Abstract
Total P was found to be low in 60 surface soils of western Nigeria savannah formed on igneous and metamorphic rocks; 41% was organic P. Average mobile P fractions were 4.1, 9.2, 19.6 and 10.0 mg kg 1 for saloid-bound, AI-, Fe- and Ca-P, respectively. All P forms except non-extractable P correlated significantly with organic matter and P uptake. Measures of active inorganic P were more related to P uptake than measures of organic P, and correlations indicated an order of P utilisation as saloid-bound P > AI-P > Ca-P> Fe-P. Six extractants were evaluated for assessing P availability; 0.05 N AHDF (pH 7.0) which extracted least P had least correlation with P forms and P uptake. New Mehlich, Bray's P1 and 0.1 N AHDF extractants had high correlations with P uptake and inorganic P forms and are good indices of P availability in the soils. Lower correlations show that these extractants do not extract organic P and that mineralisation of organic P, especially in soils with high organic matter content, affects P availability in tropical soils.
