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Soil Compaction

The dreadful weather conditions experienced across the country in 2012 has had a serious impact on many grass and forage crops. Many fields have suffered from severe soil compaction and as a consequence the whole subject has been brought into sharp focus with a surge in demand for an array of equipment such as slitters, aerators, sub soilers and sward lifters. Compaction is where soil has been squashed into a solid, impermeable layer, either at the surface or within the topsoil. This band restricts the movement of air, water, and nutrients down through the soil profile.
This type of damage leads to poor root growth, which stresses the plant and reduces its response to nitrogen. Applying fertiliser to compacted soils is a waste of time and money, as the plant will not be able to fully utilise it. The risk of fertiliser run-off will increase by as much as 50-60%.
Compaction can also cause temporary waterlogging. Wet soils stay colder for longer reducing the number of available grazing days. They can also make harvesting difficult, which is likely to reduce the quality of the resulting silage.

Soil-compactionASSESSING COMPACTION

Ideally, where soil depth allows, grass roots should go down 30cm or more. Compaction inhibits root penetration, seriously reduces grass yield and increases the risk of soil and fertiliser run-off. Compaction will also lead to a reduction in the sown varieties in the sward, with weed grasses such as annual meadow grass developing in their place.

IDENTIFYING COMPACTION

  • Dig a hole at least a spades depth, ideally when the soil is neither excessively wet or dry.
  • Look for how far roots and moisture extend down the profile.
  • Where there is extra resistance to the spade, that is the depth of compaction in the area.

The graph below illustrates the negative impact that compacted soils can have on the potential production of swards over time.  The faster the rate in decline of the sown species the poorer the economic return as the natural ingression of shallow rooting native grasses is accelerated.

Average-content-of-preferred-species-in-swards

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