Critical factors for successful subsoiling
When a decision has been made to break up the subsoil, several factors then need to be considered. The soil moisture content needs to be in the optimum range. The depth and interval between tines should be chosen after examining the effects of a test run. If done "blind" without any examination of the effects, the effort may be wasted, or even result in worse soil structural conditions.
1. Soil moisture content
If the soil is too wet, the subsoiler will not lift and crack the soil but will instead tend to create more compaction. If the soil is too dry, large blocks of soil will be lifted but not cracked. The best time to subsoil is in spring or autumn when the soil is moist and friable. In this condition, a small block of soil will crack or crumble when pressed between the fingers.
2. Critical depth
For each combination of implement type and soil conditions, a particular depth (critical depth) exists for which subsoiling is most effective. When a subsoiler is operated at the critical depth, a crescent-shaped pattern of soil disturbance is created (Figure 6). If, however, the implement is operated below its critical depth, the amount of soil loosening is much smaller and soil around the passage of the tines can be compacted (Figure 6). The depth of subsoiling should be such that crescent-shaped disturbance is achieved. This can be gauged by digging a number of holes after test runs.
Figure 6: Pattern of soil disturbance resulting from subsoiling (left) at the correct depth and (right) below the correct depth
3. Tine spacing
In addition to creating a crescent-shaped loosening, a subsoiler should also produce a uniform pattern of soil disturbance (Figure 7). Where the soil is loosened only around the passage of tines, the tine spacing is too wide. The optimum interval can be gauged when holes are dug after the test runs. As a rough guide, horizontal tine spacings on conventional subsoilers should not be greater than 1.0-1.5 times the operating depth. Spacing of up to 2.0 times the working depth may be satisfactory for some winged implements.
Figure 7: Patterns of soil disturbance after subsoiling with (a) the correct and (b) too-wide tine spacings
Crop response to subsoiling
Subsoiling can result in penetration of roots into the subsoil so that maximum root depth and total root length of crops is substantially increased. This process increases the amount of water and nutrients available to the crop since it can extract water from the subsoil.
- Under non-irrigated conditions, increased water uptake, after subsoiling can lead to significantly higher crop yields.
- Under fully-irrigated conditions, subsoiling responses can be insignificant since, with frequent water applications, extraction of water from the subsoil is much less important.
Extent of the problem
Some degree of smearing and the formation of a thin dense layer at the base of the cultivation depth is almost inevitable on arable land as it is not always possible to cultivate when soil moisture is at optimal levels. Such a phenomenon is, therefore, likely to be present to some extent in most soils where arable farming or market gardening is a predominant land use and conventional cultivation practices are used. However, varying the depth of cultivation from year-to-year will often overcome such effects. Whether or not subsoiling will be beneficial depends on the extent to which a compacted plough pan has developed and subsequently inhibited crop growth, which, in turn, is dependent on individual paddock history. The extent to which subsoil compaction has developed can only be determined by examining the soil profile in that particular paddock.
Impact on farming
The impact of soil structural decline and soil compaction on the profitability of arable farming is difficult to gauge. As outlined in previous sections, the effects will be specific to individual paddocks. Even so, some broad generalisations on the susceptibility of arable land can be made on a regional basis. The maps on pages 18 and 19 give a broad outline of the susceptibility of arable land to soil structural degradation under poor management. The maps were generated based on data from the New Zealand Soils Database (Landcare Research) and from published New Zealand Soil Bureau maps. It takes account of factors such as texture, clay mineralogy, organic matter content, topsoil depth, aggregate stability, plastic limit, drainage and drainability and water balance. As expected, most land used extensively for arable production is susceptible to degradation under poor management.
More detailed maps for specific regions can be produced where an adequate regional database is available. Such an exercise was carried out (T G Shepherd of Landcare Research) for the Manawatu-Wanganui region and the resulting map is shown on page 20. The map was generated by considering how a number of key soil properties (similar to those used to prepare the New Zealand maps plus wind erosion susceptibility) are known to change at sites where specific studies of cropping practice have been carried out. Note that the Manawatu coastal soils shown on page 18 have a higher susceptibility to degradation when wind is taken into account (page 20). The soils have been grouped into three categories. Some 243,700 ha (64% of the arable land) are highly susceptible to structural breakdown and compaction. A further 84 200 ha (22%) are moderately susceptible. Only 55 300 ha (14%) have a low susceptibility. Scientific investigations suggest that in cases where soils have been severely compacted by poor cultivation practices, at least 20 years under well-managed ryegrass-clover pasture are required to restore their original structure. It is significant that two-thirds of the region’s arable land is highly susceptible. It is important to note that the map refers to the susceptibility of soils to structural breakdown and compaction. The degree to which such breakdown has or will actually occur is principally dependent upon management practice. Farmers, therefore, need to consider soil management practice as an integral part of their management strategies if they are to sustain economically viable cropping.
Economic cost
As discussed in previous sections, the detrimental effects of soil structural breakdown include:
- increased fertiliser requirements,
- increased irrigation requirements,
- poor seed germination and emergence,
- poor plant growth and vigour,
- delays in sowing,
- resowing of poorly performing paddocks and subsequent uneven ripening of crops,
- increased susceptibility to root diseases and pest attack,
- delays in harvest, and
- reduced crop yields and grain quality.
Overall, these factors lead to increased costs because of an increased number of tractor passes for seedbed preparation and sowing, and increased fertiliser and irrigation requirements. In addition, revenue is lowered through decreased crop yields. For example, under maize in the Manawatu, the structure of some soils has been destroyed by inappropriate equipment or poorly timed cultivation. On severely compacted soils:
- tillage costs (hours required for cultivation and cost of running equipment) are up to 340% higher,
- fertiliser costs (extra amount needed to ensure crop growth) are up to 280% higher,
- yields are up to 45% lower, and
- profit margins are up to 100% lower,
- with soils where a good tilth has been maintained.
On the severely compacted soils, production costs are typically higher than $2500/ha. Maize yields, of 7.2 to 8.3 t/ha, are at or slightly above the 6.9 to 7.8 tonne that farmers need to break-even at current crop prices. On the soils where good tilth has been maintained, careful cultivation keeps production costs down to $2000/ha or less, and sustains yields at 12.2 t/ha or higher. These figures equate to a gross return of more than $3100/ha, and a profit margin of more than $1100.
Similar impacts to those cited above have also been measured on susceptible soils in several other regions. For instance:
- 23 to 43% declines in maize yield under continuous cropping with conventional tillage in the Waikato,
- 30% decline in squash yield on soil cultivated and harvested when too wet, Gisborne, and
- 5-25% declines in wheat yield under continuous cropping in Canterbury and North Otago.
The cost of compaction can also be gauged from the benefit that amelioration of the problem has (e.g. subsoiling a paddock). Under dryland conditions in North Otago where subsoil compaction restricts water availability to the crop during summer, subsoiling has been shown to give a 0.7-1.2 t/ha increase in wheat and barley yields and a 0.8-1.5 t/ha increase in pea yields. An increase in yields of 1.0 t/h for milling wheat, malting barley and peas would result in respective increases in income of about $280, $220 and $300/ha. By contrast, the additional cost of subsoiling would be about $150/ha.
Compaction can usually be traced to pressure applied to the soil when it is wet and soft. Compaction below the depth of cultivation can result in a compacted soil layer in the subsoil (see above). (Photo from British ADAS/MAFF collection.)
To deal with subsoil compaction a subsoiler is pulled through the soil loosening compacted layers by lifting and cracking them (see below). (Photo by K C Cameron, Lincoln University.)
Susceptibility of arable land to soil structural degradation under poor management
(click thumbnail for full map)
Compiled from the 1:1000000 DSIR soil map, by T.G. Shephers (Landcare Research Palmerston North) and D.L. Hicks (Ecological Asssociates Auckland), June 1994.
Susceptibility of arable land to soil structural degradation under poor management
(click thumbnail for full map)
Compiled from the 1:1000000 DSIR soil map, by T.G. Shephers (Landcare Research Palmerston North) and D.L. Hicks (Ecological Asssociates Auckland), June 1994.
Susceptibility of arable land to soil degradation under continuous cultivation: Manawatu-Wanganui region
(click thumbnail for full map)
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