What is soil biology and why is it important? Soil is the physical infrastructure for landscapes and ecosystems. Within the physical infrastructure soil chemistry works in such a close relationship with plant and animal organisms, some say the soil operates as a living biological system. Soil biology is important because it is a dominant factor in nutrient availability. Soil contains plants, micro-organisms, and invertebrate and vertebrate organisms which all work together in creating biochemical transformations essential for life.
There are over 4,000 trillion micro-organisms (microbes) such as bacteria, fungi, algae, mold, and protozoa in one acre of soil. Many of these microbes have a symbiotic relationship with plants and other organisms. All of the microbes are competing for a limited supply of nutrients and carbon (their food source). Microbes quickly multiply when there is sufficient soil organic carbon. When the soil organic carbon is limited the microbe population is diminished. Therefore, if the landscape soil is designed properly, there are trillions of microbes helping it function. And, if properly used as a stormwater system, all of these microbes have been put to work as a biological system.
When I design a soil based stormwater treatment system, I like to create a fluctuating water table between 0.0 to 1.0 feet below the surface. The rise and fall of the water table allows bacteria to create a chemical reaction that is referred to as “denitrification.”
Denitrification occurs naturally in soil environments where carbon and nitrate nitrogen are available during periods of restricted oxygen availability. Globally denitrification account for over sixty percent of nitrogen gas production in ecosystem. Over 150 bacterial species are responsible for denitrification. The population of these species account for much as 5% of microbes in soil; so an acre of soil would contain as much as 200 trillion denitrifiers per acre.
This process converts nitrate nitrogen in the water to nitrogen gas which is released into the air. Denitrification is one of the principal ways to reduce the amount of nitrogen in stormwater. This biological process can be designed into bioretention soil or filtration media in the landscape.
Sometimes we design the soil biological system to occupy very large areas in the landscape. The natural or man-made depressional areas can be modified to create the fluctuating water table to drive denitrification.
I typically specify mixing highly decomposed and finely divided (about the size of fine sand) humic soil organic matter into the soil. Humic matter is the most stable portion of organic compost. Therefore, I want to use organic compost that has at least 25% humic matter. The humic matter will provide a stable food source for the bacteria for many years. However, I want to jump start the denitrification process by adding an immediately available food source for the microbes. I typically use granular glucose sugar at a rate of about twenty pounds per acre. The granular glucose is broadcast over the soil and mixed into the upper 0.5 to 1.0 foot of soil by rotary tillage at the same time as other soil amendments are added to the soil.
When the soil system becomes saturated to the surface during a rainstorm event, the aerobic microbes uptake and transform the nitrogen until the oxygen supply is gone. As the water table recedes over several hours or days air will flow into the unsaturated zone above the water table and the nitrogen gas will be released to the atmosphere.
Thus I use a natural soil biological process enhanced by amending the composition of the soil landscape to more effectively treat stormwater. As landscape architects we can enhance soil biology to improve stormwater treatment over broad areas of the landscape. With this approach the stormwater treatment system is the landscape, so it looks naturally beautiful.
by Barrett L. Kays, Ph.D., PLA, FASLA, Landscape Architect and Soil, Hydrologic and Environmental Scientist