THE HUMIC ACIDS EXTRACTED BY VARIOUS REAGENTS FROM A SOIL

Abstract

SummarySolutions of o.5N NaOH, o.1M pyrophosphate (pH 7), and o.5N Na(CO2−3/HCO−3) [2:1] extract humic acid and organic matter from a soil with decreasing effectiveness.Pre‐treating the soil with o.1N HC1 increased the yield of humic acid obtained with the alkaline extractants. An additional pre‐treatment with a mixture, which was normal with respect to HC1 and HF, gave a slight reduction in yield.Increasing the temperature of extraction increased the yield of humic acid.The total C extracted was usually in excess of the humic acid recovered. The difference was obtained as ‘humins’.The sum of the Fe2O3, SiO2, and A12O3 contents of the humic acids was always less than 2 per cent. Where the extraction was carried out at room temperatures the SiO2/Al2O3 ratio suggested that they might be present as clay mineral. When extraction was carried out at an elevated temperature this ratio was altered.Humic acids of low ash content (0.1–0.5 per cent) could be obtained by the use of hot reagents. Of all the extractants used at room temperature, pyrophosphate produced the humic acid of lowest ash content (∼ 0.2 per cent).The Fe2O3 content of the humic acids was not correlated with their SiO2 or A12O3 content.The N‐content of the humic acids was substantially independent of the method of extraction.The cation‐exchange capacities (C.E.C.), average pK values and range of pK values, have been determined from the titration curves of the humic acids. These quantities vary with the method of extraction.There are good correlations between cation exchange capacity and both average pK values and the range of pK values.The within‐molecule variation of pK values appears to be greater than the between‐molecule variation.No correlation exists between C.E.C. and Fe, Al, Si, and N content of the humic acids.