The good dirt?

 _{March 2024} 

Our cities' green spaces provide many important environmental services. They absorb and filter stormwater, improve air quality, provide shading and cooling, and offer habitats for biodiversity. These services will become increasingly important as we face the impacts of climate change.

To work well, green spaces need enough nutrient-rich soil. But during residential developments, urban soil is often stripped from site, dumped, paved over or reduced to a thin layer in small city gardens.

The latest report from the Parliamentary Commissioner for the Environment,  Urban ground truths: Valuing soil and subsoil in urban development , investigates what happens to soil during residential developments in cities and makes recommendations for how to better protect this important resource.

Healthy soil is made up of a layer of dark, carbon-rich topsoil and a deeper layer of well-draining subsoil.

At adequate depths, healthy soil will support a wide range of plants, including shrubs and trees, to grow. When soil depths are limited or subsoil is compacted, vegetation growth will be restricted.

This illustration shows that the depth, volume and quality of topsoil and subsoil impacts the different types of vegetation that can grow. The more soil there is, the larger plants can grow.

Manaaki Whenua – Landcare Research found that minimum recommended soil depths for landscaping varies greatly across territorial authorities.

This graph shows minimum soil depths required by 14 territorial authorities, the NZ Transport Agency Waka Kotahi and Standards New Zealand (no soil requirements were found for Hutt City Council or Porirua City Council).

The research also found that most councils do not provide guidance about subsoil depths. Without this advice, topsoil is more likely to be placed over compacted fill instead of subsoil.

Compacted fill has minimal capacity to store and retain rainfall, and restricts the growth of tree roots.

As our urban green spaces shrink, we need to ensure that those that remain are high functioning and are supported by deep, healthy soil.

There can be concerns about planting larger trees in cities as roots can damage pavements, roads and foundations, and interfere with pipes and cables. But solutions to managing roots within urban environments are available.

Barriers can be installed to channel and contain roots, and structural soil can be added beneath roads, carparks and permeable pavements to increase soil availability and minimise compaction.

In his report,  Urban ground truths , the Parliamentary Commissioner for the Environment makes several recommendations on how to protect urban soil. These include:

The Minister for the Environment should provide national guidance on urban soil and the services it provides. This would include guidance to support councils’ management of urban soils.

The Ministry for the Environment should provide guidance to councils on the recommended depths, volume and profiles for soil, as well as recommended practices for soil decompaction.

Councils should use tools like integrated catchment management plans to better coordinate stormwater management, urban development and soil protection. These plans should be in place before further new developments are consented.

Regional councils and other agencies should look at ways to encourage developers to conserve and protect soil. These could include initiatives like green-score techniques where developers are rewarded in some way for providing certain soil volumes or reducing the extent of impervious surfaces on a site. Rewards could include reduced processing times, development contribution rebates or a 'credit' to be redeemed against other planning obligations.

For more information on our urban soils and other environmental investigations visit  pce.parliament.nz