Minjingu phosphate rock extractability by white lupin and chickpea in Njoro and Kiambu sub-counties, Kenya

Declining phosphorus (P) fertility is a major production constraint in Kiambu and Njoro subCounties of Kenya. Use of Minjingu phosphate rock (MPR) to rebuild soil capital P is limited by its insolubility in water. Field experiments were conducted in Kiambu and Njoro, in the short (SRS) and long rain season (LRS) of 2012, to evaluate phosphorous extractability from MPR by white lupin and chickpea. The set up was a split plot arranged in a randomised complete block design. Main plots were cereal monocrops, legume- cereal rotations and legume- cereal intercrops. Subplots were triple super phosphate (TSP) and minjingu phosphate rock fertilizers. Soil available P was measured with plant development. Effect of P source, cropping system and growth stage were significant for soil available P. In Kiambu, soil available P was higher in white lupin-maize rotation at seedling in the SRS with MPR application, a trend also observed in the LRS, but within white lupin/maize intercrop at flowering and harvest stages. In Njoro soil available P values ranged f rom 14.2- 88 mg kg-1 for TSP plots and 18- 89.7 Mg kg-1for MPR plots in lupin sorghum experiment and 6.1870.8 mg kg-1 for TSP plots and 10.7- 71.8 mg kg-1 for MPR plots in chick pea sorghum experiment. Soil available P was lowest at the flowering stage in both legumes and seasons and highest in the intercropping system. Comparing legumes, white lupin led to increase in soil available P than chickpea and is recommended for increased extractability of MPR.