Chemical studies on soil humic acids

The levels of alcoholic and phenolic hydroxyl, methoxyl, carboxyl, and carbonyl groups of 33 to 38 humic acids obtained from various types of soils were determined to analyze the relationships between the amounts of these functional groups and the degree of humification or the types of soils. The amounts of various oxygen-containing functional groups examined were all proven to be significantly different among the various types of humic acids by analyses of variance. During humification. generally, the carboxyl and carbonyl groups increased while alcoholic and phenolic hydroxyl and methoxyl groups decreased. Linear and logarithmic regression analyses of carboxyl group contents on RF values (optical density of the alkaline solution of humic acids at 600 nm) gave very significant positive correlations. The carboxyl group contents of Rp type humic acids and humic acids from calcareous soils were largely distributed in the upper side of the regression curve. The carbonyl group contents showed a very significant linear positive correlation with carboxyl group contents. and both of them showed high linear positive correlation with RF values. Phenolic hydroxyl group contents decreased with humification in Rp(l). B. and A type humic acids. Alcoholic hydroxyl group contents showed a significant negative linear association with _RF values. Methoxyl group contents decreased rapidly with increasing humification in low humified humic acids, and their negative correlation with _RF values were proven to be very significant by logarithmic regression analysis. The complicated relationship between oxygen content and RF value which was reported previously (7) has been accounted for by the results obtained in the experiments conducted here.