Healthy Soil = Healthy SuDS
By Kevin Barton, Robert Bray Associates
A recent article by Carolyn Lebel in Resurgence linking soil fertility with infiltration has reminded me of the importance of soil health, not just in terms of supporting vigorous plant cover, but also the difference it makes to water infiltration rates and soil erosion.
With a background in Landscape Architecture, I am used to the level of protection, careful treatment, storing and amelioration that needs to be specified and enforced on site in order to preserve the delicate and invaluable resource that is our topsoil.
“This dark domain harbours a quarter of all known biodiversity on Earth, and a staggering 80% of its biomass… Our underground biota may largely be single-celled, but their communities administer the very building- blocks of life, delivering food, inspiring medicine and purifying water.” 1
Once damaged through careless handling, trafficking, contamination or denudation, the intricate relationships between ants, bacteria, fungi, microarthropods, nematodes, earthworms, protozoans and yeasts that makes the soil enable to sustain us are broken down and are very difficult to reestablish. Damaged or denuded topsoil can seriously hamper the establishment and long-term health of shrub planting and grass swards that generally means places don’t look as pretty and verdant as they could.
However, for SuDS landscapes, it can also mean much more. Soil fertility can very significantly affect the rates of infiltration as well as the rates of erosion that silt up our watercourses. In a study by Richard O. Meeuwig, findings showed that :
“plant and litter cover accounted for 73% of the variance in the amount of water retained by the study plots” and“Organic matter content is the most important soil factor influencing soil erosion… For example, if organic matter content is 10%, the weights of soil eroded are only one-half of those [encountered in soils of the average organic matter content of 5%].” 2
So, whilst we commonly test infiltration rates on the underlying soil strata, or soil that has been transported to a lab and squashed into a tube, this is only half the story. For the majority of rainfall events, it will be the topsoil layer and the fauna and vegetation living in it that will determine how much water infiltrates and how much runs off from the site.
Looking after and promoting the health of a site’s soil is as important as any other aspect of SuDS design and requires clear instruction to contractors to preserve it in-situ wherever possible, carefully strip and store it in shallow aerobic stockpiles, limit site trafficking, protect from construction contamination, ameliorate with organic matter where appropriate and carefully re-spread on landscape areas.
“On a given parcel of land, the fertility of soil determines whether water is more likely to infiltrate the Earth’s crust, slowly percolating down to the aquifer, being purified along the way by microorganisms specifically designed for the task, or whether it will run off the land, potentially washing away largely non-renewable topsoil into rivers and waterways. This kind of soil erosion represents a permanent loss of soil capital.” 1
1. Living Soil, by Carolyn Lebel – Resurgence 274
2. Infiltration and Soil Erosion as Influenced by Vegetation and Soil in Northern Utah, by Richard O. Meeuwig