Determining phosphorus sorption and potential costs of phosphate fertilisers used in selected phosphate-fixing soils in the Central Highlands of Kenya

Phosphorus sorption in highly weathered tropical soils reduces fertiliser efficiency, limiting the phosphorus availability for crop uptake and increasing production costs for farmers. This study quantitatively characterised phosphorus sorption behaviour, standard phosphorus requirements, and associated fertiliser requirements across four major soil types (viz. humic Andosols, humic Nitisols, rhodic Nitisols, and orthic Acrisols) in the Central Highlands of Kenya, and evaluated the role of compost in mitigating phosphorus sorption and associated fertiliser costs. Phosphorus sorption isotherms were determined under laboratory conditions using six phosphorus concentrations (0, 6.46 × 10⁻⁴, 1.29 × 10⁻³, 1.94 × 10⁻³, 2.58 × 10⁻³, and 3.23 × 10⁻³ mol dm⁻³ P) and fitted to the Langmuir isotherm model. Linear regression analysis was used to relate sorption parameters to soil properties. Humic Nitisols and humic Andosols exhibited significantly higher phosphorus sorption (76–81%) and sorption maxima (up to 1000 mg kg − 1 ) compared to rhodic Nitisols and orthic Acrisols (~ 46% and ~ 392 mg kg − 1 ). The standard phosphorus requirements ranged from 65 to 249 mg kg − 1 , demonstrating substantial variability in fertiliser requirements and highlighting the limitations of blanket fertiliser application. Regression analysis identified gibbsite, kaolinite, calcium, and iron as significant predictors of sorption maxima. A field study on humic Andosols and orthic Acrisols further evaluated the role of compost in mitigating phosphorus sorption and reducing fertiliser-related costs. Phosphorus sorption under field conditions ranged from 57 to 79%, with fertiliser cost losses of US$47.83–54.52 ha − 1 that reduced to US$31.00–41.84 ha − 1 when triple superphosphate was combined with compost. The findings provide a soil-specific fertiliser management and demonstrate the role of organic amendments in improving phosphorus use efficiency and reducing costs in smallholder farming systems.