Effects on some clay soil qualities following the passage of rubber-tracked and wheeled tractors in central Italy

There is increased use of rubber-tracked tractors for ploughing on clay soi l (Vertic Cambisol) in central, south and insular Italy instead of metal-tr acked tractors, because they allow travel on public roads. Field tests were carried out on arable soil previously ploughed and harrowed to compare two types of tractors, one rubber-tracked (CAT Challenger Ch 45) and one wheel ed (New Holland 8770) in order to establish the compacting effects resultin g from 1 and 4 passes of the tractors in the same track. The following para meters were studied: soil penetration resistance, bulk density and its incr ement ratio, soil shear strength, soil macroporosity and hydraulic conducti vity. Multiple passes made by the two tractors induced very similar effects on the soil in regards to soil penetration resistance. Mean values of pene tration resistance (0-0.20 m depth) were 1.15 MPa for the rubber-tracked tr actor and 1.11 MPa for the wheeled tractor; mean values of penetration resi stance (0.21-0.40 m depth) were 1.07 MPa for the rubber-tracked tractor and 1.17 MPa for the wheeled tractor. The decrease in macroporosity, in partic ular that of elongated pores in the soil surface layer (0-0.10 m depth) was greater in treatments involving the rubber-tracked tractor (from 20.2 to 2 .7%) than for the wheeled tractor (from 20.2 to 10.3%). Following traffic o f the two tractors, hydraulic conductivity decreased and the following valu es were found for the five treatments: control, 18.48 mm h(-1); wheeled tra ctor 1 and 4 passes, 11.15 and 7.45 mm h(-1), respectively; rubber-tracked tractor 1 and 4 passes, 3.25 and 1.1 turn h(-1), respectively. Highly signi ficant correlations between shear strength and dry bulk density, and betwee n hydraulic conductivity and elongated pores and total macroporosity were f ound. Significant linear relationships between macroporosity and penetratio n resistance for 1 and 4 passes of both tractors were found in the soil lay ers (0-0.10 m). A significant difference was found between tractors and for correlations of penetration resistance values above control values. Howeve r, in the soil layer (0-0.20 m depth), with respect to the higher degree of macroporosity and low values of penetration resistance, treatments involvi ng wheeled tractor (1 pass) showed a lower degree of soil compaction than w as observed after 1 pass of the rubber-tracked tractor. (C) 2001 Elsevier S cience B.V. All rights reserved.