Soil Selection and Volume for Optimal Tree Growth

Urban sprawl is taking over our green spaces. With an ever increasing population it cannot be helped and is no-one’s fault. It does however mean that survival is a battle […]

Urban sprawl is taking over our green spaces. With an ever increasing population it cannot be helped and is no-one’s fault. It does however mean that survival is a battle for both our new and existing urban trees.

To appreciate this fully, we need to understand the factors that affect urban tree planting. Lack of space for healthy root growth is undoubtedly the most limiting factor when it comes to a tree’s water and nutritional requirements. In the urban landscape this space is severely compromised by not only buildings and hard structures that we can see, but also the plethora of service lines and pipes below ground. We may know how much rooting volume we need; whether we can achieve it or not is another matter.

Firstly, let us debunk the myth that a tree’s profile above ground is reflected below ground. This is not the case at all. Most UK species are shallow rooting, with ninety per cent of root growth within the top 500mm of soil. So rather than forcing root growth downwards, we need to give the tree as much lateral growth room as possible. To support this, British Standards quote that topsoil should be no more than 300mm deep. Any deeper risks the soil becoming anaerobic and the organic matter within dying off, causing more harm than good. There are certain British Standard top soils, such as Green-tech’s Green-tree topsoil, that is tested and approved to a depth 400mm due to its open texture. One thing that we would stress, make sure any soil used is tested to British Standard BS3882:2015 for top soil, or BS8601:2013 for subsoil.

Calculating the ideal soil volume for any particular tree is not a precise science. One of the key roles of the soil is to hold on to, and supply water/nutrients as and when the tree needs it. Many aboriculturists will adopt a weather-based methodology, and base tree pit size on the water requirement of a particular tree. This can be affected by a multitude of factors such as whether the tree is in shade or full sunlight, or exposure to prevailing winds. However, as a general guide, we would recommend the following volume to anticipated canopy ratio, based on a 1m deep tree pit:

3m canopy – 5m3 (2.25 x 2.25m)

5m canopy – 12m3 (3.5 x 3.5m)

8m canopy – 30m3 (5.5 x 5.5m)

In a rural, soft landscaping environment rooting space is not usually an issue. However, in the urban landscape, care needs to be given to creating a controlled and consistent growing environment, hence we use engineered tree pits. These pits, when designed correctly, give our urban trees the best chance of healthy establishment and growth.

The key factor is to not only support the ground above, but also protect the rooting area topsoil from compaction. For this we would recommend a geo-cellular rafting system such as Green-tech’s Geocellular Raft system. Based on very simple but effective engineering principles, Geocellular Raft system

acts as a sub-base replacement layer, able to carry the weight of pedestrian or vehicular traffic. At the same time, the rigid cells not only protect the soil from compaction, but also act as an aeration layer below the surface. This allows a good quality, friable topsoil to be used, with no plastic in the actual rooting zone.

The health and structure of a tree is a reflection of the soil in which it grows

Topsoil – the Premier League of soils, this is where all the exciting stuff occurs! A good top soil is critical to the healthy growth and well-being of a tree, or anything else that happens to be grown in it for that matter, and it is crucial that you select your soil from a trusted source.

Not all soils are created equal, and if you do not have clean, natural soil on site to use (still ensure it is tested for contaminants or unexpected seed bank), then we recommend a quality manufactured topsoil such as Green-tree. Green-tree have been manufacturing soils for over 15 years, to create as near perfect a growing medium as possible. It is a free-draining sandy loam that can be consolidated without losing structural integrity. The organic content of the soil comes from the blended PAS100 green compost, and the advantage when considering manufactured soils is consistency; the hundredth tonne will be the same as the first. It will also be environmentally clean.

Below the topsoil sits the subsoil layer. Although most subsoils are used for raising levels, infill and contouring, they can also build up the lower section of an urban tree pit, giving support not only to the topsoil above, but providing a stable anchoring point for tree anchor systems. Often the poor relation, subsoil still plays a vital role, having a big influence on water absorption/retention and drainage. Without it, every time it rained, the topsoil would become very boggy and detrimental to healthy root growth. Made up of a blend of UK sand and quarried minerals, with a higher clay and silt content than the more friable top soils, any batch should be certified BS8601:2013.

On occasions when space or budget is limited, Green-tree Amenity Tree Soil is a good substitute. Otherwise known as ATS or Amsterdam Tree Soil, structural tree soil is a long standing, tried and tested method, and is the simplest and least costly option for providing a stable tree pit environment.

Ideal for semi-mature trees, ATS has half the amount of clay content as subsoil, and much higher sand content.

Its naturally open structure copes well with compaction, whilst still facilitating aeration and water to the tree’s root system. Although ATS has a small amount of organic matter, from the blended PAS100 green compost, it is still classed as low in nutrition. With this in mind, you would be advised to think of supplements such as Terracottem Arbor, to encourage healthy establishment, or an annual application of slow release Terracottem Complement.

So when considering your next urban tree planting project, think about not only the quality of the soil that you are providing the tree, but also the space for healthy growth. A little extra at the front end will pay dividends for many years to come.

Geocellular Raft system being used on the set of Coronation Street at Media City, Manchester. The shallow profile was invaluable to stay above the mass of cabling and service lines underground.

The attached images show the benefit of having unrestricted rooting volume. These five trees in Battersea share one large tree pit, supported by Geocellular Raft system. As the roots are not restricted to a small area, after just two years, you can see how well the trees are flourishing.



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