Firstly, let us debunk a common myth that a tree's profile above ground is reflected below ground. 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.
Calculating the ideal soil volume for any tree is not a precise science. One of the key roles of the soil is to hold on to and supply water and nutrients as and when the tree needs it. Many arboriculturists will adopt a weather-based methodology and base the 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 the shade or full sunlight, or exposure to prevailing winds. As a general guide we 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 an urban landscape care needs to be taken to create a controlled and consistent growing environment so tree pits are commonly used. When combined with good quality soil, our urban trees have the best chance of healthy establishment, growth and survival.
Starting at the base, immediately below the topsoil, and above the substratum (formation ground), sits the subsoil layer. Although most subsoils are used for raising levels, infill and contouring, they can build up the lower section of a tree pit, giving support to the topsoil and providing a stable anchoring point for tree anchoring systems. If you use subsoil it is important to ensure what you choose is certified BS8601:2013
Good topsoil 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. It is crucial that you select your soil from a well-established, trusted source, and that it is regularly tested to British Standard BS3882:2015 by a recognised industry laboratory.
So, when considering your next tree planting project, think about not only the quality of the soil that you are providing for the tree, but also the space for healthy growth. A little extra at the front end will pay dividends for many years to come.