An investigation was carried out to characterize and study the nature and properties of humic fractions of organic residues (FYM, Vermi compost, Poultry manure, green leaf manure and rice straw) upon incorporation in an Alfisol and a Vertisol, incorporation of organic residues also with RDF using maize as test crop in an Alfisol and a Vertisol (greenhouse study) and in a Alfisol (field experiment).

Humic acid production increased with incubation in all the treatments, while fulvic acid production initially increased up to 40 days and thereafter decreased. Production of humic acid was more when compared to fulvic acid in all the treatments. The humification index was found to increase with incubation. Among the sources of organic residues, humic fractions were maximum from the treatments involving vermicompost followed by poultry manure and farm yard manure while minimum was recorded in rice straw treatment.

As incubation proceeded there was an increase in total acidity, carboxyl group and phenolic-OH group. The functional group content of fulvic acid was found to be higher than humic acid at all the intervals of incubation. Maximum total acidity and carboxyl groups were contributed by treatment involving vermicompost followed by poultry manure and farm yard manure and minimum was recorded in rice straw treatment. The phenolic groups showed no major changes and the difference between the treatments was not significant.

With incubation percent organic carbon, C/N ratio, E4/E6 ratio of humic fractions decreased while total nitrogen content increased. The highest carbon content of humic acid was obtained in treatment involving rice straw and poultry manure while highest carbon and total nitrogen content of fulvic acid were obtained in vermicompost treated posts. The total nitrogen content of humic acid was maximum in the treatment involving poultry manure followed by vermicompost and minimum was observed in rice straw treatment. Humic fractions of Glyricidia recorded lower carbon content, whereas the humic fractions of rice straw showed the lower nitrogen content. The C/N ratio of humic acid were slightly higher than that of fulvic acid. The E4/E6 ratio of fulvic acid was higher than that of humic acid. The E4/ $6 ratio of humic fractions were maximum for rice straw followed by Glyricidia while minimum E4/E6 ratios were observed in poultry manure and vermicompost treatments.

In case of soil incorporation, control recorded the lower amount of humic fractions (HA and FA), whereas treatment involving incorporation of vermicompost, followed by poultry manure and farm yard manure showed higher amount of humic fractions. The total acidity and carboxyl groups were higher in treatment involving vermicompost, followed by poultry manure; while control recorded the lowest total acidity and carboxylic content. The organic carbon and total nitrogen content of humic fractions were minimum in control followed by RDF treatment whereas G/N ratio was maximum for the treatment involving incorporation of rice straw and control and minimum was recorded in vermicompost and poultry manure treatments.

The humic acid content, organic carbon and total nitrogen content, total acidity and carboxylic groups were higher in Vertisol amended with different organic residues, whereas fulvic acid content, phenolic groups and E4/E6 ratio were higher in Alfisol treated with different organic residues. The results were similar in field experiment and greenhouse study.

The production of soil humic fractions upon incorporation of organic residues were lower when compared to the incubation of different sources in the absence of soil.

The potentiometric and conductomertic titrations curves of humic fractions indicated a weak acidic character of these fractions and the UV-spectra of humic fractions was featureless, with a decrease in optical density with an increase in wavelength.

The organic carbon, available nutrients, available (DTPA) micronutrients and dry matter yield were higher in vermicompost treated pots/plots followed by poultry manure, farm yard manure, Glyricidia and rice straw.