Introduction
Compaction reduces soil porosity, thereby restricting water and air movement in the soil profile. It affects seed germination, impedes root growth, restricting nutrient and water uptake, leading to reduction in crop canopy area which then affects yield and quality.  Increased fertiliser application and irrigation will partially overcome the effects of soil compaction but will increase input costs.   Plants in compacted soils are also less resistant to pests and disease.

Poorly drained soils are more at risk of compaction, which in turn increases the risk of waterlogging, erosion and runoff.

The use of increasingly larger and heavy machinery in agriculture over the years has increased the mechanical surface load exerted on soils, often resulting in compaction.  Compaction of the topsoil horizon is created mainly by high ground pressure and can be alleviated by wider/low pressure tyres.  In ploughed fields a hard pan (10-20cm band) is frequently formed below the topsoil, at just below plough depth.  Soil compaction is more persistent the deeper it gets.

Investigation of the soil profile to identify areas of compaction is fundamental and remedial actions should be based on the findings.  A spade is an old fashioned but very useful tool.

Compaction as it applies to different sectors

Potatoes
The potato crop is highly sensitive to compaction as it impedes root growth which restricts nutrient and water uptake, leading to reduction in crop canopy affecting yield and quality. Increased fertiliser application and irrigation will partially overcome the effects of soil compaction but increase input costs.

• A survey by Cambridge University Farms of 602 commercial fields between 1992 and 2004 revealed that two thirds of fields have a serious impediment to rooting within the potential root profile
• In a BPC survey of 200 growers:
  • Compaction was identified as the most likely soil structural problem for potato growers
  • Compaction was often a result of having to cultivate land for seedbed preparation in less than ideal conditions.

Horticulture
Compaction reduces soil porosity, restricting water and air movement in the soil profile. It affects seed germination, root development, plant growth, yield and quality.  Plants in compacted soils are less resistant to crop diseases and pests and there is a reduced uptake of soil nutrients and response to applied fertilisers.  Poorly-drained soils are more at risk of compaction, which in turn increases the risk of waterlogging. Investigation of the soil profile to identify areas of compaction is fundamental. The most important soil cultivation operations are those that minimise and remove compaction. Bed formers and powered cultivators should be set to run as slowly as possible to avoid over-cultivation and pulverising the soil more than necessary. Once soils have been loosened by cultivation they are prone to compaction. This can be minimised by use of low ground pressure tyres or cage wheels which can be fitted to tractors or other equipment, or tracked vehicles can be used. Timely subsoiling to break up soil pans will reduce the risk of waterlogging.

Cereals and Oilseeds
The use of increasingly larger and heavy machinery in cereal and oilseed cropping has increased the mechanical surface load exerted in fields.  This creates stress on the soil which is reflected in compaction - the soils reaction to stress.  Compaction of the topsoil horizon is created mainly by high ground pressure and can be alleviated by wider/low pressure tyres.  In ploughed fields a hard pan (10-20cm band) is frequently formed below the topsoil, at just below plough depth.  This type of compaction is detrimental to infiltration and root growth and to the activity of those soil organisms that contribute to soil quality and are responsible for the soil formation processes.  Soil compaction is more persistent the deeper it gets and may require remedial actions every three or so years to minimise its impact on crop production.  But the need for remedial action must be based on soil examination.

Sugar
With sugar beet being grown on lighter soils it is essential that the roots can penetrate to depth and exploit the available water in the subsoil. If pans have been created during cultivations or harvesting of previous crops rooting will be impeded. It is therefore essential that the soil in fields where the crop is to be grown is examined for the presence of pans.  If pans are identified remedial action must be taken. If pans have not been disrupted, their existence may be spotted as areas showing wilted crop on hot days in summer.

Compaction affecting seedling emergence can occur during seedbed preparation.If too many cultivation passes take place on soils which are slightly too wet for cultivation. In this case, judgments have to be made as to whether a delay to sowing will be more deleterious than the possibility of restricting early root growth.

Beef and Lamb
As with other agricultural sectors, machinery has become larger and heavier, so there is an increased risk of compaction when harvesting forage.  Consideration needs to be given to the timing of harvesting, for example could harvesting be earlier or later in the year to avoid soil damage?  Once the threat of compaction has been realised then practices can be altered to avoid or minimise further compaction.  Yields from compacted soils will be reduced, so it makes economical sense to avoid compaction.

Dairy
Maize is extremely susceptible to compaction with crops grown on compacted soils unable to develop a deep root structure. A yellow stunted crop with poor yields is evidence that compaction is likely to be a problem. Subsoiling  is carried out as part of routine cultivations on some farms especially where continuous maize is grown on heavier soil types. Secondary cultivations should be carried out using the minimum number of passes possible to avoid re compacting the soil after subsoiling and ploughing. Compaction and soil pans also produce similar problems in grass although they are less visual.  Poaching causes compaction at around 5-10cm (2-4") with plough pans and heavy machinery pans occurring at 10-15cm (4-6"). To identify compaction dig holes with a spade and examine the vertical profile of the soil for consolidation and poor root penetration.

Pigs
Both pigs and the machinery used for feeding and moving them can cause soil compaction.  The use of carefully planned permanent roadways can however limit vehicle compaction.  Surface ponding of water is often a visible sign of soil compaction and, when soil conditions permit, sub-soiling can provide a solution.  Some producers walk pigs between enclosures to avoid vehicles travelling on compaction prone soils. Soil cover crops again can reduce the level of compaction.  Soil compaction coupled with sloping ground can be a major cause of soil loss from outdoor pig fields during prolonged wet conditions.

Useful publications
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