Characters of humus formed under rice cultivation

To characterize the nature of humus in paddy soils, a comparison was made between the properties of humus in paddy soils and those in adjacent unflooded arable soils. Rice cultivation generally brought about a considerable increase in organic matter and in the PQ-value, with the exception of Andosol-paddy soils in which organic matter tended to decrease somewhat and the PQ-value remained virtually unchanged. The humification degree of humic acid as judged from Δ log k and RF values was generally lowered by rice cultivation except in the case of Yellow soil-paddy soil in which humic acid was originally low in the degree of humification. The accumulation of poorly humified humic acid may be a characteristic feature common to all paddy soils. These changes by rice cultivation are observed only in the upper part of the profiles, and seem to be associated with seasonally flooded conditions ot paddy soils. Iron oxides accumulated in subsurface soil have virtually no effect on the properties of humus.