Chemical studies on soil humic acids

Abstract The levels of amino acids, phenolic substances, sugars, and total hydrolysable organic matter of 37 humic acids obtained from various types of soils were determined after acid-hydrolysis. The levels of amino acids, hydrolysable phenols and hydrolysable organic matter increased from Rp(2) type to RP(l) type, and decreased with increasing RF values among Rp(l), Po. B, and A type humic acids. Very significant positive linear correlations were found among the levels of these acid hydrolysates. Hexose an duronic acid contents decreased with increasing RF values. The linear correlations between the hexose and uronic acid contents and RF values were very high for calcareous soil humic acids but low for the humic acids from other soils. Amino acids, phenols, and sugars were the main constituents in the acid hydrolysable fraction in humic acids. It was found that absorbance at 210 nm of the acidic supernatant of the hydrolysate was useful for determining the total organic matter content.