The water-soluble decomposition products of different plant materials and soils were separated into three fractions according to their molecular weight, namely smaller than 500,500-1000 and larger than 1000, and the concentration of complexing agents and the stability constants with Fe²⁺ or Mn²⁺ ions were determined. The three fractions accounted for 31%,36% and 33% on average of the total cotnplexing agents respectively, the fraction of a larger molecular weight was higher in proportion in the decomposed materials of plants having more stems. The stability constants of Fe(Ⅱ)-and Mn(Ⅱ)-complexes for the larger molecular weight fraction were comparatively larger by an order of magnitude of 1-2. The logk values for Fe(Ⅱ)-complexes were larger than the corresponding Mn(Ⅱ)-ones.The complexing agents in the decomposition products of plant matericals carried both negatives charges and positive charges. Fe(Ⅱ)-complexes showed a high stability against air-oxidation.