CHARACTERIZATION OF ORGANIC AND INORGANIC PHOSPHORUS IN THE HIGHLAND PLATEAU SOILS OF ETHIOPIA

Surface horizon samples from two Vertisols, an Andisol, and an Alfisol were collected from farmers' fields, research station farms, and from uncultivated/nonfertilized areas to characterize the organic and inor ganic forms of phosphorus (P) in the highland plateau soils of Ethiopi a using the Hedley soil P fractionation scheme. The total P values ran ged from 226 mg . kg(-1) in the Akaki Vertisol samples developed on al luvial deposits to 1570 mg . kg(-1) in the Andisol samples, where the HCl fraction dominates the inorganic soil P pool. The Alfisol samples contained 400 mg . kg(-1) of total P, with the NaOH and residual P bei ng the dominant P fractions. The resin inorganic phosphorus (Pi) acid bicarbonate Pi fractions generally accounted for less than 15% of the total P in all soils, and were positively correlated with organic C. T he NaOH P fraction, which was most prominent in the surface horizon sa mples of the Alfisols, accounted for 4-15% of total P. The HCl P fract ion, ranged from 1% in the Alfisols to 46% in the Andisols, and positi vely correlated with pH, All of the P fractions were negatively correl ated with clay and extractable Al contents. The organic P (Po) fractio ns were positively related to organic carbon (C) and dithionate extrac table iron (Fe). All samples collected from farmers' fields showed a l oss of P from the residual, and HCI fractions as compared to the uncul tivated/nonfertilized samples. There is also a decrease in the labile P (resin Pi, bicarbonate Pi, and Po) fractions, except for the Akaki V ertisol samples. However, under research station management, the amoun t of labile P fractions either increased or remained at the same level as the uncultivated/nonfertilized samples, except for the Andisol. Ad dition of P fertilizer at the recommended rates to the Debre Zeit rese arch station Vertisol appears to have resulted in a slight increase in the labile P and prevented loss of P from the HCl P and residual P fr actions.