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

摘要

A laboratory experiment was conducted to determine the extent and the importance of phosphorus (P) fractions of some agriculturally important Sudanese soils on P availability. The soils were gathered from different locations in Sudan, to represent three different orders: 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 authenticated procedure. The results showed that the soils had low total, organic and available P contents. 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. The Ca-P fraction constituted 60% in Hosh soil, about 40% in Hagu soil and 20 % in Um Dum soil. The available P in the top soils studied was positively related to the percent sum of Al+Fe -P of the total P, and was negatively related to the percent Ca-P fraction content of the total P. It has been proposed that the P sorption starts by exchange with singly coordinate Fe or Al- OH clay (OH? edge group) and quickly reorganizes into more stable and less soluble ring forms especially at pH more than 7.0, Alkaline pH more than 8.0 in soil like Hosh, will in presence of Ca, favour with time, the formation of inactive less soluble form like octacalcium phosphates and apatites at expense of the initial Al and Fe-P forms which are less stable at alkaline pH.