THE EFFECT OF VEHICLE WEIGHT ON THE DISTRIBUTION OF COMPACTION WITH DEPTH AND THE YIELD OF LOLIUM TRIFOLIUM GRASSLAND/

Although research has demonstrated that traffic by vehicles on soils w ith high moisture contents causes subsoil compaction and a negative im pact on grassland productivity, knowledge is lacking regarding the int eractive effects of using a light tractor with a high number of passes or a heavier one reducing wheeling on the field. Research was conduct ed in 1994-95 in Buenos Aires State, Argentina to establish the effect of the tractor axle load on the distribution of subsoil compaction in dependently of the influence of ground contact pressure and to evaluat e the effect of densification upon grassland production. The main spec ies present were annual ryegrass (Lolium multiflorum Lam.) (60%) and w hite clover (Trifolium repens L.) (38%). The grassland was trafficked with two tractors of the same design and similar pressures on the soil contact area of the drive wheels, but with one of them having 1.8 tim es the mass of the other, 4.2 Mg and 2.3 Mg, respectively. The work wa s performed on a fine clayey, illitic, thermic Typic Argiudoll (Centen o series), at a high moisture content using three different numbers of passes for each tractor: one, five and ten. Cone index and bulk densi ty to 600 mm in the profile were measured to identify changes in soil compactness. Dry matter yield per hectare was used to indicate the res ponse of grassland production. Independent of pass numbers, the heavie r tractor always produced greater increases in bulk density for the 30 0-600 mm depth range. Grass responses to compaction were significant f or all treatments, either in the lane of the tractor, where the averag e decrease in dry matter production was 74%, or adjacent to tyre impri nts, where the grass yield was reduced by 18%. For the same number of traffic passes, the heavier tractor increased bulk density at the 300- 600 mm depth compared with the lighter tractor. However, the lighter t ractor, with a larger number of passes, produced as much or even great er compaction than the heavier tractor with fewer passes.