Phosphorus fractions of soils from the Didessa watershed, Western Ethiopia

Soil samples were collected at four sites along a 53-km long Didessa river watershed in Ethiopia to get comprehensive information about the distribution of phosphorus (P). The soils represented Vertisols, Alfisols and Ultisols and four different land uses were included (arable, grazing, fallow, agroforestry). The soil samples were analyzed for basic properties (texture, carbon, pH, cation exchange capacity) and the P fractions according to the Hedley procedure plus direct determination of total P. The major outcome was that all the soils were rich in total P (>1600 mg/kg). The soils were very scarce in easily soluble P but 73-80% of P was found in recalcitrant forms, i.e.. in forms extracted with concentrated hydrochloric acid (occluded P) and concentrated sulfuric acid plus hydrogen peroxide (residual P). The results show that all soils representing advanced pedogenesis are not depleted in P. In this case, the outcome shows how soils formed of P rich parent material can be enriched with P, due to occlusion of P released from parent material upon weathering into secondary iron and alumionum compounds. Major reason for concentration is likely abundant leaching of silica and subsequent accumulation of other soil constituents.