Subsoil compaction
Loosening of compacted topsoils is a generally accepted role of conventional cultivation. Where slumping and hardsetting in the cultivation layer are a problem then management needs to be changed. Compaction below the depth of normal cultivation is, however, a particular concern. Root growth is normally restricted to the soil above the compaction zone and this can then limit crop growth and yields (Figure 3). It is preferable to try and avoid the problem rather than try and fix it once it has happened. The four main considerations are:
- compaction can usually be traced to pressure applied to the soil when it is wet and soft. It is therefore important to avoid working the soil when it is wet,
- contact pressure on the soil is increased directly with the inflation pressure of the tyre. Reducing inflation pressures may reduce the risk of compaction,
- with a given load, using larger tyres or fitting dual wheels during cultivation reduces contact pressure on the soil and reduces compaction. However, it should be noted that the larger contact area also ensures that if compaction does occur it happens to a greater depth, and
- the depth of cultivation should be varied from year-to-year. This will help avoid formation of a plough-pan.
Subsoil compaction can have varying effects on crop performance depending on the weather and, in particular, the dryness of the season.
- Where roots are restricted in their downward growth by compacted soil, water reserves below the compacted layer are unavailable to the crop. Thus, under dryland conditions, crops may suffer from water stress and this can cause yield depressions.
- Under such conditions, irrigation water needs to be applied in small amounts at frequent intervals to avoid topsoil waterlogging. This requires a high level of management, particularly in dry years.
- Under wet conditions, drainage may be impeded by the compact layer and this may lead to saturation of the soil in the plough layer. Topsoil waterlogging and associated root diseases may restrict crop growth.
Figure 3: Growth of wheat in (a) a well-structured soil and (b) a soil containing a compacted subsoil layer
The role of subsoiling
To deal with subsoil compaction, a subsoiler is required. Subsoiling (also called ripping, chiselling or aerating) is the loosening of the soil below the depth of normal cultivation using a rigid tine instrument. The instrument can be of "conventional" or "winged" construction (Figure 4). They are normally operated at depths of 30-50 cm.
When a subsoiler is pulled through the soil it loosens compacted layers by lifting and cracking them, creating a network of interconnected pores (Figure 5). Many of these pores extend from the depth of loosening up to the soil surface. They can, therefore, act as pathways for root penetration and for transmission of water and air.
Subsoiling is expensive in terms of power, fuel and labour and should be reserved for paddocks or areas of paddocks where evidence justifies its use. Several holes should be dug in the paddock to a depth of 50 cm and a detailed examination of the pit wall made to determine the presence or absence and/or extent of a compacted subsoil layer.
Figure 4: There are two main types of subsoilers - conventional tined and winged tined instruments
Figure 5: Loosening a compacted plough pan with a winged tine subsoiler. Redrawn from McLaren and Cameron (1990)
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