EFFECTS OF ORGANIC MATTER REMOVAL AND ADSORBATE SOLUTION COMPOSITION ON PHOSPHATE SORPTION BY SELECTED SOILS OF KWARA STATE, NIGERIA.

The effects of organic matter removal and adsorbate solution composition on phosphate sorption by selected soils of Kwara State were studied. Organic matter was removed by treating the soils with hydrogen peroxide. These were the 'treated' samples. The 'untreated' samples were those to which hydrogen peroxide was not applied. The adsorbate solutions used were: KH PO + KCl, KH PO (NH ) HPO + KCl and (NH ) HPO The phosphate sorption data were fitted into the Langmuir II sorption model. The r values from the Langmuir II sorption models were 0.95 for Ilorin, Omuaran and Erin-ile soils while Odoowa, Malete and Lanwa had 0.94, 0.93 and 0.89 respectively for the untreated soil samples. The r values for the treated samples were 0.96 each for Omuaran, Erin-ile and Malete while Ilorin, Odoowa and Lanwa had r values of 0.95, 0.94 and 0.91 respectively. There was no significant difference in the r values of the treated and untreated samples except for Ilorin soil. The r values obtained for the treated samples were generally higher than their untreated counterparts. The r values for KH PO + KCl adsorbate solution were higher than those for other adsorbate solution compositions for both Omuaran and Lanwa soils. The highest adsorption maximum for the untreated soils was recorded for Omuaran, followed by Erin-ile, Malete, Odoowa and Lanwa in that order. In the treated soils, the values of the adsorption maxima were slightly greater than for untreated samples. Adsorption maxima and buffer capacity values were higher in KH PO + KCl and (NH ) HPO + KCl adsorbate solutions than in KH PO and (NH ) HPO adsorbate solutions. The k values were also higher in both KH PO + KCl and (NH ) HPO + KCl adsorbate solutions than in KH PO and (NH ) HPO adsorbate solutions for Omuaran and Lanwa soils. The k constant values were higher in Omuaran, Erin-ile and Ilorin than in Lanwa, Odoowa and Malete for both treated and untreated samples. Additions of KCl to the adsorbate solutions enhanced phosphate adsorption by the soils studied. Soils with higher phosphate adsorption will require increased phosphate fertilizer application to satisfy the phosphate needs of the crops