Phosphorus sorption capacity as a guide for phosphorus availability of selected Sudanese soil series

Crop response to phosphorus application is often erratic in most soil types in Sudan. This inconsistent response is believed to be due to the alkaline nature of most soils in the country. Phosphorus adsorption isotherms and buffering capacity are powerful tools for predicting response of different soil types to apply P fertilisers. This study was carried out to investigate the degree of P adsorption capacity in some soils in Sudan and the consequential effect on P availability. A laboratory experiment was conducted on six soil samples of selected soil series from different sites in Sudan. Samples were equilibrated with different P concentrations, prior to evaluating the sorption potential and binding intensity. Results indicated that at any specific P concentration, the amount of P sorbed by the Entisols was higher than for the other soils. The order of soils according to their adsorption capacities using Freundlich equation were 917, 330, 315, 269, 214 and 80 mg P kg-1 soil for River Nile banks (Entisols), Dongola 2 (Aridisols), Permanent Fallow (Virgin Vertisols), Remitab (Cultivated Vertisols), Khashm elgirba (Vertisols) and Dongola1 (Aridisols), respectively. The soil’s ability to sorb added P increases due to increase in affinity sites of clay and iron content.