COMPACTION BY A COMBINE HARVESTER OPERATING ON MOIST, LOOSE SOIL

Soil dry bulk densities were measured in' and out of combine wheel tracks after harvesting spring barley and showed significant increases in the tracks compared with out of the tracks to 240 mm depth. The Scottish Centre of Agricultural Engineering soil compaction model predicted similar increases in bulk density, except where the initial bulk densities decreased with depth. No increases in bulk density were measured below a depth of 240 mm as the subsoil was already compact. The model predicted that only if the subsoil was deep-loosened would increases in bulk density occur in the subsoil. With the use of larger tyres on both axles, the model predicted benefits of reduced tyre inflation pressures and, hence, tyre/soil contact stresses and surface soil compaction. However, the model predicted no improvement to the subsoil compaction without reductions in the axle loads. The optional tyres which could be fitted to the combine harvester would also reduce the risk of tyre damage and would increase the purchase price of a new combine harvester by only 2%.