Changes in some physical properties of a clay soil in Central Italy following the passage of rubber tracked and wheeled tractors of medium power

The aim of this study was to evaluate the compacting effect of rubber tracked tractors in comparison to that of the traditional wheeled tractors. Macroporosity, pore shape and size distribution, bulk density, penetration resistance and saturated hydraulic conductivity were analysed in a clay soil (Vertic Cambisol) near Rome (Italy) following one and four passes on the same track of rubber tracked and wheeled tractors of medium power. The soil structure attributes were evaluated by characterising porosity by means of image analysis of soil thin sections prepared from undisturbed samples. Macroporosity decreased in the 0–10 cm layer of compacted soil, particularly after four tractor passes, due to a large reduction in the proportion of elongated pores and in their vertical continuity. The rubber tracked tractor had a more pronounced compaction effect in the surface layer (0–10 cm) than the wheeled tractor both after one and four passes; the latter treatment producing the lowest soil porosity. The same trend was observed for hydraulic conductivity, which showed a highly significant correlation with elongated pores. In the 10–20 cm layer the porosity was significantly decreased following traffic, apart from in the soil under one pass of the rubber tracked tractor. Again in this layer, the lowest values of porosity were found in soil after four passes of the rubber tracked tractor. Single and multiple passes made by the two tractors induced different effects regarding soil penetration resistance and bulk density. Increment ratio of penetration resistance after tractor passes with respect to the control was: 12.5 and 49.9% with the wheeled and 34.4 and 39.8% with the tracked after one and four passes, respectively. Increment ratio of dry bulk density values after tractor passes with respect to the control was 7.9 and 11.7% with the wheeled and 7.5 and 8.3% with the tracked after one and four passes, respectively. The tractor passes transformed the initial subangular blocky structure into a massive structure with sometimes a platy structure in the upper few centimetres. The results indicated that soil compaction following traffic with the rubber tracked tractor was generally the more pronounced. However the compacting effect of this tractor after one pass seemed to be limited to the surface layer only.