Abstract
Humic substances (HS) are the most abundant organic constituents present in soil and aquatic environments. These substances result from a humification process that involves microbial and chemical transformation of organic debris. On the basis of varying solubility in acid and alkali media, the HS are divided into three fractions: (i) humic acid (HA)-alkali soluble but acid insoluble, (ii) fulvic acid (FA)-soluble in both alkali and acid, and (iii) humin-insoluble in both alkali and acid. Information on the nature of HS synthesized in a fixed cropping sequence is lacking. This study was conducted during the 28th year of a long-term fertilizer experiment established in 1971 with a rice-wheat-cowpea sequence on virgin land classified as Mollisol at Pantnagar (29o N, 79o 3' E) in northern India. The initial soil characteristics were pH 7.3, EC of-0.35 dS m-1, and -1.48% organic C (Nand Ram 1995).
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