Generally, soil compaction is found at the top 30cm of the soil, which coincides with the region where most roots grow. Where possible, soil compaction should be prevented to maintain an environment suitable for root development.
Soil strength is measured by a penetrometer which measure force required for a probe to push into the soil. Penetrometer readings in excess of 2 MPa (2 megapascals = 20 bars) will restrict root elongation. As bulk density increase, soil strength is more likely to increase in step, and even more likely in drier soil than wetter conditions. Most roots are unable to penetrate moist soils of a bulk density > 1.4-1.6g cm-3 in high clay soil and 1.7g cm-3 in more sandy soil.
Low pore space is harmful to roots and microorganisms as it would lead to hypoxic (low oxygen) conditions and reduced nutrient uptake. Tree nutrient deficiency would result even if nutrient level is good. Compacted soil can lead to both water deficiency and water logging. Erosion due to compacted surface will increase nutrient leaching and remove topsoil, which decrease tree stability.
In a classic case of a tree growing in compacted soil, a decline of the crown can be observed. In consideration of existing and newly planted trees, soil should be prevented from compaction by means of physical barriers or routing paths away from rooting zones.
Decompaction - Before trees are planted, the site could be cultivated using hand tools or equipment. As for de-compaction an area with existing trees, the best method to use air soil-excavation tools. They can facture compacted soil to around 30 cm depth and minimise disturbance to roots.
Reference:
1. International Society of Arboriculture's Arborist News, Volume 29, Number 5, October 2020
