Chemical studies on soil humic acids

Abstract

Relationships between elementary compositions and types of humic acids or the "degrees of humification were studied statistically using 39 humic acids prepared from various types of soils. Mean values of C%. H%. N%, 0%, H/C, N/C, and O/C of the different types of humic acids (A. B. Rp(l), Rp(2). and Po) were compared. The elementary compositions of various types of humic acids were proven by variance analyses to be significantly distinct. Linear regression analyses of C%, H%, N%. 0%, H/C, N/C, or O/C on RF or AlogK values were also carried out on 35 humic acids excluding the P_+~+++ type humic acids. The linear associations were found to be significant between C%-RF (5% level), H%-RF, H%-?logK, H/C-RF and HIC-?logK (0.1% level), N/C-RF(O.1% level). N%-RF(l% level), N%-?logK N/C-?logK(5% level), and ?logK (1% level), while no significant relationship was found with regard to C%;-?logK, 0%-?logK (both 5% level) and O/C-RF and O/C-?logK (l0% level). Carbon and oxygen contents of humic acids may be apt to reflect the different conditions of soils. The deeper the visible light absorption of humic acids and so the higher the degrees of humification, the lower the hydrogen contents of humic acids were. Though nitrogen content showed a trend similar to the trend of hydrogen content against RF values as a whole. the nitrogen content of less humified humic acids (Rp type) varied from very low to very high values. suggesting the enrichment of nitrogen into humic acid molecules in the early stage of humification. In the H/C versus O/C diagram, humic acids occupied an area reserved for the oxydation products of lignins. The area was fairly wide and “J”-shaped, that is, in the early to the middle stage of humification humic acids were arranged in the direction of dehydrogenation or demethanation, and in the later stage they were arranged in the direction of dehydration.