Soil Analysis for Customized Fertilizer Application in Nyamira County, Kenya

Soil nutrient depletion is a major constraint on smallholder agricultural productivity in the Kenya highlands. This study evaluated soil nutrient status and pH variability in Borabu and North Mugirango constituencies of Nyamira County, Kenya, to develop site-specific fertilizer recommendations for smallholder farmers. Composite soil samples were collected from representative wards, air-dried, sieved through a 2 mm mesh, and digested using aqua regia. Potassium (K+) and calcium (Ca2+) were quantified by flame photometry; magnesium (Mg2+) by atomic absorption spectroscopy (AAS); phosphate (PO43-) and nitrate-nitrogen (NO3--N) by UV-Vis spectrophotometry using ammonium molybdate and Griess reagents respectively. Measured concentrations in both constituencies fell significantly below FAO crop-specific thresholds. K+ averaged 48.6 ppm in Borabu versus the FAO minimum of 120-200 mg/kg for maize; PO43- averaged 3.2 ppm versus the FAO minimum of 15-30 mg/kg. Soil pH averaged 5.8 (Borabu) and 6.0 (North Mugirango), suitable for maize and bananas but marginal for tea. Calibration models yielded R2 >= 0.97 for Ca2+, Mg2+, PO43-, and NO3-, confirming high analytical precision. Borabu soils showed uniform severe depletion; North Mugirango showed moderate spatial variability. GIS-driven, variable-rate fertilization integrating rock phosphate, DAP, dolomitic lime, and split nitrogen with leguminous cover crops is recommended to close nutrient gaps, improve yields by 30-50%, reduce input costs by 20-30%, and mitigate nutrient leaching into the Sondu-Miriu River Basin.