When a farmer takes a drain spade (or “sharpshooter”) and scoops a sample of soil from the field, they produce a composite of all the elements in their land. A soil testing lab would be able to tell them exactly what it consists of phosphorus, potassium, cations and micronutrients, even carbon and nitrogen. All of the necessary components for healthy soil can be present in that single handful, but that does not tell the whole story.
Perhaps mere chemical identification would suffice years ago, but researchers like Dr. Andrea Jilling, (a soil scientist and Assistant Professor of Environmental Health Sciences at the University of South Carolina) are busy zooming in on the smallest particulars. Dr. Jilling states that the bulk soil gives one “an average of what’s going on… but you’re not catching what’s happening at these fine localized scales or environments.”
The fine localized scales and environments that Dr. Jilling is primarily interested in is known as the rhizosphere–that eighth-inch area surrounding a plant’s roots. While bulk soil may contain some root systems, some organic nutrients, some microbial bacteria, and fungi, it is in the rhizosphere that the biogeochemical reaction takes place. It is through zooming in on this specific area that we can better understand the genius of plants and how we can see them thrive.
Plants have always absorbed carbon dioxide (CO2) from the earth’s atmosphere and fixed it into their tissues, converting it into biomass; today we know that on average roughly 30% of photosynthetically fixed carbon is allocated to belowground growth. Some of this carbon is respired by roots, some released as exudates (which includes simple compounds like sugars, organic acids, amino acids, metabolites, etc. ) and some enter soil through sloughed-off root cells, root fragments, all of these in some way fueling the rhizosphere processes. At one time, some of these processes were thought to be a waste of resources and energy.
When conducting research, it is important to remember Heisenberg’s indeterminacy principle–that even with all the supposed components of data that go into claiming empirical evidence, one can only narrow the margins of estimation. A stranger can answer all the banal questions about their name, their occupation, and where they are from, but that does not qualify as knowing. To know that stranger, you would have to ask more detailed and focused questions and know what to ask those detailed and focused questions about. Soil researchers were not asking the right questions. The simple awareness of plants “leaking” CO2 is not sufficient evidence for understanding what is truly going on, no more than pointing out that major cities just happen to have rivers or ports nearby. There is an economy at work in the rhizosphere.
Researchers like Dr. Jilling have begun to explore this topic closely and more thoughtfully. Asking why the plants are doing this. “This fueling and pumping of carbon out into the soil is stimulating microbes and fueling their growths and activity, so these microbes are feeding on the carbon from the plants and producing enzymes and growing in their microbial biomass.” When the microbes increase their biomass, they produce more enzymes that provide nutrients, primarily in the form of nitrogen, for the plants to absorb and ultimately increase their biomass. Dr. Jilling explains that the plants are investors. “The plant’s investment below ground is tied in space and in time to the actual supply of that nutrient. So, you have more synchronicity within the soil. Whereas unhealthy soil creates an asynchronous environment and relies more on fertilizer.”
Carbon is the energy currency of the soil. Make no mistake, plants are aware of its value. They know that to fuel their processes, they need to absorb carbon, but they are also in tune with the economy of the rhizosphere. For plants to thrive to see fertility, they know that they must stimulate production around the root system. Fertilizer may be compared to the stimulus package of this economy–the investment in the corporations of plants themselves–but a diverse and thriving base of organic life is what yields the most resilient soil and best results for growth. Dr. Jilling and other researchers observe this when plants first take root. Often the early stages of plant growth are when they pump the most CO2 through their root systems, an effort to secure a microbial environment and fortify a healthy economy.
The most stable forms of organic matter within the soil (with associated carbon and nitrogen) have been located in these tiny mineral-associated clay and silt particles; this matter has existed for centuries. However, we are beginning to understand that plants and microbes recycle and use even these very stable sources of carbon and nitrogen. These storehouses of carbon, nitrogen, and other minerals are vital to the reduction-oxidation reactions that drive biological processes and energy in the soil, but it has been up to plants to spread the wealth. This is where researchers, like Dr. Jilling, are hoping to affect change.
To better understand the economy of the rhizosphere, researchers are exploring ways in which they can better support the processes that are already occurring. While plants and microbes have been observed exchanging nutrients, Dr. Jilling aims to discover where farming methods may be improved. “From the moment a leaf hits the ground, microbes begin to make it smaller and smaller until it can be associated with a silt and clay particle,” she states. A thriving ecosystem of microbial life results in a quicker leaf-to-particle conversion, replenishing the nutrients in the soil. “The nutrients can stay in that form for centuries,” says Dr. Jilling, “You want the storehouse of organic matter, but also a faster cycling form.”
Synchronicity is the key. Researchers are attempting to find the optimal balance to promote a synchronous environment. What conditions make these nitrogen stores available to plants? What effect will it have when it also means accessing carbon? Dr. Jilling’s research is dedicated to finding the massive implications of these microscopic questions. To uncover the answers that will improve farming techniques everywhere, she continues to focus on the eighth of an inch around the roots, where plants seem to have a functioning farm of their own.
 At the time of the interview, Dr. Jilling was an Assistant Professor of Environmental Soil Chemistry at Oklahoma State University but has since moved to the University of South Carolina.
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