THE JOURNAL FOR THE SOCIETY OF GARDEN DESIGNERS

Take topsoil seriously


Leave soil selection to chance, says soil scientist Tim O’Hare, and you might have big problems


Soil is the foundation of every garden. It may not be the most exciting element, but if you ignore the soil, the full potential of any scheme may not be achieved. The garden could fail to establish or be left with long-term problems that are difficult and costly to repair.

If there are existing soils on site, these should always be ‘investigated’ in the first instance to see if they can be re-used. The cost of disposing of soils has vastly increased in recent years, and buying in new soil doesn’t always guarantee it will perform any better.

A soil assessment should ideally consider the condition (soil structure, compaction, topsoil depth, watertable depth) and composition (particle size, pH, organic matter, plant nutrients) of the existing soils. The end result should be a better understanding of its attributes, but also, more importantly, its limitations. Armed with this, a decision can be made as to whether the existing soils can be re-used.

A project I worked on last year summed up the problems when no such thought is put into existing soils. The site subsoils were clay and the garden was profiled into a series of cut embankments and flat planting beds. The combined poor drainage of the clay and the ‘adjusted hydrology’ of the site resulted in more of a pond than a garden. A simple drainage system sorted it out, but not before all the plants had died from water logging.


On foreign soil

If the site soil is unsuitable for re-use, or if there is a shortfall, imported soil will be required. Sourcing good-quality topsoil and subsoil is still seen as a major risk. We hear horror stories of ‘screened muckaway’ being used as topsoil, and ‘concrete crusher fines’ or ‘recycled sand’ being offered as subsoil. These types of material are noticeably cheaper, which should ring alarm bells. They contain chemical and physical contaminants, such as heavy metals, hydrocarbons, excess alkalinity, asbestos and glass shards, which render them completely unsuitable for gardens.

One of the best ways of ensuring imported soil is of the required standard and composition is to use a robust Soil Specification, which can be issued to a contractor or directly to the soil supplier. But don’t use the same soil for every scheme - recently I received a Soil Specification (to test soils against) that I actually wrote back in 1996 for a very unique project. The horticultural parameters were way off, and the contamination testing criteria has since been updated three times.

The British Standards for Topsoil (BS3882:2015) and Subsoil (BS8601:2013) are specifically intended to assess the quality of soils that are moved and traded. Compliance with the relevant standard should therefore be a prerequisite for selection, but just because a soil meets the standard, doesn’t mean it’s suitable for every project. Many testing laboratories don’t realise both standards require soil to be tested for human health contaminants – particularly important for gardens. Don’t accept any soil analysis that only covers the horticultural properties.

The use of imported soil means a new soil profile will have to be made. The main distinction between using imported soils and soil already in-situ is the additional disturbance the soil receives during importation and placement. Many soils, especially silt- or clay-based ones, can’t withstand this disruption and collapse and self-compact when placed. Sand-dominant soils are best for such projects, as they’re more resilient to excessive handling, are easier to spread and work, and are more forgiving in wet weather. It’s always advisable to buy soils from a dedicated soil-supply company. The industry has grown over the past 10 years, and good-quality sandy soils can now be found throughout the UK. As a bare minimum, any reputable supplier should be able to provide details about their firm and the soil materials they sell. The soil should be regularly tested (at least every three months) by an independent company (rather than an ‘in-house’ lab) and the results should be accompanied by an interpretive report to point out any restrictions or limitations that a soil may hold, and to advise on what ameliorants may be required before planting, turfing or seeding.