Abstract
Abstract Kumada (1) has proposed that humic acid can be grouped into four major types: A, B, Rp, and P types. Rp type is defined in his classification diagram of humic acid as the one located in the region where the value Δlog k is larger than 0.7 and RF is smaller than 40. This type of humic acid represents the main humic fraction of the following soils : Ao horizon of various forest soils, red-yellow soils and peat soils. Also, the humic fraction of farmyard manures (Kyuhi) and composts (Taihi) belongs to this type. It is presumed that Rp type humic acids are formed in the rotting processes of plant residues, and that this type of humic acid constitutes the initial stage of humification. This stage is labile and it undergoes further decomposition in soils, leading to the formation of other types of humic acid. It is therefore valuable to get information on the nature and properties of Rp type humic acid in relation to their role in soil formation and plant growth.
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