Effects of coal derived humic substances on water retention and structural stability of Mediterranean soils

The potential for using coal-derived humic substances to improve the available water holding capacity (AWC) and aggregate stability of typical Mediterranean soils was evaluated in the laboratory using an agricultural surface (0-20 cm) soil from each of three regions of Italy, (Sicily), Tuscany and Venetia) and five rates of humic acids (HA), 0, 0.05, 0.10, 0.50 and 1.00 g/kg. There were significant (P < 0.05) differences between the field capacity (FC), permanent wilting point (PWP), and available water capacity (AWC) values of the controls and those treated with 0.05 g/kg of the HA. Beyond this rate, differences in these properties were not significant. At the 1.00 g/kg HA rate, the relative improvements in AWC over the three controls were 30%, 10% and 26%. Low rates (0.05 to 0.10 g/kg) of HA were also needed to obtain a 40 to 120% improvement in aggregate stability of these soils relative to the controls. These results indicate that the addition of highly humified organic matter such as coal-derived humic substances can improve the structural and water retention properties of degraded arable soils. However, since there is not yet any direct evidence that these humic materials can ameliorate soils under field conditions, field studies will be needed to validate these results.