What Every Landscape Architect Should Know About Soil…
What do you think is the most important information to have about soil and soil management, as a landscape architect?
A landscape architect should at the very least equip themselves with the basic scientific information about soils: they should have a knowledge of soil classification, soil texture and structural properties, soil water movement, soil fertility and soil ecology (micro-biology); fundamentally they should understand how soil properties strongly influence the make-up of vegetation in a particular landscape.
Landscape architects should also have a basic knowledge of the engineering properties of soils and how even minimal disturbance to soil could negatively affect the structural and ecological functioning of that soil. In this regard, knowledge of soil remediation techniques is also useful to landscape architects.
How as a landscape architect can you avoid operations likely to cause damage to soil on the site you are working on?
As a landscape architect, the best way to avoid damaging soil operations on a site that you are working on is to devise sound and pragmatic soil handling specifications and write these into the contract document for the landscape or civil engineering contractor to work to in advance of any work happening on the site. The value of your contract specifications also relies on an intimate knowledge of the soil on your site, which is ascertained at the survey stage i.e. before design and specification.
The second way to avoid damaging operations is by close site supervision of works throughout the duration of the project. Critical to this is a site meeting with the landscape or civil engineering contractor in advance of excavation and/or soil handling operations where you, as landscape architect, lay down your requirements for soil protection.
You should also make sure that no site operations take place when the soil is wet or icy as then soil structure is easily destroyed; remember that a contractor may be tempted to continue work if subject to financial or project programming pressures.
Most importantly, you will also need to ensure that your client understands that you need to perform this role.
Have you had any experiences/problems with soil in past projects?
Yes, hence my understanding of the need for all of the above! However, the main problems with soil that I run up against are where I do not have any control over the soil handling operations, for example on commercial development sites or infrastructural schemes. Here the soil operations are usually under the auspices of the civil engineering consultant and or building contractor, rather than the landscape architect. The soil handling specifications are devised by engineers and, in my opinion, the soil is often treated like a building material rather than as a future medium for growing plants or for regenerating a particular type of vegetation or habitat.
This type of specification can lead to poor soil handling, soil damage and consequently poor plant growth due to compaction, damaged structure, poor drainage or deficient soil biology. Much better to have the landscape architect consulted in the first place.
How, in Ireland, do you go about the process of soil storage/movement today?
I don’t think the situation in Ireland is or should be different to any other part of the world. However, I would recommend that due process be applied as outlined above.
The key principles should be that soil movements and soil storage time periods are minimised. If soil does have to be moved then it should, firstly, be analysed to understand its composition in terms of topsoil and subsoil and the different layers handled separately. Soil handling will immediately result in a reduction of soil porosity due to compaction and shearing forces imparted during handling and will impact on the ability of plants to root through the soil as well as compromising water and gas movement through the soil.
Soil storage is where the most profound changes can occur that will subsequently affect plant growth. These include compaction resulting in loss of soil structure by particle disaggregation, chemical changes as a result of the development of anaerobic conditions in the soil store, this, turn, can result in changes to soil pH, availability of nitrogen after respreading of the soil and also the accumulation of phytotoxicelements.
Furthermore, there are immediate and significant changes to soil biology with a loss of beneficial soil fungi and fauna and an alteration in the bacteriological content of the soil.
The size and longevity of the soil mound will determine the deterioration of the soil with the effects varying at different depths within the soil store.
Consequently, my recommendations would be to:
1) If possible, pick up and move soil from donor area to receptor are in one movement avoiding the transition stage and of course avoid tracking over areas of soil yet to be stripped.
2) If that is not possible then soil should be stored in low mounds, subject to availability of space. The texture of a soil will affect the resulting degree of compaction with clay soils likely to compact more easily and even low mounds up to 3m will result in the development of debilitating anaerobic conditions in the centre of the mound. Sandy soils having greater voids between the soil particles are less likely to compact. Thus restoration treatments post- re-instatement will vary for different soils.
3) Thirdly, try to minimise the amount of time that the soil is in storage, as long term storage will generally exacerbate soil degeneration.
4) Finally and obviously, it is important that the soil should always be stripped in layers, following the soil profile and is put back in the same order. Mixing of topsoil and subsoil layers should always be avoided.
I would love to see priority being given to proper soil handling on all development projects, as I don’t think this issue is being taken seriously at the moment.
What is the role of the landscape architect in creating and utilising soils?
The role of the landscape architect in creating soils should be limited by necessity as, I believe, that the conservation of existing soils should be the starting point. However, there are situations, for example in urban design projects, where new soil needs to be created to support planted vegetation.
Urban soil or structural soil is the term given to soil that is manufactured to support plants and, in particular, trees in circumstances where there is limited scope for root development and/or where the secondary function of supporting surrounding paving, tarmac, concrete or other surfacing material is required, for example in car parks or along street verges. These soils are usually made up of a mix of stone, topsoil, and compost and may also contain a bonding agent to tie- the soil together.
Obviously there are situations where landscape architects deliberately design new contours and seek to create soil mounds of various shapes and sizes in order to fulfil design functions; the same principles should be followed, as outlined above, where this is the case.
A final point is that we, as Irish landscape architects, need to understand the particular properties of Irish soils, the pressures that they are under from differing forms of land use and how important it is that their properties are preserved in order that both local and wider ecosystem characteristics are maintained.
Amongst copious volumes, some key references would be:
- Brady and Weil (2008) “The nature and properties of soils”
- Harris, Birch and Palmer (1996) “Land Restoration and Reclamation”
- Books and articles on land restoration by A.D. Bradshaw et al.
- British Standard Specification for topsoil BS 3882:2007