Department of Soil Science, G. B. Pant University of Agriculture and Technology, Pantnogar, Uttar Pradesh, 263145
*Present addresses: National Bureau of Soil Survey and Land Use Planning (ICAR), Seminary Hills, Nagpur, Maharashtra, 440006
**Present addresses: National Academy of Agricultural Research Management (ICAR), Rajendranagar, Hyderabad, Andhra Pradesh, 500030
Stability constants are indices which are indirect measures of availability of nutrients. Stability constants at pH 7 and 8 for humic acid metal complexes and at pH 6 and 7 for fulvic acid were determined at 30 and 40°C. The humic and fulvic acid metal complexes showed little variation in stability (log K values) with pH or temperature. In general, humic acid complexes showed higher degree of stability than the latter indicating greater number of active sites in the former ones. The order of stability of the humic acid complexes was: Cu2+>Zn2+>Ca2+>Mg2+ and Cu2+>Ca2+>Zn2+>Mg2+ in case of fulvic acid metal complexes, irrespective of pH or temperature of the equilibrated systems. Higher stabilities were observed in copper humic acid complexes (log K varying from 6.68 to 8.65), whereas less stability was seen in the magnesium fulvic acid complexes (lon K 2.51 to 3.87).
Stability constant, humic acid, fulvic acid
