Soil compaction and soil management - a review

Soil compaction is an important component of the land degradation syndrome which is an issue for soil management throughout the world. It is a long standing phenomenon not only associated with agriculture but also with forest harvesting, amenity land use, pipeline installation, land restoration and wildlife trampling. This review concentrates on the impact of soil compaction on practical soil management issues, an area not previously reviewed. It discusses in the context of the current situation, the causes, identification, effects and alleviation of compaction. The principal causes are when compressive forces derived from wheels, tillage machinery and from the trampling of animals, act on compressible soil. Compact soils can also be found under natural conditions without human or animal involvement. Compaction alters many soil properties and adverse effects are mostly linked to a reduction in permeability to air, water and roots. Many methods can be used to measure the changes. In practical situations, the use of visual and tactile methods directly in the field is recommended. The worst problems tend to occur when root crops and vegetables are harvested from soils at or wetter than field capacity. As discussed by a farmer, the effects on crop uniformity and quality (as well as a reduction in yield) can be marked. By contrast, rendzinas and other calcareous soils growing mainly cereals are comparatively free of compaction problems. The effect of a given level of compaction is related to both weather and climate; where soil moisture deficits are large, a restriction in root depth may have severe effects but the same level of compaction may have a negligible effect where moisture deficits are small. Topsoil compaction in sloping landscapes enhances runoff and may induce erosion particularly along wheeltracks, with consequent off-farm environmental impacts. Indirect effects of compaction include denitrification which is likely to lead to nitrogen deficiency in crops. The effects of heavy tractors and harvesters can to some extent be compensated for by a reduction in tyre pressures although there is concern that deep-seated compaction may occur. Techniques for loosening compaction up to depths of 45 cm are well established but to correct deeper problems presents difficulties. Several authors recommend that monitoring of soil physical conditions, including compaction, should be part of routine soil management.