Use of humic substances as soil conditioners to increase aggregate stability

In many agricultural soils in the semi-arid and arid mediterranean climates: exposure to cyclic wetting and drying (WD) can reduce aggregate stability. The extent to which soil pretreatment with coal-derived humic substances (HS) can increase aggregate stability in soils exposed to cyclic wetting and drying (WD) was evaluated in this study. The soils studied are an Acireale silty clay loam from Sicily, a Principina silt loam from Tuscany and a Bovolone loam from Venetia in Italy. On each soil right rates of the HS (0, 0.001, 0.01, 0.05, 0.10, 0.50, 1.00 and 10.00 g/kg of soil) and four WD cycles (0, 3, 6 and 9) were used to assess any changes in Stability. On the Principina and Bovolone soils dominated by illitic and smectic clay minerals, successive WD cycles reduced aggregate stability. On the Acireale soil, dominated by kaolinitic clay mineral, after initial decrease in stability following three WD cycles, the aggregates regained stability as cyclic WD continued. Amending the soils with low rates of HS (equivalent to 100-200 kg/ha) not only improved aggregate stability significantly (p = 0.05) on all the three soils, but also reduced substantially the disaggregating effects of WD cycles. This work indicates that exogenous humic substances have a potential as soil conditioners in conservation practices aimed at increasing the structural stability of soils.