Landscape Architecture Australia
Designing with soil
Healthy soils are critical to plant growth, but modern building practices destroy soil through compaction, contamination and destruction of fungal and microbial communities. What can landscape architects do about it? —
How can landscape architects work with planting design to address urban soil issues? Article by Alistair Kirkpatrick.
Soil is fascinating. It is simultaneously biotic and abiotic, its qualities can radically shift within a square metre and it has remarkable plasticity. Top soil is critically important, as this is where the majority of soil life resides. Top soil allows for nutrient cycling through interactions with subsoil and gaseous exchanges.
While metropolitan Melbourne’s top soil is generally shallow and typically hydrophobic, the depth of the top soil in this region ranges from almost absent on the steep hills around Warrandyte to a metre deep in the bayside suburbs. Soil specifications, in all cases, need to be site specific.
An interesting case study is to compare Melbourne and London’s climatic conditions. Melbourne’s annual rainfall is higher than London’s, yet unlike Melbourne, metropolitan London supports lush Quercus and Betula forests. There are two key reasons for this – soil type and evaporation rates. London’s soils are typically younger than Melbourne’s (aside from the soils derived from weathered basalt) and younger soils contain more bioavailable nutrients and minerals. Nutrients absorbed by plants also need to be water soluble – conversely, this means that when soils dry out, the nutrients contained within it are no longer available.
Evaporation is critical to soil moisture levels; London’s rate of evaporation is far lower than Melbourne’s as the former has a cooler climate with greater cloud cover. The result is that London soils generally have more nutrients available, a condition that supports the development of broad-leaved plant communities, including the Quercus. Broad-leaved plant communities in the northern hemisphere are typically closed canopy forests, meaning minimal light reaches the forest floor. This, in turn, is a further contributor to the city’s typically lower evaporation rates. In London, hydrophobia is far less prevalent and worm, fungal and microbial activity – all processes essential to the breaking down of organic matter into nutrients – is higher.
Melbourne’s pre-European vegetation relied on a different nutrient-cycling process. Worms hibernate when the soil dries out, so much of the nutrient cycle in these earlier times was performed by ants and other insects. To combat hydrophobia, many plant species found in the pre-European Melbourne biome also produce compounds known as saponins, which act as wetting agents. In Melbourne’s highly varied urban conditions, it would be prudent to assess each site’s soil condition “facsimile” – in other words, to match the urban site’s current soil qualities and microclimatic conditions to a naturally existing nonurban biome. This can help us to better understand how the soil is functioning, how we might work with the soil to achieve particular outcomes and what plantings might be appropriate to achieve such effects.
A suburban park with minimal impermeable surfaces requires the application of wetting agents. The reason for this is that such parks generally function as facsimiles of dry sclerophyll forests. Dense urban areas, on the other hand, often operate as facsimiles of valley floors and the areas at the base of cliffs. In new dense urban developments, a reduction of evaporation could be achieved through the careful consideration of built form. By designing buildings and their landscapes to lower evaporation rates, we can help to reduce plant attrition.
Healthy soils are critical to plant growth, especially in the hostile growing conditions found in many parts of Australia. The ratio of sand to silt to clay can dictate whether a non-irrigated tree survives a drought or perishes. Landscape architects work within the urban environment and modern building practices commonly destroy the
soil though compaction, contamination and destruction of fungal and microbial communities. Alkaline soils are a common side effect of construction, an effect of the use of concrete and cement. Remediating the soil by lowering its pH is an expensive undertaking – budgets invariably blow out and soil remediation can be perceived as a low priority on major projects. With Melbourne currently experiencing an infrastructure boom, soil remediation can be considered a pressing issue, especially with the urgent need to increase tree biomass to capture carbon and combat the urban heat island effect. A radical restructure of construction practices seems unlikely, so what possibilities are there for landscape architects to work with urban soils? The following suggestions are intended to be propositional and provocative.
One possibility could see the redistribution of landscape architecture budgets into remediation, taking funds from visible design elements. This idea could be extremely unpopular as it would remove the visually tangible elements of a design – however in the long term it could foster opportunities for exploring more complex and nuanced planting design. With the world in climate crisis, is it still appropriate for landscape architects to be designing triangulated concrete sculptures swimming in seas of granitic sand? As the climate becomes increasingly more extreme, attrition rates of our urban vegetation are likely to dramatically increase, already it is near impossible to sit in the full sun on a Melbourne summer day. The establishment of vegetation canopy will only become more critical.
Another idea would be to work with soil contamination and degradation by looking to plant communities that have evolved to grow on subsoil or sodic/alkaline soils or soil impacted by compaction. Embracing the philosophy of “there is no such place as away,” it could provide interesting design challenges to work with the conditions of each site. By investigating plant communities in hostile conditions in Australia and overseas, we could start to generate some nuanced and resilient planting designs that minimize the necessity for large volumes of material being brought to site or removed. The key to this approach is the acceptance of time: systematic or passive remediation is a very long process especially in dry climates. The urban landscape does however offer certain advantages that accelerate the process of soil accumulation. The phenomenon of soil development can be seen along any of Melbourne’s tram routes – sand is dumped by the trams to brake when it’s wet, that sand gets washed or blown into crevices in the road, which are then colonized by mosses and spontaneous urban plants, binding the material and preventing further erosion. If there were no maintenance regime to clear these soil pockets, a viable depth of soil could form relatively quickly.
The final idea is a rethinking of planting design via soil manipulation. Along Merri Creek, between High Street and Heidelberg Road, it is possible to observe suites of plants growing in response to particular soil conditions and types. The creek’s northfacing bank is a constructed slope built from building rubble. Although this north-facing bank hosts species of vegetation that are shared with those of the parent soils (parent soils being the soils that have weathered from the underlying geology rather than being imported), on the south-facing bank, the composition and spatial arrangements of the vegetation differs. This could be an interesting idea to replicate in parks with low funding and traditionally high plant attrition rates. The current maintenance model dictates plant growth, distribution and species by mowing, spraying and pruning. This idea, by contrast, proposes soil manipulation at the parks inception, to guide growth, distribution and species by influencing the soil porosity, pH and moisture content.
Ultimately soil is critical to any plant community and should be at the forefront of landscape architectural practice as a way to generate meaningful, sustainable designs for a mercurial and tumultuous future.