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Specifying Top Soil – by Janine Pattison MSGD
Time spent designing the most fabulous planting schemes and sourcing perfect plants will be wasted if the soil they are being planted into is poor. The garden designer needs to make sure the planting areas are correctly prepared before reaching for the plants.
On many sites, topsoil is either completely absent or there is insufficient quantity to meet
landscaping needs. When site preparation commences (even on sites that have adequate
topsoil) space constraints might necessitate disposal off-site and importation of new topsoil later in the project. When it is necessary to source topsoil from outside the site, it is worth understanding the materials that are available and the potential implications of using them for landscaping purposes. The term ‘topsoil’ can mean a variety of things and many designers still believe that all topsoil comes from green fields, but in reality there are now a broad range of ‘soil-based’ materials sold as topsoil in the landscape industry, and not all of them are suitable for the intended purpose.
Essentially, there are three main varieties of topsoil available: Natural Topsoil, Manufactured Topsoil and Skip Waste Soil. Over the last few years, the last two of these have increased in availability because of changes in legislation to promote recycling and waste recovery, and reduce the reliance on landfill. There has also been a decline in the availability of natural topsoil, as less greenfield sites are developed, while organic soil improvers such as green compost are increasingly produced.
Natural topsoil
Natural topsoil is derived as a by-product of development of greenfield sites and, sometimes, from undisturbed and uncontaminated areas of brownfield sites. It is illegal to remove topsoil (other than for turf-cutting) from agricultural or forestry land without planning consent.
When sites are developed with the benefit of a consent, topsoil is normally stripped off before construction begins, and any surplus removed for reuse elsewhere. As a consequence, the availability of natural topsoil fluctuates, as it only becomes available at the time of a topsoil strip and might not be available at all in urban centres remote from greenfield development.
Not all natural topsoil is suitable for landscape use, as it can range from a crumbly loam to an acid, nutrient-deficient sand or a strongly alkaline clay. While many topsoils are suitable for general landscape applications, those with more extreme characteristics are not, and their use can result in widespread plant failures. If in any doubt, inspect the soil before delivery and get the supplier to provide some form of certification as to the quality of the product (check it is recent and authentic). All soils traded should be tested in accordance with BS3882:2007.
Manufactured topsoil
Manufactured topsoil is formed when two or more components are mixed to form a rooting medium. Traditionally, these have been termed ‘rootzones’, and consist of a mixture of sand and natural topsoil at varying proportions, (for example, 70 per cent sand and 30 per cent topsoil). These are mainly used for sports applications, where they provide an ideal playing surface and free-draining rooting medium. As these are processed materials using high value components, they are expensive. More recently,a broader range of manufactured topsoil has become available, usually as the result of blending soil and subsoil with various bulky organic materials, such as green compost.
In many instances, these materials provide a good alternative to natural topsoil as long as the correct components have been selected and blended to correct ratios. Their attributes can potentially include consistent composition, year-round availability, and freedom from weeds and contaminants. However, the exact opposite can be the case where unsuitable soil or additives are used, or the manufacture process does not have the necessary quality control measures. Check that the soil has been tested in accordance with BS3882:2007 as BS3882:1994 has now been withdrawn.
BS3882:2007 provides for a multi-purpose soil which is suitable for a wide range of planting applications as well as a range of specialist soils outside the scope of this article.
Skip waste soil
Unfortunately, one of the main alternatives to natural topsoil or properly manufactured topsoil is ‘skip waste soil’. This is offered to the unsuspecting landscaper as ‘general purpose topsoil’ or ‘turfing soil’ or ‘screened topsoil’, from suppliers who have no supporting technical information to confirm the quality and suitability of their ‘product’. This should not be confused with natural topsoil that has been screened to remove larger stones, as a means of improving its composition.
Skip waste soil is the result of screening materials derived from the ‘muck away’ site clearance operations. The coarse element is screened out and used as recycled aggregate while the fines are sold as ‘topsoil’.
Skip waste soils are often reused without the necessary Environmental Permit exemptions in place and usually consist of a mixture of topsoil, subsoil, clay and numerous fragments of building waste materials; brick, concrete, mortar, ash, clinker, and possibly asbestos, glass, metal, wood and plastic. In terms of its physical and chemical soil properties, the material is usually extremely alkaline, saline, infertile, and can often contain elevated levels of chemical contaminants and ‘sharps’ such as glass and ceramic shards. This material often has a fine granular structure and may slump or cap when used as a landscape soil, leading to drainage and growth problems. This garden designer should insist that this soil is avoided.
Summary
It is well worth a visit to the production facility to see how the soil is produced and to satisfy yourself it is suitable for your project’s needs. Collect a sample of your selected soil so it can be compared to the delivered material. In my experience, there is no substitute for being on site to meet the delivery and checking the load before it is tipped.
Do
Don’t
Sources:
Paul Brewer of Terragen Environmental Consultants Ltd
www.terragenenvironmental.co.uk