Based on their solubility, humus has been often divided into three fractions, i.e. fulvic acids, humic acids and humins. On the other hand, it seems to be generally accepted that humus can be divided into two groups (1) (2), non-humic substances and humic substances. The term humic substances (3) is used as a generic expression to describe the dark coloured, amorphous material in humus, or any of its above-mentioned fractions.
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Kyoichi Kumada (1965) studied this question.