Soil Structure – protecting your best asset

Date published: 25 July 2024

Trevor Alcorn, CAFRE Dairy Adviser remarked: "Grazing of stock, slurry applications and silage harvesting have all been carried out, in less than favourable conditions. This combined with heavier machinery, and a lack of maintenance of field drainage systems, has led to a breakdown of soil structure and increased compaction. The result will be decreased yields, reduced soil drainage, increased weed infestation and poor nutrient uptake from fertilisers."

Jane Sayers, CAFRE Dairy Adviser explained: "Compaction is where soil has been squashed into a solid, impermeable layer, either at the surface or within the topsoil. The compacted layer restricts the movement of air, water and nutrients down through the soil profile. This leads to poor root growth, which stresses the plant and reduces its response to nitrogen."

Compaction can also cause temporary waterlogging. Wet soils stay colder for longer, reducing the number of available grazing days. This can also make harvesting difficult, which is likely to reduce the quality of the resulting silage.

1. Assessing level of soil compaction

Dig a test hole approximately 50 cm square and at least 40 cm deep. This will allow you to see the extent of the problem.

Compacted soils will have:

• A blocky structure that is hard to break up.
• Signs of shallow roots growing horizontally
• Very few or no worms present
• A bad smell due to anaerobic conditions.
• A grey colour with brown mottling.

It is also important to identify the depth of the compacted layer. This will determine the type of machine that should be used to rectify the problem.

2. Reducing compaction

Once the level of compaction has been assessed, it can be determined whether it has been caused by farm machinery or livestock.

• Compaction by farm machinery

The potential for wheeled vehicle compaction in any given situation is controlled by the following factors:

• Total axle weight – the higher the weight the greater the possibility of compaction.

• Tyre width, diameter and type – the larger the footprint (width and diameter) of the tyre on the soil surface the less likelihood of deep compaction. Correctly inflated radial tyres have been shown to do least damage.

• Inflation pressure – affects the footprint of the tyre on the soil surface, with high inflation pressures meaning little or no flex in the tyre and subsequent potential for deep compaction.

• Wheel slip – even low levels of wheel slip, has been shown to produce significant compaction down to 5 cm.

• Number of passes – the higher the number of vehicle passes the higher the degree of compaction. However, 50 to 80% of the soil compaction may be caused by the first wheel pass, with only slight increases after the fourth pass.

• Compaction by livestock

At low to medium soil moisture contents animals cause compaction at or near the soil surface. At high soil moisture contents, trampling results in poaching (penetration of the soil surface by the hoof). As the hoof penetrates the soil, the structure of the soil is damaged and soil at the base of the hoofprint is compacted. Animal induced compaction does not generally extend beyond 10cm, although with a combination of heavy stock and poor conditions compaction can be deeper.

To put pressures into context, a standing sheep exerts a static pressure on the soil of approximately 80 kilopascal (kPa) or 12 pounds per square inch (psi), increasing to 200 kPa (29 psi) when the sheep is moving, while for cattle, static pressure is 160-192 kPa (23-27 psi) and this pressure at least doubles when the animal is walking. This compares to 60-80 kPa (9-12 psi) pressure exerted by a typical unloaded tractor.

The potential for animal induced soil compaction in any given situation is influenced by the following factors:

• Total mass of the animal – again the higher the weight the greater the possibility of compaction.

• Stocking density – higher stocking densities are associated with increased soil compaction.

• Rate of rotations – Rotating stock more frequently reduces the degree of compaction.

• Sward type and condition - High tillering dense swards such as those in long-term or permanent pastures are more resistant to poaching than the open swards which develop under heavy crops of silage.

• Local issues – such as placement of drinkers, location of entry points etc.

3. Alleviating Compaction

• Spiking/ aerating can alleviate shallow pan (variable response).
• Subsoiling is only recommended where compaction is evident (not a substitute for drainage!)
• Soil MUST be dry at working depth.
• Best time is autumn to allow the soil to rest/ recover over the winter.

BUT it is possible to do more harm than good!

• Only subsoil as deep as required (assessed from test hole).
• Recently loosened soils are more easily re-compacted.
• Try to reduce/ minimise any future compaction.

In conclusion, plan now to repair damaged soils which will help to improve the performance and output from grassland in the future. Try to minimise compaction where possible from livestock and machinery. Where compaction has been identified, soil alleviation should only be done when conditions are suitable, preferably in the autumn and fields should be carefully managed to prevent problems reoccurring in the future.