Green Roof Soil - Great Opportunities Ahead!

by Dr. Anna Zakrisson on Tuesday, January 15, 2019

green roof soil

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Green Roof Soil – great opportunities ahead!

Green roof soil biology – why should I care?

Traditionally, green roof soil biology (or soil health) has not been considered a central factor for improving stormwater management or nutrient runoff in green roofs. This life-giving layer has merely been viewed as a compartment for water, and the intricacies and complexities of the biological systems have often been ignored.

We think that this one-dimensional view results in the loss of great opportunities. We are convinced that improving the roof’s soil biology helps creates a sustainable ecosystem, which can have profound positive impacts on vegetation stability, nutrient runoff, and stormwater retention, all leading to significant cost reductions for the end user.

green roof soil media on living roof

What do we mean by “sustainability”?

Sustainability is a modern buzzword, but what does it mean?

The dictionary defines the word like this:
“Sustainability - avoidance of the depletion of natural resources in order to maintain an ecological balance.”

“Ecological balance” sounds nice, but what does it mean in real-world terminology? Why should you care about this?

Let me ask you this: Would you paint your house with a coat that will flake off in a few years, or would you instead use a paint that will last for several decades and which you can purchase at a similar price as the inferior paint? I guess that you’d go for the higher quality paint.

A sustainable roof is like that good paint. It will last for years with low maintenance costs. Furthermore, improving the soil to create a more sustainable roof may be achieved with little to no upfront cost.

System sustainability means that the system can buffer for environmental variation. Periods of drought or flooding will have less effect on the roof, and fewer inputs of fertilizer or toxic pesticides are needed. Plants will thrive and be less prone to collapse.

A sustainable roof will require lower maintenance and will comply with environmental regulations, e.g. regarding nutrient runoff. The result is a win-win where there is a real financial gain to good-doing.

A truly sustainable system looks beyond just ecology - it also includes finances and engineering.

green-roof-soil-post.jpg
Soil is far from a dead pile of dirt. Look at all the stuff going on in this short clip. This is just the stuff a human eye can observe! Imagine all the hidden stuff!

What is a green roof soil?

Until recently, few biologists have been involved in the development of green roof systems. This is unfortunate and has resulted in the German Landscape Research, Development and Construction Society (FLL) soil guideline standards not having been questioned or improved much over the past years from a biological perspective.

However, this means that a closer look at soil biology could be a tremendous opportunity for the entire green roof market. Globally.

green roof soil purple-roof
Green roof soil media

Natural soil vs. green roof soil media

There is a difference between natural soil and what is currently used on a standard green roof. Often, the green roof soil is referred to as “media” to distinguish it from natural soils.

Currently, most green roofs are built using a media that strictly follow the German FLL soil guidelines. FFL developed a test called ASTM 2397 which has the single objective of establishing the maximum retention weight of the media at a 10 cm (~ 4 inches) soil column tested. This and the vertical Hydraulic Flow-through rate seems to be the focus of FLL.

These guidelines have helped standardize many aspects of the green roof industry which has been positive. However, the FLL guidelines have mostly been focused on the engineering objectives of keeping media light and well-drained with low organic composition. Thus, the biological aspect has not been fully explored.

Biological systems, such as soil systems, show greater stability with increased diversity. Hence, ensuring diverse micro- and macro-biota will make the system less vulnerable to different types of pests. It’s also possible to improve the cycling of nutrients, with reduced runoff, with some closer research attention to the nutrient cycling between the different soil compartments, as well as the soil-medium nutrient ratios (stoichiometry).

We believe that it is possible to create a healthy soil ecosystem on green roofs with tighter nutrient cycles and lower requirements for fertilization. The possibilities are near limitless if the green roof industry develops soil and material guidelines targeted to specific climatic regions. Such guidelines would make green roofs an even better financial investment than they already are, positioning green roofs to become a standard solution for runoff mitigation, stormwater management, the greening of cities on a global scale.

The role of green roof soil in retention and detention

Green roofs can retain a lot of rainwater. Small and medium storms are generally not a problem. A roof can retain about 80% of the frequency of all storms, but these storms are routine rain events, and they only make up about 50% of the total rain volume. The remnant 50% is what becomes a flooding risk.

This 50% is the Achilles heel of the current green roof industry but is an issue that is intensively researched, and many ingenious solutions are about to hit the market.

Extreme storms are expected to become more frequent over the years to come. Thus, if green roofs are to become the best solution for stormwater management, we need to look at these matters from many different perspectives, and that includes the often overlooked, but central, topic of soil biology.

green roof soil water cycle
The natural water-cycle showing transpiration and evaporation

Soil, evapotranspiration & plant happiness

Evapotranspiration is the main driving factor for retention. Evapotranspiration is defined as the sum of plant transpiration and evaporation processes.

Adding more of the currently used, and well-drained, soil medium to a roof profile will only increase retention to a degree, albeit with a diminishing return. The mean water table in a 4’’ (about 10 cm) soil is approximately the same as in an 8’’ (about 20 cm) soil. Hence, the main difference being that shallow-rooted plants can’t access this water table when it’s 6” down in the soil.

However, if a vegetation mixture interspersing shallow-rooted plants with more deeply-rooting species is used, increasing the depth of the soil medium profile might increase retention significantly.

Soil water retention and water availability can be further improved by incorporating different types of soil amendments and ensuring a healthy microbiota.

Increasing the humus percentage of the soil by a mere 1 % can greatly increase the amount of water held by the roof, which is then available for evapotranspiration – the primary vehicle through which green roofs ultimately “recharge” their stormwater absorption capacity.

Furthermore, healthy soil biology means more lush vegetation, and lush vegetation - in turn - equals higher transpiration rates. Hence, despite the soil’s only partial direct effects on retention, it has strong secondary effects that should not be ignored.

A better understanding of the interplay between different soils, plant succession, and climate is essential for creating the perfect green roof for each individual customer worldwide.

Detention, however, happens in the drainage layer of the green roof. Detention is the key ROI for green roofs. You can read more about this important topic in our articles on detention.

green roof soil photosynthesis

Soil and vegetation

Sedum plants are very commonly used as green roof vegetation. One of the main benefits of sedums is their extreme tolerance to drought. Sedum plants make use of a neat little variation of photosynthesis called crassulacean acid metabolism (CAM). This modification makes it possible for the plants to close their gas exchange openings (stomata/guard cells) during the hot daytime and open them during the colder night when less water is evaporated.

Some sedums even switch between CAM and “normal” photosynthesis (C3) depending on environmental conditions. These plants are referred to as facultative CAM plants.

Both broadleaf and ruderal sedums are used on the roofs for different purposes. Deciduous broadleaf sedums add to the natural cycle of life more so than the ruderal species due to the addition of biological material to the soil when they drop their leaves. However, the fast growth of the ruderal species also has its benefits, particularly when it comes to rapid plant establishment, a key function in protecting the soil from wind and weather.

green roof soil diverse sedums
Diversity makes the system more stable

Soil & nutrient runoff

Plants need different types of nutrients to be happy. In addition to the macronutrients phosphorus, nitrogen and potassium, plants require a whole range of micronutrients such as iron (Fe), boron (B), chlorine (Cl), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), and nickel (Ni).

Generally, the old strategy has been to apply ample amounts of fertilizer on fields and roofs to ensure that the plants have enough nutrients to enable fast growth. While this might work wonders short-term, it isn’t a good long-term strategy. Fertilizer application requires maintenance, is costly, and will result in very high nutrient runoff from the roofs.

Much of the applied nutrients will not stay on the roof but end up in ponds and other local water bodies downstream. This runoff may also go against local environmental regulations, so a new soil strategy is required to prepare the green roof industry for a blossoming future ahead.

plant nutrition

Phosphorus and nitrogen runoff

One major issue for the green roof industry is phosphorus (in the form of phosphate) runoff from the roofs. It still amazes us that people use over-the-counter fertilizers that contain copious amounts of unnecessary Phosphorus. Why? This excess phosphorus can be of severe environmental concern as phosphorus excesses, particularly in freshwater systems, can lead to toxic algal blooms and contamination of drinking water deposits.

As this problem is expected to become more severe in the years to come, we should be prepared for future strict regulations.

There are many exciting ongoing research projects currently looking at ways to mitigate phosphorus runoff. Many make use of phosphate-binding materials that hold the phosphate in the soil and prevent it from being flushed out in the runoff.

However, too tight a bond between the added materials/chemicals and the phosphorus can result in the phosphate also being unavailable for plant uptake.

Perhaps, biology can add value to this engineering approach?

We firmly believe in the possibility of creating natural and sustainable systems on the roofs. In the end, phosphorus cycling in nature is a well-functioning system. A healthy soil ecosystem can retain the available phosphorus in the biomass of micro- and macro-biota and plant tissue, and so enable efficient cycling of nutrients between these compartments.

The currently available data on nitrogen runoff from green roofs is conflicting. Some studies claim that roofs are sources for nitrogen, while others have found that green roofs are sinks. This uncertainty around the behavior of nitrogen in green roofs soils underlines the necessity for the further development of standardized mesocosm research to understand these complex processes further. Only then can we, as an industry, provide specialized roof soils for specific climate regions.

retention basin
Nutrent-fuelled algal bloom in a retention basin

Green roof soil - a summary

A healthy green roof soil means efficient nutrient cycling, less fertilizer applied to achieve lush growth, lower maintenance costs, less plant replacement, and lower nutrient runoff. A healthy green roof soil means compliance with environmental regulations.

A green roof ecosystem includes not only detention and retention but also soil and vegetation. All of these parts work together to create a future roof that makes our cities sustainable and protected. We need closer collaborations between engineers, architects, biologists, business & finance.

Together, as an industry, we can make a profitable and resilient future possible!

Reading tip: Green roof soil

Akther, Musa, et al. "A review of green roof applications for managing urban stormwater in different climatic zones."Sustainability10.8 (2018): 2864.

Audio article

This article is available as an audio version on all major podcast directories such as iTunes, Spotify and many more.

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