LIMITATIONS OF ALKALI-EXTRACTABLE ORGANIC FRACTIONS AS BASES OF SOIL CLASSIFICATION CRITERIA

Soil samples, mainly from eastern and northern Canada, were extracted with sodium hydroxide-pyrophosphate, and the centrifuged extracts were fractionated into humic acid, fulvic acid and the polyphenolic component of the fulvic acid fraction. Details of the extraction and fractionation procedure were altered systematically, and the effects of these changes on amounts of C in the various fractions were studied. A tenfold increase in the ratio of soil to extracting solution resulted in a marked increase (nearly double in some cases) in the ratio of humic acid C to fulvic acid C (Ch/Cf). Increasing the centrifugal force from 1150 .times. g to 8200 .times. g had little effect on Ch, but decreased Cf values. Dissolving and reprecipitating humic acid usually resulted in a decrease in Ch/Cf. Different shaking speeds and times and extraction under N rather than air had minor, if any, effects on the Ch/Cf ratios. Most of the podzolic B horizons tested could be distinguished from the other 60 horizons by the combination of C ext (extractable C), Ch/Cf and Ca/Cf (C content of the polyphenolic component of the fulvic acid fraction/fulvic acid C). Extractable organic fractions have some promise as bases of soil classification criteria, but their potential is limited at present. The procedures are time-consuming and exacting, and results are not comparable between laboratories. Rigorous standardization of methods and interlaboratory comparisons of data might lead to more useful criteria based on organic matter composition.