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Researching Soil Types for Rain Gardens
A Three-Year Study

by Jim Funai, Assistant Professor of Plant Science and Landscape Technology at Cuyahoga Community College, Cleveland, Ohio

Researching Soil Types for Rain Gardens

An aerial view of the test plots in Cleveland, Ohio at the Cuyahoga Community College.

My research is focused on rain garden planting soils. Namely, the current specification being used across the nation by engineers and landscape architects (most between 60 - 80% sand) is completely useless to a majority of landscape plants.

I come from the horticulture side, so of course I simply blamed it on the architects and engineers for not understanding plants at all. The current recommendation, and much of the work done by horticulture minded researchers, was to find plants that could "survive" in this hostile growing environment. Survive? How are we looking for plants that can simply survive? I want to see plants THRIVE.

It wasn't until attending a training session at the Interlocking Concrete Pavement Institute, where it all hit me. I sat there, listening to their Director of Engineering, Robert Bowers, describe soil. A phrase he said will stick with me forever. "What's the best way to improve a soil?" Of course, I said it likely needs aeration and a soil test to determine chemical properties and organic material needs.... He smiles, "No, compact it."

COMPACT IT? Yes. From his perspective, to build a functioning pavement system and reach the Proctor Density needed by their calculations, soil compaction is vital. From the horticulturist perspective, compaction is likely the biggest challenge we have to growing plants in the urbanized environment.

Researching Soil Types for Rain Gardens

Soil Plot 1 with an experimental blend of haydite and compost.

Researching Soil Types for Rain Gardens

Soil Blend Plot 2 containing an experimental blend of 60% Haydite, with 40% in-situ clay loam by volume.

Researching Soil Types for Rain Gardens

Soil Plot 3 includes the Ohio Department of Natural Resources' specified soil for rain gardens with >70% sand content.

This is when I realized that we need to communicate better between all of our trades in order to provide the beautiful and functional landscapes needed to combat the negative impacts of a growing world population.

My Research
Because the engineering perspective of soil is structural, there isn't as much attention payed to the materials that pass the #200 sieve. They simply call them "fines" and try to keep them to a specified minimum, often below 10%.
In horticulture, we define and measure below the #200 sieve, as we are interested in how much and what type of clay is in the soil, to create a better environment for root systems. Many landscape plants in the Midwest (and many other regions) do much better with clay contents between 15 - 40%, even well into 50 or 60%.

Traditionally, much of the research done on stormwater management practices has been by engineers who tested the infiltration rates, saturation rates, and dewatering times. They tested physical filtration capability, chemical filtration and the results pointed to high sand, low fines. This makes sense to them, as the goal is to filter the water quickly, and move it along.

Somehow, some horticulturists got on board and said, "OK, we'll find some plants that will tolerate this planting condition to make a "rain garden."" So here is the condition: periods of extreme flooding where the plant must endure up to 48 hours of saturation (anaerobic soil). The soil quickly drains to field capacity, then depending on the season and location, these plants must then survive a period of time where the soil rapidly reaches, and remains at, or below, the wilting point. That happy time in between field capacity (when gravity has pulled all it can out of the soil) and wilting point (plant cannot pull any more water out of the soil) is called Plant Available Water (PAW). Horticulturists spend their whole life trying to maintain landscapes within this PAW window. Too much water, the plant can't breathe; too little water, the plant can't drink.

Researching Soil Types for Rain Gardens

Freshly installed soil plots. From right to left - Plot 1: BioG Mix, Plot 2: 60:40 Haydite: in situ, Plot 3: Ohio Department of Natural Resources' Standard soil.

Researching Soil Types for Rain Gardens

This is the Double Ring Infiltrometer by Turf Tec International. It measures the downward flow of water through the soil and includes a timer to correlate irrigation time to infiltration rate. This test was from July 2018 in Plot 1, which maintained original infiltration rates, while Plot 3 silted over and lost over 75% of the original infiltration rate.

Researching Soil Types for Rain Gardens

This is Winterberry holly in Plot 3 after a storm. Plot 3 ended up with 3" of standing water, while Plot 1 and 2 had no standing water after the storm.

Knowing that I couldn't walk into the conversation and say, "hey, clay is awesome at holding water and nutrients unlike sand, so let's make a rain garden soil with a bunch of clay!" (if you see figure 1. The PAW represented by the orange arrow, of Sandy Loam is the same amount as pure clay). I knew I had to meet the engineer requirements for infiltration rates, filtration capacities, and load bearing capacity. I also wanted to meet the horticultural needs of keeping as much PAW in the soil as possible.

Well, three years later, here I am analyzing my results and we've found a winner. Utilizing the impressive capability of lightweight expanded shale, combined with some compost and pine fines, there is a soil recipe for stormwater bioremediation that is able to keep the "bio" in bioremediation and meet all engineering specifications for chemical and physical filtration.

For my experiments, we utilized a local product in Cleveland called Haydite BioBlend. Haydite is a expanded shale that has great properties for this type of use, including: high absorption rates, nutrient holding capacity, improved air exchange, high surface area for microbial activity, and as you guessed it, very high plant available water holding; all while draining as fast, and sometimes faster, than the high sand blends we are used to.

Researching Soil Types for Rain Gardens

The trouble with a high sand soil is the window between field capacity, and wilting point is the absolute smallest of all soil types. While there are a number of ways to measure this, a simple measure is the percent of water in the soil matrix. In a high sand soil, say a Sandy Loam, like most rain garden recipes out there, we must manage a soil water content between about 8-10% and 20%. Above 20% and gravity pulls it out due to sand's inability to hold onto water. Below 10% and the plant cannot physically pull the water off the soil matrix.

Researching Soil Types for Rain Gardens

View across Plot 2 into Plot 3 in July 2018, showing plant failures.

Researching Soil Types for Rain Gardens

The graph shows soil moisture readings taken every 30 minutes from June 1, 2018 to July 1, 2018, three years into establishment of these gardens. This box and whisker plot shows the range of readings (the smaller the box, the closer each reading is to one another thus less fluctuation in moisture, thus less stress to the plants). The dark blue arrow indicates the range of plant available water in the Haydite Bioblend and the traditional sandy loam. It is clear that the Sandy Loam mix experiences much wider range in soil suction (how hard is the soil fighting the plant for water) and spends most of the time above the Plants Available Water (PAW) limit of that soil. Conversely, the expanded shale creates a soil environment, which maintains a near constant level of soil moisture well within the PAW range. Furthermore, the expanded shale allows a much wider range of soil suction without losing PAW.

Comparing this alone to the survivorship of plants in each garden after three years of growth and the story becomes quite clear; with a 48% survival rate in the sandy loam soil, versus 96% in experimental soil A (60% expanded shale, 20% compost, 20% pine fines), or even 76% in experimental blend B (60% expanded shale, 40% in situ soil, in this case a silty clay).

We'll be releasing the full study soon for publication showing a number of stress factors, including poor PAW, and render the high sand soils we are used to as inferior media blends for stormwater bioremediation. The hope is to spur conversation and feedback into the design and specification process of bioremediation.

Let's talk, horticulturists and architects and engineers. We all want the same thing, a more beautiful and functional world.

As seen in LASN magazine, January 2019.

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August 26, 2019, 1:18 am PDT

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