Apr 07, 2020

Understanding Soil & Storm Water

This article originally appeared in Storm Water Solutions March/April 2020 issue as "Laying Groundwork"

Terry Ryan

We have a lot in common with plants. Consider this–we both need dirt. Your life depends on it because your body is made of the nutrients you consume. Whether eating meat or a plant-based diet, all animals depend on harvesting the nutrients that plants take from the soil. Since our lives depend on soil, perhaps we need to know more about it.

Soil has several layers, called horizons, and it is the top layer of A horizon topsoil, which actually feeds our plants, which in turn, provide our main sources of food, including food for the pollinators required for growing food. This thin layer is composed of minerals, rock particles, organics, such as decayed plants and animals, as well as air, gas, water and living organisms. 

Unfortunately, we are depleting topsoil throughout the world at records rates, primarily through modern agricultural practices. Linda Qia, a writer for National Geographic, estimates that it would take 200 years to restore soils in the U.S. to bring them back to pre-Columbian depths. The Rodale Institute estimates that if organic, regenerative farming replaced modern chemically-based farming, climate change could be reversed. Our urban developments, which destroy healthy soils through erosion, compaction and displacement, could also do a better job of soil conservation by protecting existing soils in place and providing enhancement recipes for manufactured soils.

Healthy soil requires a full complement of life, including the microbial content and community dynamics that comprise an ecosystem–things that thrive in organic matter (OM). For a deeper dive into healthy soils, I suggest reading "Growing a Revolution; Bringing Our Soil Back to Life" by David R. Montgomery.

Storm Water Solutions

We also need healthy soil to retain storm water and minimize erosion. The Rodale Institute researchers estimate that each pound of carbon can retain up to 40 pounds of water. OM is about 40% carbon. So, for example, they say that if topsoil with only 1% OM is spread 6 inches deep over 1 acre, that equals 320,000 pounds of water or about 40,000 gallons. This is why landscape architects specify organic matter for storm water management solutions.

Organic matter benefits the control of erosion and sedimentation. It holds water, which keeps soil particles from moving and results in better plant health by further holding soil in place by means of root networks. When soil is healthy, deeper, denser root networks establish and exude carbohydrates, which then feed soil organisms, initiating a virtuous cycle. Therefore, to minimize sedimentation and runoff, it may be just as important to establish healthy soil-plant relationships upstream of engineered solutions. 

We should be implementing regenerative practices in our upstream landscapes to capture and sink rainwater where it falls. At a recent Green Build conference, Zolna Russell and Stu Schwartz suggested that we should generally minimize tilling, which changes soil structure, releases carbon and opens soils to erosion. They recommend that we feed the soil to feed the plants, build a robust insect habitat through plant diversity, substitute compost tea for chemicals and cover the soil with plants and mulch. In addition, we can also inoculate the soil with mycorrhizae for increased water and nutrient absorption and add bio-char to enhance both mechanical and biological processes. 

Native plants function best in these healthier systems due to deeper roots and because native organisms can find food and shelter, thereby increasing habitat richness. If we employ regenerative methods, Russell and Schwartz suggest that even traditional lawns can absorb more water than they do with conventional practice.  

About the author

Terry Warriner Ryan, FASLA, is a partner at Jacobs/Ryan Associates, Landscape Architects. She can be reached at tryan@jacobsryan.com.