EFFECTS OF PHOSPHORUS CONCENTRATION AND pH ON PHOSPHATE RETENTION BY ACTIVE ALUMINUM AND IRON OF ANDO SOILS

Abstract

We measured phosphate retention by 18 Ando soil samples at different pH values (4 to 8.5) and phosphorus concentrations (10-3 to 2 M). A general regression equation held for each soil: log Y = α pH + β log [C] + c, where Y = phosphorus retention and [C] = phosphorus concentration. By multiple regression analysis, the Y values were correlated with the status of aluminum and iron analyzed by selective dissolution using pyrophosphate, dithionite-citrate, and oxalate-oxalic acid as reagents. Aluminum bound with humus, Alpyr, reacted with phosphate depending little on pH and markedly on phosphorus concentration, whereas aluminum and iron in allophane, (Al + Fe)oxa, depending markedly on pH and moderately on phosphorus concentration. Aluminum and iron in allophanelike constituents (Al + Fe)dit-pyr showed features similar to aluminum bound with humus. The molar reactivity of aluminum and iron toward phosphate (P/Al or Fe) at 0.5 M increased in the order: (Al + Fe)dit-pyr (0.3) < (Al + Fe)oxa (0.4) < Alpyr (0.5) < Fepyr (0.8) at pH 5.0, and (Al + Fe)oxa (<0.1) < (Al + Fe)dit-pyr (0.3) < Fepyr (0.5) < Alpyr (0.6) at pH 8.0.