An IR spectroscopic study of soil humin and its relationship to other soil humic substances and fungal pigments

Humin, with a relatively low ash content, was isolated from a sandy soil by exhaustively dissolving humic and fulvic fractions in cold 0.5 M NaOH, and the mineral components in 20% HF. The infrared spectrum of the insoluble humin was similar to that of humic acid derived from the same soil, and resembled those of the humic-like pigments obtained from the soil fungi Coniothyrium minitans and Rhizoctonia solani. It was less like the spectrum of the pigment from Aspergillus niger which closely resembled that of the melanin pigment from Sclerotinia sclerotiorum. The brown debris remaining after extracting some of the fungi with NaOH, produced, on acid hydrolysis, fractions having IR spectra similar to acid hydrolysed humin. It is concluded from the similarity of humin, humic acid and fungal pigments, that well humified soil organic matter is to a large extent microbial in origin. When 14C-labelled humin from a sandy loam was incubated with fresh soil, about 20% of the radioactivity was lost after 3 months and 25% after 6 months, but subsequently there was virtually no further loss. A similar pattern of stability was obtained using 14C-labelled humic acid and this result, in conjunction with those from infrared spectroscopy, indicates a close relationship between humin and humic acid in terms of their structure, and their turnover in the soil. It is concluded that the composition of a soil humin is likely to be determined by the soil's microbial population.