PHOSPHORUS FORMS AND FIXATION CAPACITY OF REPRESENTATIVE SOILS IN AKWA-IBOM STATE OF NIGERIA

The P forms and fixation capacity of seven representative soils in Akwa Ibom State were evaluated. Total P varied widely from as low as 79 mg kg-1 to as high as 1410 mg kg-1 with the values decreasing with depth in some of the soils. Organic P also varied from 5 to 434 mg kg-1 decreasing generally with depth and accounting on the average for 24.0% of the total P. The C/P ratios which influence the mineralization of organic P were generally low, being below 200 in most of the soils. The relative abundance of the active P fractions were in the order of Fe-P > Al-P > Ca-P > Sal-P in all the soils excepting those derived from shale and river alluvium where Ca-P was more abundant. Inactive P fractions or reverted P accounted for between 1-7 and 62% of total P. On the basis of the relative contents of reverted il in the soils. it has been inferred that soils derived from coastal plain sands and sandstone parent materials are more developed than others. Available P estimates indicated that the Bray P-2 method removed larger amounts of P than Bray P-1 and the soils differed markedly in their contents of available P, ranging from 2 to 112 mg kg-1 using the Bray P-1 method. The soils differed widely in their P adsorption capacity with a mean range of 0.4 to 44.9 expressed as an adsorption index (X/log C). Soils derived from shale and river alluvium possessed the highest capacity to sorb P compared to those of coastal plain sand, sandstone and beach sand parent materials. The P sorption capacity was significantly correlated with both clay and-silt contents and with oxalate-extractable Al2O3. The significance of the adsorption tendencies of these soils is that those with very high sorption capacities may require larger P fertilizer dressings for optimum crop yield than others with low capacity. Routine soil testing before P fertilizer recommendations and adequate organic matter applications are management imperatives for these soils.