Promiscuous Soybean in Kenya

“…However, compared to countries like Brazil (4 t ha −1 ) [3], soybean yield in Kenya is relatively low (< 0.5 t ha −1 ). The main reasons for low crop yields include poor soil fertility associated with poor organic content as a consequence of little or no nutrient inputs (both organic and inorganic) and P-deficient soils [4,5]. East African soils are highly deficient in P [6].…”