Effect of Heating on Some Physical and Chemical Parameters Related to Soil Aggregation and Erodibility

To clarify the effect of the heat wave that accompanies the passage of a fire on some physicochemical parameters related to soil erodibility, we subjected two soils, classified as a sandy loam and a silty clay, to an artificial heating in controlled conditions. The heating program was arranged according to the true thermal reactions occurring in the soils previously detected by differential thermal analysis. Heating up to 220°C had only little effect on organic matter content, particle-size distribution, and plastic and liquid limits. Beyond this value the organic matter was burned up, the soils lost their plasticity, and the textural sand fraction increased. The aggregate stability of both soils increased continuously also when the generally acknowledged cementing agents, such as the gel forms, the organic matter, and the organometallic cements, were destroyed. This means that heating promotes new forms of cementation in which iron and aluminum are involved in a process very similar to soil laterization. The porosity of the sandy loam soil decreased continuously, as the heating increased, the clearest evidence of this being in correspondence of the dehydration of the gel forms; on the contrary, the porosity of the silty clay soil increased up to 460°C. At this temperature the porosity sharply decreased, because the heating produced the loss of the OH groups from the clay materials and the internal structure of the clays collapsed.