SUBSOIL COMPACTION BY VEHICLES WITH HIGH AXLE LOAD EXTENT, PERSISTENCE AND CROP RESPONSE

Extent and persistence of soil and crop responses to subsoil compaction caused by vehicles with high axle loads are reviewed and methods to protect the subsoil from permanent deterioration are discussed. Traffic by vehicles with high axle loads on soils with high moisture contents generally causes deep subsoil compaction. At an axle load of 10 Mg, compaction typically penetrates to a depth of 50 cm. With still higher loads, compaction to a depth of 1 m has been reported. Subsoil compaction is very persistent. At depths of more than 40 cm it is virtually permanent even in clay soils in regions with annual freezing, Deep subsoil compaction also causes persistent and possibly permanent reductions of crop yields. Complete amelioration by mechanical loosening is usually impossible and definitely expensive. From a soil productivity point of view, limits for mechanical stresses in the subsoil are needed. These may have the form of axle load limits for the vehicles or a combination of limits for the axle load and for some other important factors, such as the ground contact pressure of the running gear or the per cent water saturation of the soil at the time of trafficking. Guidelines for such limits should preferably be worked out in an international joint effort.