What is compaction
Soil compaction occurs when the airspace in the soil is displaced. This is caused by excessive traffic, such as footfall, mowers, machinery and other traffic. A force such as a machine or a persons feet pushes the soil particles together, this displaces the pore space between the particles.
When the air is displaced the roots become shallow and weak as they can’t penetrate into the soil. Grass growth becomes then becomes thin and weak, weed grasses and broad-leaved weeds start invade the thin areas of the lawn. Clay and silt soils are more susceptible to compaction than sandy soils, however the latter is still not immune it.
Symptoms of compacted soils
The most common symptoms of compaction include:
- Extended periods of water logging during periods of substantial rainfall. The water is slow to drain away because of the lack of pore space in the soil. Therefore it sits on the surface of the lawn, instead of draining away.
- Browning off, of the turf during prolonged hot and dry weather, due to the lack of root growth. Drought resistant turf has deep roots, enabling to tap into the moisture reserves in the root zone. When a soil becomes compacted root growth is restricted and as the top layer of soil dries out, then there is no moisture available for the grass plant.
- Shallow rooting weeds and weed grasses such as plantains, pearlwort and annual meadow grass invade as the expense of the deeper rooted fine grasses.
- Thatch build up is increased as the micro-organisms or bacteria that are responsible for its breakdown can’t survive without sufficient air.
Testing for compaction
Be on the look out for the obvious signs of compaction. Is the lawn puddling up more and taking longer to drain off after heavy rainfall than in previous years? Does the grass seem to turn brown quickly in hot, dry periods than before? Is the grass looking thin and are certain weeds becoming a nuisance?
If the answer is ‘yes’ to any or all of these, then it is very possible the lawn is suffering from soil compaction and further investigation is required. Using a long screwdriver, push it into the lawn. If the soil is compacted it will be difficult to push in. You can try this on different parts of the lawn, to gauge where the compaction is most severe.
Following this test, use a spade to remove a square of turf, from what seems to be the most compacted area. If the spade is difficult to push into the ground, this will also confirm the soil is compacted. When you dig out a piece of turf, inspect the thatch layer and the root depth.
If the thatch layer is over 13mm thick then action is required. If the roots only penetrate between 25 – 50mm then the soil is likely to be compacted and action is again, required. A health root system should extend between 100 – 150mm or more.
How to relieve compaction on turf
The best way to relieve compaction is by aerating the lawn. In most cases hollow tining is most effective way of relieving compaction. However, hollow tining at a depth of 50mm may help alleviate surface compaction, but is of little benefit if the soil is compacted further down. In this case a machine or implement will be needed that penetrates deep enough to break up the compacted layer.
Hand held hollow tine forks are perfect for this task if the area is relatively small, as they can be pushed in relatively deep. The main drawback is they are time-consuming and hard work on larger areas. On larger lawns a powered machine is best option. The main disadvantage is the cost of the machine. In almost all cases it is better to hire a machine or enlist the help of a lawn care specialist.
Other types of aeration will also relieve compaction. A traditional garden fork is one of the best implements at breaking down soil compaction, as they penetrate very deep. A special technique can be used with a garden fork to create fissures in the soil, which creates lots of air space, significantly reducing compaction. For this technique to work the soil needs to be as dry as possible, yet still able to receive a garden fork.
When the fork is inserted into the ground, press back on the handle to lift the turf a little and create a heave effect in the ground. This heaving action cracks the soil and creates more airspace than normal aeration. However this is very laborious work, but highly effective on severely compacted areas. Professional groundsmen / greenkeepers use specialized machinery (verti-drain) that creates this heaving effect on compacted areas.