A Study on the Nature and Composition of Humus in Major Soil Orders of Rajasthan

Eight profiles representing major soil orders in different agro-climatic regions of Rajasthan were studied for humus composition. Nature of humic and fulvic acid was characterized by optical density ratios (E₄/E₆) and coagulation threshold values. The Alfisols and Mollisols were characterized by the preponderance of humic acid whereas in Entisols and Vertisols less of humic acid and significantly higher amounts of non-humic and fulvic acid fractions were observed. Narrow E₄/E₆ ratios and lower threshold values of humic acids of Entisols, Mollisols, Alfisols and Vertisols of Banswara profile indicated their well condensed structure but wider E₄/E₆ ratio and higher coagulation threshold values of humic acid of Kota soil suggested their less condensed aliphatic structure. Fulvic acid of all the profiles studied had wider E₄/E₆ ratio and higher coagulation threshold values. Thus, the studies suggest that though there were differences in the composition of humus in different soils, the nature of humic and fulvic acids in these soils were not very much different except the humic acid of Kota and fulvic acid of Banswara rofile.