Phosphorus (P) sorption capacity of surface soils of five different Land use types (Cultivated land, Grazing land, Homestead land, Wood land and Eucalyptus Plantation) of Assosa district in Benishangul Gumiz Regional States, Northwestern Ethiopia were studied. The purpose of this study was to investigate the effect of land use on soil phosphorous sorption characteristics of surface soils, determine the external phosphorous requirements of the soils (EPR) and identify factors contributing to P sorption at Assosa district. One gram of the processed soil from each sample was equilibrated with 25 ml of 0.01M of CaCl2 in 100 ml capacity flasks to which 0, 2, 4, 10, 20, 30, and 40 mg PL-1 were added as KH2PO4 with 0.01M CaCl2 solution. The flasks were shacked for 24 h at room temperature and the P in the supernatants was determined using ascorbic acid method. The P-sorption data were fitted well with both Langmuir and Freundlich models with average r2 values of 0.94 and 0.78 respectively. Comparing the two-adsorption models, the Langmuir model was better for cultivated land, Grazing land, Homestead land and Wood land while the Freundlich model was slightly better in Eucalyptus plantation in predicting P sorption. The adsorption maximum (Xm) obtained from the Langmuir isotherm ranged from 97.09 mg P kg-1 soil in Eucalyptus plantation to 253.85 mg P kg-1 soil in Cultivated lands. The constant related to bonding energy (K) calculated from the Langmuir adsorption model varied from 0.6 to 0.81 L mg-1. The Freundlich adsorption parameters also ranged from 0.23 to 0.51 mg kg-1 for the Freundlich isotherm constant (Kf) and from 0.83 to 0.95 Lkg-1 for slope of the Freundlich isotherm (b). The EPRL values ranged between 109.48 to 175.03 mg P kg-1 for soils of the study area. 46.62 % of the soils of the study area had EPRL values >150 mg P Kg-1 and then were classified as high P-fixing soils. Among the soil properties clay content, CEC, pH, OC and EA were positively correlated with Xm. The result of path analysis revealed that pH, OM and CEC had direct effect on P-sorption parameters of both models. The EPRL of the studied land use soils were 40.63% to 62.83% times greater than the banket recommendation or Assosa agricultural research center P fertilizer rate recommendation. It is concluded that P-sorption models can effectively be used to discriminate soils based on P-fixation ability. The result further indicates that current P fertilizer application rate of 46 kg P ha-1 being practiced across all land use types needs to be revised after validating the models and EPR values estimated in this study for each Land use soils both in greenhouse and in the field at Assosa district.
Keywords: Fixation, batch experiment, external P-requirements, Langmuir & Freindulich equations, and Isotherm