Effects of phosphorus recapitalisation and agroforestry on soil, water and

Abstract

In the highlands of western Kenya, poor crop growth due to severe nutrient depletion over the past two to three decades has exacerbated soil erosion. The potential benefits of phosphorus (P) recapitalisation with a large one-time application of 500 kg P ha-1 and agroforestry in terms of soil, water and nutrient conservation were investigated on P-deficient soils in western Kenya for 3.5-years. The agroforestry systems compared were one-season sesbania (Sesbania sesban) fallow followed by annual crops, and contour hedgerows of calliandra (Calliandra calothyrsus) plus Napier grass (Pennisetum purpureum). Continuous cropping without P fertiliser produced an average runoff of 367 mm (37% of rainfall) and soil displacement of 105 t ha-1 per season, with a loss of 159 kg of total N and 46 kg of total P ha-1. Phosphorus application reduced runoff by 31% and soil loss by 59% compared to non-fertiliser applied cropping. In P-replenished soils, neither oneseason sesbania fallow nor contour hedgerows affected water runoff but one-season sesbania fallow reduced soil loss by 5% and contour hedgerows reduce soil loss by 20%, over P alone. Nutrient losses due to erosion occurred mainly through sediment movement, and losses through water runoff were negligible. Sesbania fallow and contour hedgerows prevented surface nutrient losses proportionate to their effects on reducing soil erosion. Integrating P and short-duration sesbania fallows in Pdeficient soils increases crop yields and can also contribute to soil, water and nutrient conservation by providing a rapid and prolonged ground cover with good crop growth and increased water infiltration into soil. The hedgerows have the added advantage of yielding fodder besides effectively reducing soil and nutrient losses.