Summary
The review assessed, through international literature, the incidence and impact of soil compaction in cropping systems. Primarily the compaction studied is that imposed by vehicles working the land and tending and harvesting crops. The literature was further scrutinised to determine the likely outcome if the compaction exerted by vehicles were isolated into narrow ribbons within cropped fields, frequently described as controlled traffic.
Soil effects
During the compaction of soils energy is absorbed. This leads to an increased bonding between particles and aggregates making them more difficult to separate by tillage. Tillage of compacted soils therefore uses more energy, loses more moisture and often results in coarse, dry and unsatisfactory seedbeds that lead to poor crop establishment. Different types and intensities of tillage on compacted soils tend to have a very similar outcome, regardless of energy input. Further compaction of soil that is moist during these loosening processes is particularly damaging. Avoiding all soil compaction tends to avoid all the negative outcomes.
Wheel loads at the soil surface are now so high that it is increasingly difficult to keep pressures low enough to avoid stresses reaching deeper into the profile. Because these often exceed historic values (such as those created by in-furrow ploughing), they are changing the subsoil and reducing its ability to function.
It is clear from the literature that sands, sandy loams and some silt soils are more vulnerable to compaction than clays. They tend to have less natural structure that will resist loads and are more likely to develop an implement pan at operating depth, or a traffic and implement pan at ploughing depth. Although they are more easily repaired by cultivation, they have little ability to restructure naturally. Compaction at depth in all soils persists for long periods and on sandy soils often indefinitely. Mechanical loosening (usually subsoiling) is difficult to time correctly and if carried out successfully, may make the soil more vulnerable, often to a greater depth.
Compaction is particularly damaging in terms of drainage, aeration and erosion. This is demonstrated by improved infiltration (84%-400%) if compaction is avoided and through more plant available water (6%-34%). Because compaction reduces infiltration, runoff can increase by around 40% and this increases nutrient and soil loss by a similar amount, even in the presence of crop cover.
Wheel loads of just 5 Mg can reduce the saturated hydraulic conductivity of many subsoils by around 100%. On lighter soils this can happen even with lower loads and likewise topsoils can suffer a 4-5 fold reduction in conductivity. This decline is brought about because both pore size and number are reduced. Typical reductions in pore space due to traffic are around 10%, but up to 70% reduction at 0.5 m depth has been recorded. These decreases in pore size mean that there is less space for water and that water is held more tightly through capillary attraction. Fields therefore return to field capacity earlier but equally, they also run out of plant available water more quickly.
Other than very light firming of seedbeds, compaction has a negative impact on nutrient supply and mobility. Nutrient uptake is impaired through restricted crop rooting, lack of oxygen and greater losses (denitrification) from the soil system that can lead to diffuse pollution. Denitrification is greatest in wet conditions when fertilizer is applied to heavily compacted soils. Sediment losses triggered by compaction and associated poor infiltration increase the consequential loss of P & K in particular. Compaction is also likely to increase losses to the atmosphere in the form of carbon dioxide and methane. Overall, avoiding compaction can increase nutrient recovery by up to 20%.
Although cultivated soils contain less organic matter than virgin soils, the effect of compaction on soil organic matter (SOM) seems to be neutral. Within cultivated soils the level of SOM is determined primarily by cropping and this is confirmed by long-term trials, despite contrary research showing increased short-term loss with greater tillage intensity. Some experiments may not have taken full account of the redistribution of organic matter through the whole soil profile, which can be to a substantial depth, even with zero tillage. Where stratification of SOM in the upper horizon occurs, for example with minimum or no till systems, compaction can increase emissions of greenhouse gases such as nitrous oxide and methane because oxygen supply is reduced.
Because compaction increases the strength of both the soil mass and the aggregates within that mass, tillage for loosening the profile and creating seedbeds invariably needs more draught and energy. In rare prolonged dry conditions where tillage follows tillage without intermediate compaction, there may be little difference in energy requirements between traffic systems.
The effects of soil compaction on soil function and quality are almost exclusively negative. Light firming of loose seedbeds on the other hand is often beneficial, as may be more substantial firming of the profile in dry conditions when capillary rise of water from deeper in the profile can be enhanced.
Crop effects
Crop yield responses to the avoidance of vehicle compaction are invariably positive and range from 82-190% compared with conventional traffic systems. These variations can often be explained if appropriate factors are considered, all of which relate to the correct timing and supply of water, nutrients and air, both during and after crop establishment. These requirements are more likely to be satisfied if crop roots are able to explore the soil profile without hindrance. Excessive compaction tends to preclude this free exploration and the consequential reduction in water and nutrient uptake is often the cause of yield depression. There is a considerable body of evidence to suggest that wheel loads in excess of 5 Mg will cause a permanent 2.5% reduction in yield due to subsoil damage. Although many East European countries have identified optimum soil bulk densities for maximum crop production on different soils, no clear relationship between soil type, yield and compaction could be established from data in this review.
Machinery effects
Zero traffic reduced the draught requirements for shallow (10 cm) primary tillage by up to 60% and for mole ploughing (at 55 cm) by 18%. At intermediate depths (20-25 cm), zero traffic reduced implement draught by up to 48%. Energy demands for seedbed preparation fell by up to 87%, while power requirements for primary and secondary tillage were reduced by 45% and 47% respectively. Practitioners of controlled traffic in Australia have responded to these reduced energy demands by selecting smaller rather than larger replacement tractors. Wear on the soil engaging parts of implements is likely to be reduced in line with draught requirements, but no specific data on this subject were found.
Wheel tracks and soil erosion
In experiments designed to assess the optimum orientation (up/down or across slope) of controlled traffic wheelways, suspended sediment loss was 4.52 Mg ha-1 with across slope and 6.74 Mg ha-1 with up/down orientation. Soil loss on the other hand totalled 6.2 Mg ha-1 with across slope and 4.5 Mg ha-1 with up/down orientation. Similar relative soil losses were recorded at another trial site where the equivalent figures were 15.1 Mg ha-1 for across slope and 5.2 Mg ha-1 for up/down slope orientation. These data prompted the conclusion that up/down orientation may increase sedimentation losses but reduce total soil loss. None of the trials made a direct comparison with any conventional traffic systems. Separate infiltration and run-off (but not soil loss) data including traffic comparisons all suggested a lower potential for soil loss with controlled traffic.
Prediction of the potential for increased or decreased erosion from controlled traffic farming on vulnerable soils in the UK only provided information relative to existing practice rather than in absolute terms. Data suggested a 5-200 fold increase in infiltration on non-trafficked compared with trafficked soils; this implies a reduced risk of run-off and overland flow into permanent wheelways. It is also probable that intermediate but cropped permanent wheelways would moderate the concentration of any overland flow. Calculated flows down permanent wheelways based on directly intercepted rainfall result in relatively modest volumes whose erosive power can be estimated from established formulae.
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