Relationship between phosphorus fractions of some selected Sudanese soil orders to phosphate availability

Abstract

A laboratory experiment was conducted to determine the extent and the importance ofphosphorus (P) fractions of some agriculturally important Sudanese soils on P availability.The soils were gathered from different locations in Sudan, to represent three differentorders: Aridisols (Um Dum soil, North Kordofan state), Alfisols (Hagu soil, Senar state),and Vertisols (Hosh soil, Gezira state). The soil P was fractionated using an authenticatedprocedure. The results showed that the soils had low total, organic and available Pcontents. Most of the inorganic soil P was present in the Fe-Al fraction in Um Dum soil (>50%), and about 40% of this fraction in Hagu soil, but, only 20% in alkaline Hosh soil. TheCa-P fraction constituted > 60% in Hosh soil, about 40% in Hagu soil and 20 % in Um Dumsoil. The available P in the top soils studied was positively related to the percent sum ofAl+Fe -P of the total P, and was negatively related to the percent Ca-P fraction content ofthe total P. It has been proposed that the P sorption starts by exchange with singlycoordinate Fe or Al- OH clay (OH− edge group) and quickly reorganizes into more stableand less soluble ring forms especially at pH more than 7.0, Alkaline pH more than 8.0 insoil like Hosh, will in presence of Ca, favour with time, the formation of inactive lesssoluble form like octacalcium phosphates and apatites at expense of the initial Al and Fe-Pforms which are less stable at alkaline pH.