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

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Abstract
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 carbon 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 carbon to fulvic acid carbon (Ch/Cf). Increasing the centrifugal force from 1150 × g to 8200 × g had little effect on Ch, but decreased Cf values. Dissolving and reprecipitating humic acid resulted usually in a decrease in Ch/Cf. Different shaking speeds and times, and extraction under nitrogen 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 carbon), Ch/Cf and Ca/Cf (carbon content of the polyphenols component of the fulvic acid fraction/fulvic acid carbon).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. Key words: Humic acid carbon, fulvic acid carbon, polyphenols carbon, extractable carbon, classification criteria

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