The aim of the study was to determine humus quality in hydromorphic soils of the island of Rab. The study was conducted on 16 samples collected from the anthropogenic top-soil horizon in which were determined total organic carbon according to Walkley-Black method (1934) and fractional composition of humic substances following the procedure given by Schnitzer (1982) using 0.1 M NaOH and 0.1 M Na4P2O7 as extraction solution. Spectroscopic characterization of isolated humic substances was carried out by measuring absorbance in VIS spectral range of 400-700 nm and optical indices (Q4/6, E4/6) were calculated. Investigated soils had sandy silty loam texture, acidic to alkaline pH, low content of physiologically active phosphorus and potassium, hydroameliorated by channels and were used as pastures. The total organic carbon content was low (0.95% in average) and varied from 0.42 to 1.86%. The average C/N ratio was 12.7. The humic acid content varied from 0.09 to 0.38% and fulvic acid content varied from 0.10 to 0.55%. The humification degree was low to medium, in average 21.5%. The average HA/FA ratio was 0.84 that indicated the domination of fulvic-humic type of isolated humic substances. The average optical indices Q4/6 and E4/6 were 5.92 and 5.62, respectively, which corresponded to the presence of more aliphatic and fewer aromatic compounds. The obtained results indicated low humus quality of investigated hydromorphic soils.

humic substances, humic acid, fulvic acid, humification degree, optical indices

Published by University of Zagreb, Faculty of Agriculture, Zagreb, CROATIA
Sponsored by Ministry Science and Education of the Republic of Croatia