Studies on Humus Characterisation and Distribution in Particle Size Separates in Some Typical Soils of Rajasthan

The distribution of humus fractions in various soil particle size separates and their characterisation by infra-red spectroscopy and differential thermal analysis in eight soil profiles of Rajasthan are reported. Infra-red spectra of fulvic acid showed a very sharp peak at 1725 cm⁻¹ band indicating preponderance of carboxyl and carbonyl groups. The very sharp and steep curves at 1630 cm⁻¹ band for humic acid and humin indicated the presence of aromatic C=C nucleus in these compounds. Differential thermogravimetry of these humus fractions showed three pyrolysis bands, relating to main reactions governing pyrolysis. Absence of high temperature peaks in humic and fulvic acids in some soils suggested openness of their structure.

The variation in the organic matter associated with particle size fractions was in the order: clay (30 to 80%), silt (9 to 82%) and sand (2 to 17%). In all soils, clay fraction accounted for bulk of organic matter with the exception of Ganganagar and Jaipur soils. In Banswara (black), Sirohi and Udaipur profiles, humic acid carbon dominated the clays in all the horizons. In Kota, Banswara (red) and Chittorgarh profiles, carbon found with silt and clay accounted for only small fraction of humus and most of it was either non-humic or infirm linkage with inorganic constituents. There was no definite relation of humus fractions with sand and silt but clay was significantly related with organic carbon and humic acid carbon.