Definition
Soil preservation protects and maintains soil structure, biological function, and ecological performance during construction and development. Protecting urban soil is crucial because it supports building construction and plant production, it is an interface with the atmosphere and hydrosphere and it is a source of key functions and services for urban systems sustainability and therefore for the wellbeing of urban biodiversity and people.
What this strategy does
Limits soil disturbance, compaction, erosion, and contamination while retaining soil as living infrastructure that supports water regulation, biodiversity, and long-term site performance.
Context
Urban development is a primary driver of soil compaction, erosion, and ecological degradation. Once soil structure and biotic function are lost, recovery is slow and costly, with direct consequences for stormwater performance, vegetation health, and downstream freshwater systems. Natural soil biodiversity may also play important roles through One Health (the interconnection between human, animal, plant, and ecosystem health), by controlling pathogens and supporting the immune fitness of plants, animals, and humans.
Technical considerations
Design considerations
Minimise soil disturbance and compaction
Avoid unnecessary excavation and trafficking. Prevention of soil damage or loss is more effective than remediation; soils should be protected throughout construction phases rather than repaired post-compaction. Raised boardwalks may be a useful way to protect soil as well as dunes, tree roots and other sensitive areas in some contexts.
Soil handling and reuse
Inspect soils prior to movement. Only relocate soil verified as clean and free of invasive propagules. Replace soil using loose tipping to avoid secondary compaction. Where soils are temporarily removed for construction, aim to stockpile horizons separately so that soil layers can be reinstated.
Vegetative cover and erosion control
Maintain continuous soil cover through using perennial vegetation planting where possible. Vegetated buffers along waterways reduce sediment and nutrient runoff. On slopes, use terracing and natural-fibre erosion control matting to stabilise exposed soils.
Mulching
Apply organic mulch at sufficient depth to protect soil surfaces, reduce erosion, conserve moisture, and suppress weeds. Avoid mulch derived from treated or contaminated timber.
Soil decompaction
Where compaction has occurred, assess soil texture, moisture, and compaction depth before intervention. Mechanical decompaction should be timed to avoid smearing or recompaction and combined with organic matter inputs for longer-term benefit.
Invasive species control in soils
Thermal soil treatment (e.g. stationary soil steaming) can effectively neutralise invasive seeds prior to soil relocation. However, this may also kill some beneficial invertebrates and microbes, so consult with an expert.
Implementation considerations
Design priority
Protect existing soils in situ wherever possible and sequence works to minimise exposure and trafficking.
Key constraint
Decompaction and mulching effectiveness vary by soil type, moisture condition, slope, and rainfall intensity.
Relevant tools or standards
Local authority earthworks and erosion and sediment control guidelines; soil visual assessment frameworks; water-sensitive urban design (WSUD) guidance for soil–water integration.
Issues and barriers
Construction sequencing risk
Works on wet soils can worsen compaction and negate remediation benefits.
Soil and landscape issues
Consult local experts and soil maps to understand local issues. For example, soils may have become acidic under previous land use, or be prone to landslip in sloping terrain.
Material quality risk
Mulches and soils without verified provenance may introduce weeds, pathogens, or contaminants.
Knowledge and capacity gaps
Limited practitioner expertise in soil ecology and multi-species interactions can result in soil being treated as inert material rather than living infrastructure.
Financial constraints
Short-term project budgets often prioritise hardscape outcomes over long-term soil performance benefits.
Synergies and opportunities
- Climate change – Improved soil structure enhances infiltration, reduces flood risk, and supports soil carbon storage.
- Disaster risk reduction – Erosion control reduces sediment-driven flood impacts during extreme rainfall.
- Freshwater security – Reduced sediment runoff improves urban stream health.
- Food security – Healthy soils enable productive urban agriculture.
- Human wellbeing – Vegetated, biologically active landscapes improve thermal comfort, air quality, and mental health outcomes.
- Waste and pollution management – Soil-integrated green infrastructure filters pollutants and reduces reliance on grey systems.
Financial case
Ecosystem services and/or performance value
Value type
Healthy soils reduce long-term stormwater infrastructure costs, improve vegetation survival rates, and lower maintenance inputs for irrigation and fertilisers.
Cost-effectiveness
Investment logic
Nature-based stormwater and soil management systems can achieve comparable or lower life-cycle costs than conventional engineered solutions when designed early and maintained appropriately.
Monitoring and evaluation metrics
Core metric
Soil organic matter, compaction, structure, and infiltration rates measured through field assessment and soil testing.
Advanced or long-term metric
Biodiversity indicators (e.g. soil biota activity, acoustic indices, NZ Biodiversity Factor–Residential scoring).
Case study
Transmission Gully Motorway
Related design strategies
Additional resources or tools
Caring for Urban Streams: Erosion
Auckland Council guidance
A Guide to Soil De-Compaction
https://www.cif-ifc.org/wp-content/uploads/2018/03/4_17-0006-Soil-Decompaction-EN_nov_29_acc-1.pdf
Auckland Botanic Gardens – Mulch
https://www.aucklandbotanicgardens.co.nz/garden-advice/garden-tips/healthy-soils/mulch/