Tree Root Compaction
by David Steg on 10/20/14
Soil compaction - practical considerations
Soil compaction can be a significant problem at undermanaged greenspaces, for example where unauthorised vehicle access has occurred, around street trees due to construction activities, or at former agricultural sites due to regular ploughing. Different cultivation methods are recommended for different types of compacted site.
Background
Soil compaction is a common problem at many brownfield and contaminated sites. It is often caused by the removal, storage and reinstatement of soil materials during mineral extraction or mining activities, but may also exist on former industrial areas where areas of hard standing have been removed, or where the ground has been subjected to heavy machinery traffic.
Soil compaction reduces the pore space within soil, resulting in a poor soil structure that restricts the development of plant roots. It also affects the soil water status, causing waterlogging during wetter periods and drought conditions during drier periods, which in turn limits root development. Poor rooting significantly inhibits plant growth on compacted soils and, in the case of trees, can also increase the risk of trees being blown over during storm events.
Practical considerations
The current guidance suggests that a rootable profile of at least 1 m depth is required for sustainable tree establishment, and at least 0.5 m depth for grass cover. A rootable profile is soil with a bulk density of less than 1.5 g/cm3 to at least 0.5 m depth, and less than 1.7 g/cm3 to below 1 m depth.
There are a number of cultivation methods that can be applied to a compacted site. Where compaction is not too severe, or is relatively shallow an agricultural ripper pulled by a tractor can be used to break up the soil profile.
Additional information
Where compaction is severe, or goes more than 0.4 m deep (as is often the case at brownfield sites), complete cultivation is recommended (see Best Practice Guidance 13 below). Complete cultivation involves using an excavator to remove the soil progressively down to a depth of at least 1 m and replace it by loosely depositing the soil back into the trench that is created. This avoids the need for traffic movement over the completed soil profile. Despite being more expensive than ripping, complete cultivation has a number of advantages.
• Is more sustainable – recompaction often occurs within the first year following cultivation by deep ripping. Where trees are planted, further cultivation is not possible and the roots will not develop sufficiently in this time period.
• Enables continued separation of soil horizons – where different horizons exist within the soil profile, complete cultivation can be used to remove each horizon separately, then replace them in reverse order to ensure that the subsoil and topsoil horizons are maintained.
• Can be combined with incorporation of soil amendments – where soil amendments are used (e.g. to improve organic matter content), they can be incorporated during complete cultivation, ensuring that a well mixed material is produced.
Where compaction occurred following tree establishment, for example around street trees following construction activities, air can be injected into the soil profile around the tree roots to ‘lift’ the soil and reduce compaction. This success of this method often depends on the soil type, for example soils with a high clay content may recompact after air injection.
Where a site is being restored following mineral extraction, the risk of soil compaction occurring can be minimised through loose tipping (see Best Practice Guidance 4 below). This involves placing the soil material on the site using an excavator to deposit the material loosely, in such a way that there is no further traffic movement over the soil. Where a site is already suffering from soil compaction, due to either poor restoration practices in the past, or subsequent heavy vehicle traffic, some form of cultivation will be necessary before the establishment of vegetation.
Case studies
Evaluating methods of loosening the ground
Forest Research is currently evaluating methods of maximising the soil depth available for tree rooting by loosening ground to depths of 1 m. A long-term trial has been established at a restored sand and gravel quarry to assess the effectiveness of deep cultivation methods on the survival, growth and root development of Italian alder, Corsican pine, Japanese larch and silver birch.
Tested cultivation methods are:
• Complete cultivation to 1.1 m
• Standard industrial ripping to 0.9 m
• Deep ripping to 0.75 and 0.9 m using a prototype ripper, the mega-lift (developed by Tim Howard Engineering Ltd)
• An unloosened control.
The effectiveness of the cultivations is being assessed in terms of:
• Depth and nature of loosening achieved following cultivation
• Improvements in soil penetration resistance (a measure of compaction) and longevity
• Improvements in tree rooting
• Tree survival in the first years following planting
• Tree growth rates and health.
Results include:
• Differences in soil penetration resistance and tree performance between treatments suggest that complete cultivation remains the most effective method of alleviating soil compaction
• No significant difference between soil penetration resistance measurements or tree performance after cultivation with Mega-Lift and industrial rippers.
