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Soils and tree roots: improving conditions for trees

In the last article we looked at the five key requirements for tree health provided by soil: oxygen, water, nutrients, space to explore and anchorage. In this article we look at how we can improve life for trees by improving soil conditions.

Disturbance to existing trees

During development, it is common to retain mature trees and build around them. Unfortunately, this can often lead to the slow decline and death of these trees. Urbanisation of the natural soil system can alter the soil characteristics to create a new environment that established trees may no longer tolerate: wetter soil, less oxygen, too much nutrients, for example.

The surface 20 cm of a soil is the most critical zone for root health, but it is also the most sensitive to urban disturbance. The most important changes instigated by urbanisation are:

• compaction by traffic – both vehicles and pedestrians – which reduces water infiltration and gas exchange
• a rise in soil pH due to lime, which is commonly used in construction
• a rise in soil phosphorus levels through manure, litter and fertiliser
• scalping of topsoil, either deliberately or by erosion, leading to loss of nutrients, organic matter and routes for gas exchange, confining roots to the poorer subsoil properties
• burial by fill brought in to raise levels and by topdressing of lawns, both of which reduce gas exchange
• reduction in the available soil volume by the burial of pipes etc.

The root system of a tree is dynamic and changes its depth of activity according to season, moisture availability and oxygen availability. But these changes are gradual, and rapid alterations to soil are often fatal, because the roots cannot respond fast enough.

Chemical alterations are better tolerated, and a slow and steady decline in topsoil nutrient levels results at first in a slowing in growth rate until the decline becomes visible.

Improving soil conditions for established trees might require the removal of added soil, the return of topsoil to the original level, the restoration of original soil water movement (by either reducing extra flow or restoring cut-off flow), de-compaction of soil or the addition of fertiliser. However, if site conditions are altered too much, it might be better in the long term to remove the tree and replant a sapling, which can adapt to the new conditions as it grows.

Improving (natural and disturbed) soils before planting

When starting afresh, the approach that works best is to reproduce a natural soil profile with coarser and more sandy material as topsoil over a more clayey subsoil or fill layer. The fill and topsoil should have a good cation exchange capacity.

Where conditions are severe – for example, salinity in subsurface fill and no possibility of rectification – the only practical solution is to choose tree species that can tolerate the soil conditions.

An important long-term aim is to allow the natural soil processes of organic matter and nutrient cycling to occur. This ultimately minimises maintenance. For example, if a healthy mulch layer is used to kick start an ecosystem on fill or excavated material, if the basic nutrient deficiencies are addressed, and if a diversity of plants, including legumes, is included in the species mix, the landscape should be almost completely self-sustaining.

Designing soils for new plantings

Sometimes SESL has the luxury of completely designing the entire soil system for an intended planting. This is necessary in difficult situations such as where there is no soil, where there is very intense traffic, or indoors. For example:

• Construction of “facsimile” soil profiles following natural soil principles at the Sydney Olympics site. This former industrial site was heavily disturbed, had no natural soil remaining, and presented only fill and excavated sandstone to work with. Working with SESL, Australian Native Landscapes created soil from compost and crushed sandstone.
• Use of structural soils and other “gap-graded” soils designed to be incompressible.
• Use of potting mixes in interior and exterior landscapes, notably Sydney’s Capita Centre and Casino. Such mixes must be designed with longevity in mind.