The amount of water-soluble organic substances varied with the kinds of green manure,soil types and incubation time.The water-soluble organic substances of decomposition products of green manure were the substances with lower molecular weight,different charges (positive and negative) and isoelectric points,containing carboxyl and amino groups.Some of these properties changed with time duration of incubation.These organic substances were the products derived from the interaction between organic and inorganic constituents (chiefly iron oxides) of soil.In the soils rich in organic matter,these substances could determine the redox potential of the system where they existed,and there was a good negative correlation between them.The appearance of Cr⁺³ resulted from the influence of organic substances (as the source of electron) on Cr⁺⁶.The reducing capacity of such decomposition products for astragalus was stronger than that for rice straw.