Photo: papan saenkutrueang
A new report has found greater protection is needed for soil in urban areas, highlighting the important role it plays in natural drainage during heavy rain and providing green spaces in cities.
Urban ground truths, the latest report from the Parliamentary Commissioner for the Environment, includes recommendations for clearer guidance for territorial authorities from the government to preserve the quantity and quality of soil in urban areas.
Commissioner Simon Upton said his main goal was to put soil on the radar, and draw attention to its benefits and the things which threaten it.
With the National government's housing growth plan aiming to fast-track developments on new land, requiring councils to zone enough land for 30 years of demand, increasing areas of untouched soil will be under threat.
Why is soil important?
Simon Upton Photo: [https://www.flickr.com/photos/eu2017ee/35725837402flickr]
Soil acts as a drainage mechanism, slowing the movement of water and filtering contaminants. With more severe weather events predicted due to climate change, this would only increase in importance, the report found.
"When we get extreme [weather] events, if everything's been hardened over and you're having to use concrete pipes for everything, there's a lot of cost," Upton said.
"Soil, to the extent that we can leave it in place, can absorb a lot of moisture."
It also supported vegetation, with deeper soil allowing for larger, more shady trees, with greater capacity for carbon sequestration and mitigating the "urban heat island effect".
The report said shade from trees could provide measurable air temperature cooling of between 0.4 and 4.5 degrees Celsius.
The country needed to make the most of the green space it had, he said, meaning allowing trees to get as big as possible.
"We are going to be living in a much hotter world, and if our cities are concrete, asphalt and built structures with very little green because there isn't the soil to support trees, we're going to be living in a very hot place."
What's threatening soil quality?
Currently, the report found, housing developments tend to remove more soil than necessary in the name of efficiency and keeping costs low.
Geotechnical engineers did not tend to focus on soil health, rather on providing a solid base for new homes.
The report found the most common foundation type, concrete slab, required a flat section on "good ground", often led to over-excavation.
Without a thick enough layer of soil or subsoil, tree growth was stunted and drainage was compromised.
In addition, the report found excavated soil was often discarded. The Ministry for the Environment estimates that of the 800,000 tonnes of "potentially hazardous" material sent to landfill, the bulk of that is made up of soil.
The report said: "To summarise, regulation and geotechnical requirements require some soil to be excavated. But there are additional drivers that relate more to the current market".
These included, maximising dwellings and efficiency of site clearing, and therefore, profit.
What does the report recommend?
Among other recommendations it says:
- The Minister for the Environment should consider providing national direction and guidance to councils addressing urban soil depth and volume
- Developers should be required to submit a landscape plan to territorial authorities in relation to any major new development, which includes information about the depth, volume, composition and profile of soils, and a planting plan
- All territorial authorities should be required to undertake prior planning for stormwater management for urban growth, such as an integrated catchment management plan
- Councils should encourage developers to conserve and protect soil, submit all consent applications - including stormwater and earthworks - at the same time, and reuse uncontaminated soils on-site.
- Increase the recommended topsoil depth for lawns to improve rainfall storage capacity
