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Fundamentals of tree pit soils

By Tim O’Hare
Too often recently planted trees fail to thrive because errors have been made in the design and construction of the tree pits and particularly in the decisions that have been made about soils. There are some basic principles to follow, and actions that can be taken to give trees the best chance of a healthy future.

The importance of trees within the landscape has been rapidly climbing up the agenda in the last few years. The efforts of the Tree Design Action Group (TDAG), the latest revision to BS 5837:2012, and the increasing number of companies promoting ever more complex tree planting systems, would appear to support this. 
So why do so many trees that leave the nursery in a healthy state still end up failing after planting? Poor soil conditions within the tree pit will inhibit basic root function. This in turn leads to a lack of water uptake and a rapid decline of the tree. The most common factor responsible for this is the development of anaerobic conditions within the pit. Far lesser problems, certainly during the tree’s establishment period, are low fertility, lack of water, the wrong pH or ground contamination. If anaerobism can be avoided, the tree has a fighting chance of survival. 
Anaerobism is the term attributed to a ‘low oxygen’ environment. In soils this is an immediate and major problem that can cause plant failure. Without oxygen, plant roots simply cannot take up water (or plant nutrients within the water), which induces a ‘drought condition’, even though there may be ample water in the soil. 
There are three main causes of anaerobic soil conditions: 
• Topsoil placed too deep 
• Lack of drainage 
• Soil compaction 
Excessive topsoil depths 
The depth to which topsoil should be placed is probably the most misunderstood element of soil management in the landscape industry. There is still a perception by many that ‘the more topsoil the better’, when in reality this can have catastrophic results. In a natural soil environment, for example a woodland, an agricultural field, a park or a back garden, topsoil depths rarely extend below 300mm and in many instances they can be as little as 150mm. So why is topsoil routinely placed to a depth of 1 metre in new tree pits? And why do so many tree pit details specify this? 
Beneath the topsoil is a variable depth of subsoil. The main difference between the composition of topsoil and subsoil is the higher organic matter content of topsoil. This is accompanied by a far greater population of soil microbes (eg. fungi, bacteria, protozoa), which among other things, are responsible for the breakdown of organic matter and the synthesis of nutrients for plant uptake. Like tree roots, these microbes require a supply of oxygen to breath (collectively known as the Biochemical Oxygen Demand or ‘BOD’). Since the oxygen is sourced from the atmosphere above ground, its greatest availability is in the upper soil layers (i.e. the topsoil). When topsoil is placed too deeply, in a place where the supply of oxygen is less, the BOD of the soil microbes becomes greater than the incoming supply. As a consequence, the soil runs out of oxygen and becomes anaerobic. 
We should not ignore the quality of subsoil. The term ‘subsoil’ is often regarded as meaning a soil that is ‘sub-standard’ to topsoil. This is not the case and subsoil is very much an essential part of any soil profile. It complements the functions of the overlying topsoil and has a number of key roles: 
• Absorbs surplus water percolating down from the topsoil layer above. In doing so, subsoil also provides a valuable ‘environmental service’ in the form of water attenuation during periods of high rainfall; 
• Acts as a reservoir of water during dry periods; 
• Provides the main anchorage and support for large shrubs and trees; 
• Supplements the topsoil with reserves of mineral plant nutrients (eg. potassium, magnesium, calcium). 
The reason that subsoil does not become anaerobic at depth is because it does not have a microbe population, and therefore has a low BOD. Any oxygen that permeates down to the subsoil is therefore exclusively available for tree roots. 
Lack of drainage 
Poor soil drainage often leads to anaerobic conditions. This is a major cause of failures in new tree planting. Incoming water to a tree pit (i.e. rainfall or irrigation water) is oxygenated, and in fact water is one of the main carriers of oxygen through the soil. If the drainage potential of the tree pit soil is sufficient to allow that water to permeate through it without impedance, then the water is of benefit. 
The problem arises when the water stops ‘moving’ through the soil. In tree pits, this can be due to a lack of soil structure in the backfill soil (see the section on compaction below), or an impervious layer below the pit that causes the pit to act as a ‘sump’ for surface draining waters. Essentially, the inputs of water exceed the outputs. The oxygen within water is readily used by the tree roots and the soil microbes. Without ‘fresh water’, the environment becomes anaerobic (stagnant water). 
Soil compaction 
A normal, uncompacted soil is made up of 50% soil particles (by volume), which are aggregated into soil ‘peds’ or ‘structures’. The remaining 50% consists of voids known as ‘pore spaces’. It is this network of pores that stores and transmits air and water through the soil and maintains an ‘aerated environment’. All but the most sandy soils (or urban tree rootzones) need soil structure to function properly. Soil compaction, for example from mishandling the soil or excessive traffick, results in the destruction of these structures and pores spaces, and the loss of a vital network for aeration and drainage. The outcome is an anaerobic soil with no water-holding capacity. 
The solutions 
It is worth treating the tree pit as the ‘transitional zone’ between the ‘nursery soil’ (in the field or a container/air-pot) and the ‘real world’. It is the rooting environment that needs to minimise transplant stress, and promote healthy root growth to optimise tree establish-ment. Various degrees of design intervention may be required to achieve this, depending on the nature of the site (greenfield/brownfield), the existing soils, the topography, the hydrology and other environmental factors such as exposure to wind, sun scorch, etc. 
Regardless of the level of design input, it is useful to stick to some basic principles for an aerated soil and healthy rooting environment. Keep tree pit design as simple as possible and minimise the amount of disturbance to the soil. 
Soil types 
Any soil that can maintain its ability to drain and aerate after soil spreading and tree planting has the potential for re-use. However, in many landscape projects where there are time constraints and inclement weather to factor in, sandy soils provide the greatest flexibility. They remain ‘non-plastic’ at higher moisture contents, and they are less prone to compaction and structural degradation. 
Heavier clay-based soils can be used provided they are reasonably dry and in a friable, ‘non-plastic’ state when handled. Silty soils are generally not suitable for backfilling tree pits as they have weak structural strength and suffer from ‘self-compaction’ even when handled carefully. 
Soil depths 
Quite simply, do not put topsoil too deep. A topsoil depth of 300mm is usually ample, and certainly 400mm should be the maximum and only provided the soil type will allow it. 
This applies to planting beds as well as to tree pits. The British Standard for Topsoil (BS3882: 2007) and DEFRA’s Construction Code of Practice for the Sustainable Use of Soils on Construction Sites (2009) both support this approach. The rootball should sit on subsoil, and with bigger rootballs, the subsoil will also sit around the lower portion of the rootball. 
Small rootballs 
For trees with a smaller rootball (up to 300–400mm deep) that are being planted into ‘in-situ’, undisturbed ground, it is far better to minimise the size and dimensions of the tree pit to limit the destruction of the soil’s structure. 
The tree pit should be as shallow as possible, and usually only requires excavation to the depth at which the rootball will sit. If machine dug, it is useful to decompact the soil in the base of the pit where the excavator bucket often causes smearing and compaction. 
After placing the rootball, the pit can be backfilled with the excavated topsoil, ensuring that any soil ameliorants (eg. green compost ) have been evenly mixed with the backfill topsoil. 
Larger rootballs 
For larger trees, there is a need to excavate a deeper pit to accommodate the rootball. This requires excavation into the subsoil. It is not always sensible to backfill with the same subsoil, especially if it is a particularly heavy-textured soil (silty or clayey). In preference, it is better to use a high-sand-content subsoil or even a quarried sand, to sit the rootball on, and to surround its lower portion. Sands and sandy subsoils will support the weight of the rootball better, and thereby prevent later settlement. A coarser sand with a narrow particle-size distribution will also be able to maintain a reason-able porosity even in this compacted environment below the rootball, thereby ensuring it will have good aeration, drainage and water storage properties. Roots will grow happily into a sand as it is full of oxygen and water. 
Tree pit drainage considerations 
To prevent anaerobism resulting from waterlogging, the principle of any tree pit design is to ensure that inputs of water are equal to or less than the outputs. 
Inputs in this instance could be rainfall, surface water run-off, shallow watertable or irrigation. These are influenced by climate, topography, hydrology, hydrogeology and maintenance. 
Outputs include tree root extraction and transpiration (only in the growing season), evaporation from the surface, percolation into the surrounding soils and underlying strata. These are influenced by the size and species of tree, climate and most notably, the drainage potential of tree pit soil and the soakage potential of the underlying strata. 
Drainage options may include mounding the pit slightly to help shed water away from the upper rootball, incorporating a mini soakaway in the base of the pit, or connecting the pit to a positive drainage outfall. 
Soil investigation, combined with a review of the landscape proposals, is essential to determine which drainage option(s) is most applicable to each tree pit. Very often more than one option can be applied to a landscape scheme to suit the variable size and location of trees. 
The decision about whether to artificially improve tree pit drainage and the method to be used must be taken at an early stage in the landscape design process. All too often drainage provision is only considered when the landscape contractor is due to start tree planting. The available options are very limited at this point as drainage infrastructure probably will not be in place.
Tim O’Hare is the driving force behind a conference entitled ‘Soil – meeting the challenges of a changing landscape’ which will be held on 14 October at the Howbery Park Conference Centre, Wallingford, Oxfordshire. 

Marking the UN’s International Year of Soils, the conference will have the following speakers: 
Tim O’Hare, principal, Tim O’Hare Associates 
Rob Askew, senior associate, Tim O’Hare Associates 
Sue Illman, Illman Young Landscape Design and Past President LI 
Johanna Gibbons, J&L Gibbons and advisor to English Heritage 
John Melmoe, commercial director, Willerby Landscapes 
George Longmuir, managing director, Freeland Horticulture 
To learn more, please email [email protected] or call 01491 822653.

BS 5837:2012 Trees in relation to design, demolition and construction – Recommen-dations, BSi, 2012 
BS 3882:2007 Specification for topsoil 
and requirements for use, BSi, 2007
Construction Code of Practice for the Sustainable Use of Soils on Construction Sites, DEFRA, 2009
Tim O’Hare is principal soil consultant of Tim O’Hare Associates LLP. He has advised on soil specification and tree pit design for numerous projects throughout the UK, including the Olympic Park and the Athletes’ Village. He has also been investigating failed planting schemes for over 20 years.

Leave a comment


Posted by Steve Jackson - February 1st, 2018
Thanks for your article and informed contribution to this continuing malady. Disturbed urban soils are particularly challenging to the establishment of newly planted trees.

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