Abstract:An experiment was carried out of persistent activating effect of citric acid on phosphorus in red soil.Results show that amount of the phosphorus activated decreased gradually with the increase in frequency of extraction,but never down to nil,and the activation intensified with the increasing citric acid concentration.Fractionations of inorganic phosphorus in the soil samples before and after the extraction show that phosphorus was activated by citric acid mostly from the fractions of Al-P,Fe-P and Ca-P,and some from the fraction of organic phosphorus,and that citric acid accelerated formation and accumulation of Oc-P in red soil.Effect of concentration of citric acid on activattion of phosphorus from Al-P,Fe-P and Ca-P was very clear.When the concentration of citric acid was 5 mmol L-1 and 10 mmol L-1,the fractions were in the order of Ca-P>Al-P>Fe-P in terms of amount of phosphorus activated,but when it was 100 mmol L-1,the order was:Fe-P>Al-P>Ca-P.