Reducing soil compaction of brownfield sites

 

Impact

Compaction affects soil by:

• Inhibiting root development
• Altering the soil moisture and oxygen status
• Causing poor winter drainage or severe summer droughtiness.

These factors reduce the ability of vegetation to exploit soil water and nutrients, and in the case of trees increase the risk of windthrow.

An average soil depth of at least 1m is recommended for tree planting on restored sites, which may vary depending on initial soil condition and location within the UK. On sites where compaction has occurred following soil reinstatement, soil will need cultivation before any trees are planted.

Evaluating methods of loosening the ground

We are currently evaluating methods of maximising the soil depth available for tree rooting by loosening ground to depths of 1m. A long-term trial has been established on a restored site to assess the effectiveness of deep cultivation methods on tree growth and root development.

Tested cultivation methods are:

• Complete cultivation to 1.1m
• Standard industrial ripping to 0.9m
• Deep ripping to 0.75 and 0.9m using a prototype ripper, the Mega-Lift, developed by Tim Howard Engineering Ltd.
• An unloosened control.

The effectiveness of the cultivations are being assessed in terms of:

• Depth and nature of loosening achieved following cultivation
• Improvements in soil penetration resistance (a measure of compaction) and their longevity
• Improvements in tree rooting
• Tree survival in the first years following planting
• Tree growth rates and health.

Progress to date

The project began in 2001 and the fieldwork was completed in 2006.  Annual measurements of tree height, diameter, soil moisture content and penetration resistance have been taken. Assessments of root development were made in 2002, 2004 and 2006.

Summary of results

• Differences in soil penetration resistance and tree performance between treatments suggest the complete cultivation remains the most effective method of alleviating soil compaction

• No significant difference between the soil penetration resistance measurements or tree performance after cultivation with the Mega-Lift and industrial rippers.

Outcomes

• Improved understanding of the performance of different cultivation techniques in relation to tree establishment on restored sites
• Improved understanding of the effect of root development on growth and survival of trees
• Development of a model predicting the critical penetration resistance value at which a significant reduction in rooting should be expected
• Development of a model looking at the relationship between two methods for measuring penetration resistance: the penetrometer and the ‘lifting driving tool’.

The ‘lifting driving tool’ is a much more cost effective and user-friendly method for the assessment of soil compaction. The modelled relationship will allow data from this tool to be related to both penetration resistance in MPa and potential root development.

Publications

Sinnett, D., Poole, J. and Hutchings, T.R.  (2006).  The efficacy of three techniques to alleviate soil compaction at a restored sand and gravel quarry.  Soil Use and Management 22: 362-371.

Sinnett, D., Poole, J. and Hutchings, T.R.  (in preparation).  Optimisation of cultivation techniques for successful tree establishment.

Sinnett, D., Williams, M., Morgan, G. and Hutchings, T.R.  (in preparation).  The relationship between penetration resistance and tree root development.