Earthy Habitats: Tim White on working with soils to achieve biodiversity net gain
Requiring developers to achieve biodiversity net gain (BNG) is a positive step towards protecting existing wildlife and creating new habitats.
But the UK government’s Statutory Biodiversity Metric is not a guide to habitat creation, rather a tool for quantifying a site’s biodiversity based on the distinctiveness and quality of the habitats present. It also factors in the difficulty of creating each habitat type.
Metric Condition Assessment criteria for many habitats set a high bar to achieve. A new sward, for example, needs to be a good representation as defined under the UK Habitat Classification System (UKHab) – so no more general purpose ‘Heinz 57’ seed mixes.
Other criteria put limits on the presence of suboptimal species, and the complete absence of certain non-native invasives. We have to assume that local authorities will audit BNG systems, with consequences for developers based on their findings, so new habitats need to be well planned and executed to avoid being rated ‘poor’, a la Ofsted.
In this context, the importance of soil fertility status for new and existing species-rich grasslands and the consequences of high fertility soils, whereby nutrients encourage the growth of aggressive plants, are well understood.
“Designing a scheme to match the in situ soils will always be easier than trying to change their inherent properties.”
Phosphorus seems to act like a growth steroid, boosting many plants to triple or more their normal size. It then becomes impossible to fit more than two or three plants per square metre, rather than the 10 per square metre required for a ‘good’ rating.
Understanding this limitation at an early stage and moving towards habitats that thrive on fertile soils, such as woodland and scrub, can prevent costly efforts to manage weeds. Beyond soil fertility, many UKHab habitats have specific soil requirements in terms of drainage, pH, carbonate, and soil thickness.
Beech woods prefer shallow, well-drained, alkaline, chalky soils, whilst neutral grasslands are found on heavy, non-calcareous, slightly acid soils with restricted drainage.
Designing a scheme to match the in situ soils will always be easier than trying to change their inherent properties so, for both on site BNG developments and off site BNG mitigation schemes, these wider soil factors should be determined early to inform the design.
A site-specific soil survey should be undertaken by a soil scientist – this includes digging small trial holes to examine the soil conditions and collecting representative samples for laboratory testing – to provide an interpretative report with the findings, enabling a clear understanding of a site’s soils.
Whilst generic soil data is available via on-line mapping services, this can be completely wrong and definitely would not pick up local variations that could, once identified, reveal opportunities to further enhance site biodiversity.
It is possible, in some circumstances, to create distinctive habitats using materials such as crushed brick and/or concrete, sand, and other demolition fines. These opportunities are best explored to create new, rather than replace existing, habitats.
Read the full February edition of Pro Landscaper here.
Tim White
Tim White, a professional soil scientist since 1999, is senior associate at Tim O’Hare Associates. He leads a specialist team providing technical soils input for BNG and has co-authored with nationally recognised Forest Research the Best Practice Guidance (BPG) document on ‘Soil Preparation for the Creation of Species-rich Grassland Habitats’. He has also contributed to several other BPG documents for specific grassland types.
