PHOSPHORUS SORPTION ISOTHERMS AND EXTERNAL PHOSPHORUS REQUIREMENTS OF SOME SOILS OF SOUTHERN ETHIOPIA

Information on P sorption characteristics of soils is important for making profitable P fertiliser recommendations and designing appropriate P management strategies for improving crop productivity. An experiment was conducted to determine the P sorption capacities and external P requirements (EPR) of some soils of southern Ethiopia. Sorption data were obtained by equilibrating 3.0 g of each sample with 30 ml of 0.01 M CaCl_2 containing various amounts of KH_2PO_4. The data were then fitted to the Langmuir and Freundlich Equations. Results revealed that both models fitted well with the P sorption data of the soils. The adsorption maxima (Xm) and bonding energy (b) values of the soils derived from Langmuir model were in the ranges of 560-833 mg P kg^-1 and 0.16- 20 L kg^-1, respectively. The Freundlich capacity factor (Kf) ranged from 237 to 1200 mg kg^-1.The EPRs of the soils predicted by both models correlated well (r = 0.99) and ranged from 54 to 667 mg P kg-1. Forty three percent of the soils had EPR values 150 mg P kg^-1, and were thus classified as low P fixing soils; whereas 57% of the remaining soils had EPR 150 mg P kg^-1 and were classified as high P sorbing soils. Exchangeable Al^3+ and Fe^2+ significantly and positively correlated with Xm, b and Kf. Aluminium and Fe^2+ are major soil properties responsible for the variations in P sorption capacities of the soils.