EFFECT OF PH ON PRECIPITATION OF HUMIC-ACID FROM PEAT AND MINERAL SOILS ON THE DISTRIBUTION OF PHOSPHORUS FORMS IN HUMIC AND FULVIC-ACID FRACTIONS

Humic and fulvic acid fractions were isolated from a mineral soil and a peat by adjusting the pH of the alkali extracts to a range of values from 0.2 to 2.5. Total inorganic and organic forms of phosphorus (P) in the acids were measured by chemical analysis and by P-31 NMR spectr oscopy. As the pH of precipitation of the mineral soil humic acid incr eased, there was an increase in the total P of the humic acid which re lated to the inorganic P component. In contrast with the peat, the inc reases observed in the pH range 0.2 to 1.5 were the result of changes in organic P. Using P-31 nuclear magnetic resonance spectroscopy, the ratio of inorganic to organic P as mono- and di-esters in the peat hum ic acid was found to increase from 1:4.8 at pH 2 to 1:19 at pH 2.5. In contrast with mineral soil humic acid, the ratio decreased from 1:6.1 at pH 0.2 to 1:1.3 at pH 2.5. The mono-ester to di-ester ratio was ab out 3 in the peat and 10 in the mineral soil and varied little with pH of precipitation. Phosphonates were detected only in the peat humic a cid precipitated in the pH range 1.0 to 2.0