Soil preservation

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 people¹.

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 systems². 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 humans³.

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 (for example by using protective mats) throughout construction phases rather than repaired post-compaction⁴, ⁵. Raised boardwalks, if designed well, may be a useful way to protect soil as well as dunes, tree roots and other sensitive areas in some contexts⁶.

Soil handling and reuse

Inspect soils prior to movement. Only relocate soil verified as clean and free of invasive propagules (such as seeds and plant fragments that enable species to spread). Replace soil using loose tipping to avoid secondary compaction⁵. Where soils are temporarily removed for construction, aim to stockpile horizons separately (e.g., topsoil, subsoil) so that soil layers can be reinstated. Appropriate plantings may be useful for interim protection of stockpiled soils.

Vegetative cover and erosion control

Soil erosion can be caused by wind and water. Maintain continuous soil cover through using perennial vegetation planting where possible. Vegetated buffers along waterways reduce sediment and nutrient runoff⁶. On slopes, use terracing and natural-fibre erosion control matting to stabilise exposed soils⁶, ⁷, ⁸, ⁹.

Mulching

Apply organic mulch at sufficient depth to protect soil surfaces, reduce erosion, conserve moisture, and suppress weeds⁶, ⁹. Avoid mulch derived from treated or contaminated timber¹⁰.

Soil decompaction

Where compaction has occurred, assess soil texture, moisture, and compaction depth before intervention¹¹. Mechanical decompaction should be timed to avoid smearing or recompaction and combined with organic matter inputs for longer-term benefit¹¹, ¹², ¹³.

Invasive species control in soils

Thermal soil treatment (e.g. stationary soil steaming) can effectively neutralise invasive seeds prior to soil relocation¹⁴, ¹⁵. 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 intensity⁷, ¹⁰, ¹¹.

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 integration¹⁶.

Issues & barriers

Construction sequencing risk

Works on wet soils can worsen compaction and negate remediation benefits¹¹.

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 contaminants¹⁰.

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 infrastructure¹⁷, ¹⁸, ¹⁹.

Financial constraints

Short-term project budgets often prioritise hardscape outcomes over long-term soil performance benefits¹⁸, ²⁰.

Synergies & 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 rainfall⁷.

Freshwater security: Reduced sediment runoff improves urban stream health.

Food security: Healthy soils enable productive urban agriculture²¹.

Human wellbeing: Vegetated, biologically active landscapes improve thermal comfort, air quality, and mental health outcomes²², ²³, ²⁴.

Waste and pollution management: Soil-integrated green infrastructure filters pollutants and reduces reliance on grey systems²³, ²⁴, ²⁵, ²⁶, ²⁷.

Financial case

Ecosystem services &/or performance value

Value type

Healthy soils reduce long-term stormwater infrastructure costs, improve vegetation survival rates, and lower maintenance inputs for irrigation and fertilisers¹⁰, ¹⁷.

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 appropriately¹⁶.

Monitoring & evaluation metrics

Core metric

Soil organic matter, compaction, structure, and infiltration rates measured through field assessment and soil testing²⁸, ²⁹, ³⁰, ³¹, ³², ³³.

Advanced or long-term metric

Biodiversity indicators (e.g. soil biota activity, acoustic indices, NZ Biodiversity Factor–Residential scoring)³⁴, ³⁵.

Additional resources or tools

Caring for Urban Streams: Erosion

Guidance on erosion and sediment control for urban sites.

Earthworks Good Practice

Regional earthworks and sediment control guidance.

A Guide to Soil De-Compaction

Practical guidance on assessing and treating compacted soils.

Auckland Botanic Gardens – Mulch

Best practice guidance for mulch selection and application.

References

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