Conclusions
Maintaining soil physical condition is an important consideration for a sustainable farming system. In any agricultural system, the most important aspect is to be aware of the potential problem so that preventative and/or remedial action can be taken. The use of this report along with participation in associated field days will increase this awareness. In systems where the soil is cultivated once or more per year (e.g. arable farms and market gardens), breakdown of soil organic matter can be a problem. Since an important role of organic matter is to bind soil aggregates together, this can result in a breakdown of soil structure and the formation of a compact plough layer. Remedial measures for such an occurrence include minimising the number of cultivations and employing measures to build up soil organic matter (using grass pasture in rotation, adding bulky organic matter, retaining crop residues and using green manures).
Soil physical condition is best assessed by digging a number of holes and examining in detail the soil profile to a depth of about 50 cm. In arable soils where structural decline has occurred, the cultivated layer is characterised by the presence of large massive clods with smooth dense faces often accompanied by a significant amount of fine structureless material. Structural decline is also indicated when the upper soil layers appear dense with weak root growth, low earthworm density and low organic matter content. Soil compaction is characterised by a high density in the compacted region which is indicated by a smooth shiny soil surface and can be further identified by probing and levering at the soil surface with a knife. Subsoil compaction can be identified by an absence of pores, fissures, penetrating roots or earthworm holes within the compacted region and accumulation of root growth immediately above the dense layer.
Soil compaction on arable soils can usually be traced to pressure applied to the soil when it is wet and soft. The agents of compaction include driving and trailed wheels, plough soles, disc edges and rotary blades. Loosening compacted top soils is a generally accepted role of conventional cultivation. Compaction below the depth of normal cultivation can, however, be a concern. Root growth is normally restricted to the soil above the compacted layer and this can then limit crop growth and yields. The major ways of preventing subsoil compaction from developing are to avoid working the soil when it is wet and varying the depth of cultivation from year-to-year. Where subsoil compaction is identified as a problem, subsoil tillage can have beneficial effects on crop growth and performance. For subsoiling to be effective, soil moisture content needs to be in the favourable range (not too dry or wet but moist and friable). The optimum depth and interval between subsoiler tines is also required if subsoiling is to be effective.
For pastoral soils, compaction can occur through the treading actions of grazing stock (pugging). It occurs when the soil is so soft that the hooves of grazing animals cannot be supported by the surface and they press into the soil. An important point is that treading effects tend to be "self perpetuating". If a soil is partially pugged in early winter then water will tend to remain on the surface for longer. As a result, the soil will remain softer and wetter for longer and hooves of grazing animals will do more damage at subsequent grazings. Where natural regenerative processes (i.e. wetting and drying cycles, growth and death of roots and burrowing by earthworms) are slow and compaction has been caused by heavy stocking over a number of years, subsoil tillage (aeration) can be beneficial. Nevertheless, where water logging of pasture is a persistent problem, drainage of the paddock is the first priority.
On orchards, some degree of compaction in the alley area is inevitable below wheel tracks since tractors, trailers and heavy spray equipment frequently pass up and down. For crops with shallow but extensive roots that run laterally into the alley (e.g. kiwifruit and most berry fruits) such compaction is a potential problem. Where compaction is severe, extension of roots into the alley area can be restricted. For crops which naturally have a fibrous root system concentrated below the tree canopy such compaction is less of a problem. Even so, over-irrigation, particularly in young orchards where crop water use is low, should be avoided since it can maintain the alley area in a wet state thus encouraging compaction to occur in wheeltracks. Where wheeltrack compaction has occurred, little can be done to alleviate the problem. Careful soil management of the row area where most roots are restricted is the practical answer.
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