Soils are alive with thousands of tiny organisms which live in their own little ecosystem, as advanced and as complex as anything above ground.

Not only this, but every soil contains its own blend of inorganic matter (sand, silt and clay) and organic matter (which break down to provide nutrients for plants). Soils are a vital part of the natural environment and should be cared for as such, argues Tim O’Hare of Tim O’Hare Associates, who hosted a Landscape Institute CPD which we recently attended in Birmingham.

It may come as no surprise that soils are an important consideration for Hosta, they are a major part of our schemes, and there’s a lot more to them than dirt. Over time we’ve come to realise that each landscape profession values soils in a different way. Engineers will be looking for structural capabilities of the soil, Ecologists will be looking at microbe activity, Horticulturists will be looking at nutrient and water-retention, and contractors will be wanting to know depth and areas. Each profession focuses on a small part of the wonderful substance which allows the majority of plants to grow. As designers and project managers we have to manage interaction on site to get the best out of the soil, which means juggling all aspects.

Tim’s talk was really interesting and gave us a specialist insight into elements that we may not have considered previously. Whilst we were aware of the importance of developing project specific soil specifications, Tim highlighted how vague the British Standard for Specification for Topsoils (BS3882:2015) is, regarding what conforms to its standards. 

Source: https://dougsarchaeology.files.wordpress.com/2012/06/textural_tri_hi.jpg

On a structural level, soils are classified by their texture which is created by the varying quantity of sand, silt and clay (see soil textural triangle). A soil that has a high clay content is likely to be very dense, heavy to dig and will drain slowly. A sandy soil, on the other hand will be much easier to cultivate and work, but will drain much quicker, which means drought tolerant planting will need to be considered. Silt soils can hold more nutrients than sandy soils, but can easily become compacted, leading to water logging and other problems.  

The British Standard for soils covers a range of different textured soils, which can obviously lead to a wide range of planting conditions. Therefore Tim recommended specifying ‘sandy loam’ or ‘loamy sand’, if more site specific soils are not required. These two types of soils will be known to keen horticulturalists as having the perfect balance to create a free-draining, water-retentive, nutrient-rich substrate.

Soil is a vital part of the natural environment, and careful consideration should be made to preserve it, particularly where it is in short supply in our urban environments. All good planting starts with the soil, and promoting biodiversity, not only above ground, but also beneath it.

Tim O’Hare Associates, is a leading independent environmental consultancy specialising in Soil Science and Landscape Engineering.

(www.timohare-associates.com)

 

Additional Reading:

- BS3882:2015: http://shop.bsigroup.com/ProductDetail/?pid=000000000030297815

- Construction Code of Practice for the Sustainable Use of Soils on Construction Sites: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/69308/pb13298-code-of-practice-090910.pdf