The aim of this research was to evaluate the change in physical properties of an arable clay soil following passage of rubber- and metal-tracked tractors in view of increased use of tracked tractors for ploughing on clay soil in centre-south and insular Italy. Field tests were carried out on arable soil cropped to lucerne (Medicago sativa L.) to compare two types of tracked tractors, one with rubber tracks (CAT Challenger Ch35) and one with metal tracks (FIAT FA 150) in order to establish the compacting effects resulting from one and four passes of the tractors on the same track. The following parameters were studied: soil penetration resistance, bulk density and its increment ratio, pressure distribution on soil along the length of the tracks, soil macroporosity and hydraulic conductivity. Multiple passes made by the two tractors induced Very similar effects in the surface soil in regards to soil penetration resistance and dry hulk density. Mean values of penetration resistance (0 to 0.20 m depth), were 2425 kPa for rubber tracked tractor and 2415 kPa for metal-tracked tractor; dry bulk density values were 1.52 and 1.48 Mg m(-3), respectively. The decrease in macroporosity, in particular that of elongated pores in the soil surface layer (0-0.10 m depth) was greater in treatments involving the rubber-tracked tractor (from 10.6% to 4.0%) than for the metal-tracked tractor (from 10.6% to 7.3%). Following traffic of the two tractors hydraulic conductivity decreased and the lowest values were found after one and four passes of the rubber-tracked tractor (1.5 and 0.08 mm h(-1), respectively). A highly significant correlation between hydraulic conductivity and elongated pores and a significant correlation with total macroporosity were found. Significant exponential relationships between macroporosity and penetration resistance for one and four passes of both tractors were found in the surface soil. A significant difference was found between tractors and for correlations of penetration resistance values above control values. However, in the soil surface with respect to the higher degree of porosity, treatments involving the metal-tracked tractor showed better soil structure quality. (C) 1998 Elsevier Science B.V. All rights reserved.
