There is a growing push toward greening cities as a response to urbanisation and climate change. This is because adding trees and other plants to urban areas provides many benefits that relieve us from the pressures of high density and congestion.

Urban greening regulates temperatures by providing shade, and cooling via evapotranspiration.

Plants also provide other ecosystem benefits by managing stormwater and providing animal habitat. Human well-being is also improved when we can see and be close to vegetation, reconnecting us with nature and improving our mental health.

These ideas are gaining traction and being supported by governments at all levels.

Local Councils in Sydney and Melbourne have green roof design guides and initiatives to encourage uptake of urban greening.

Adelaide City Council has also released a Greening section of the Adelaide Design Manual that includes Green Infrastructure Guidelines, as well as Greening Design Standards and Technical Specifications.

green infrastructure

Adelaide City Council Green Infrastructure Guide
image: ASPECT Studios

Even the federal government is getting on the bandwagon with the recent announcement from Environment Minister Greg Hunt for tools to help cities set urban greening targets.

But plants require organic topsoil to grow, and often not enough is provided for them. Urbanisation historically involves a process of removing topsoil and replacing it with sub-soil and concrete. Sub-soil does not have the necessary chemical and physical properties to support plant growth. Plants need the correct structure to not only physically support them, but also allow essential biological processes to occur.

This include air gaps in the soil, sufficient availability of water, and the presence of minerals. Healthy soils also need to have organic matter and other elements such as nitrogen and phosphorus to provide nutrients for growth. Then there are the active microbial constituents of top soil. These are the fungi and other living biological creatures that synergistically interact in the soil with the plants to enable exchanges between the soil, water, nutrients, and roots. Sub-soil does not have these properties and will not support plant growth.

Matthew Daniel from Tree Preservation Australia believes the soil we are specifying as a standard for topsoil does not have all of the necessary properties to promote healthy trees.

Daniel and his colleagues have detailed soil and plant health assessments for urban areas across Australia. They have found from these assessments that what is being constructed is often far from adequate to support a green city. Soil health is slowly decreasing to alarming levels through compaction, low soil moisture, nutrient toxicity, pollution and root zones devoid of soil microbiology.

He says “the top soil that is provided by the horticultural industry is often barely soil. It is a minimum standard that approximates natural soil, geared toward what the landscape industry can easily supply. This material is manufactured often from easily accessible green waste and other organic industrial by-products.”

He believes we need to take a new approach to our urban soils.

“Plants play a pivotal role providing energy to the microbial life in the soil and indirectly to all other life forms,” Daniel said. “The soil‐plant root associations are based on the complex interactions of uncountable microbes. Each go to ensuring the plant can continue its important role of maintaining healthy growth to then in return provide the nourishment the soil biological network needs to flourish.

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So what does this mean to how can we influence and build healthy soils?

Daniel says that for microbes to perform in the soil, the correct ratio of air space (25 per cent), water holding (25 per cent), minerals (45 per cent) and organic matter (five per cent) must be present. This is the habitat that regulates the types and amounts of soil life upon which healthy plants can then grow and function.

As he puts it, “the nutrients, physical structure and key biology of soils must be well managed to grow healthy soils and subsequently healthy plants, animals, people and landscapes.”

As simple as it might sound in principle, this new NPK concept of managing the health and function of any life form is challenging in our cities.

Changing rainfall patterns and extreme heat, not to mention the practices of construction and development itself, create trauma to the sets of living organisms within soil. It is their ability to withstand these changes that is a measure of their resilience, and that of our cities as a whole.

Daniel observes that the root to shoot ratio of trees is commonly compromised and not factored in design specifications. Along with the provision of inadequate soil volumes, he notes that the impact on tree protection zones is another major issue that is far too common on development sites to the detriment of the tree’s future.

He believes that an exciting new world is gradually being revealed as scientists learn the intricacies of the complex world beneath our feet.

He says “understanding and better managing soil and plant microbiology represents a powerful tool which people can learn to apply to great advantage in building healthier soils, plants, animals, people and landscapes.”

Along with the loss of top soil, there is a lot of competition for space both in the ground and overhead. Electricity, water, sewer, and communications services are all located in the same spaces that we need to place trees and other vegetation. In this competition, engineering services and traffic requirements usually win out over trees. Trees and planted landscapes are usually the losers in the fight for space.