Transformation of Humus Composition in Soils Derived from Limestone at Akiyoshi Plateau in Yamaguchi Prefecture. I. Humus Composition of Soils on Surface Horizon

Humus composition of surface soils derived from limestone at Akiyoshi Plateau was determined by the method of Kumada et al. (2). Fourteen soil samples were classified into 3 groups according to the soil pH, i.e., group A(pH: 7.1-8.0, 2 samples), group B(pH:6.1-7.0, 5 samples), and group C(pH:5.0-6.0, 7 samples). The humus omposition in these groups was compared each other. The results obtained are as follows: 1) The total amount of humus was generally larger in group B than in group A and C. 2) In general, the ratio of extracted humus to total gumus decreased in the order of group C,A, and B. However, the reverse was true for the ratio of humic acid to extracted humus. 3) Humic acids in group A belonged to R_P・P_O or B type and most of humic acids in group B and C belonged to B･A type and P_O・ Atype, respectively. 4) At lower soil pH, the humus was largely free form.