Characteristics of Humic Substances in Paddy Soils

In the present paper, the composition of humus and the charateHstics of humic acid from seven paddy soils were compared with those of upland (and / or natural) soils. Results show that :(1) in each group of the soil samples for comparison the HA / FA ratio of the humus of a paddy soil, in most cases, was appreciably higher than that of adjacent upland (and / or natural) soil derived from the same parent material; (2) the humic acid extracted from the paddy soils was characterized by a higher C/ O ratio, a higher content of methoxyl groups, and a lower content of carboxyl groups than those from the corresponding upland (and / or natural) flooded soils, implying that the humic acid formed under rice cultivation is in a lower degree of humification than that formed under upland (and / or natural) conditions; and (3) the humic acid of paddy soils, however, was not always characterized by a lower aromaticity than that of the corresponding upland (and / or natural) soils.