I was at the Minnesota Organic Conference many years ago, sitting in a session on testing soil biology. The presenter was showing a microscopic video of a soil sample. There were several large clods of soil in the magnified image, with numerous soil organisms moving around the screen. The presenter began to point out the different organisms, the bacteria, protozoa, and mycorrhizal fungi, when suddenly all the bugs disappeared from view. Then slowly from the bottom of the image came a large arthropod on the hunt. It was like a microscopic National Geographic documentary.
Soil organisms are vital to living soil, but they are just one part of the soil food web and the energy that provides nutrients to plants for growth.
In the early 2000s, a decade plus into my soil health journey, I had the fortune of meeting market farmer pioneer Bruce Bacon who I quickly took on as a mentor. As mentioned in a previous blog, Bruce had begun selling organic vegetables to urban restaurants in the 1970s, before local food was cool and when only hippies ate organic veggies. Bruce believed in small, organic farms, and over the decades he built Garden Farme from a degraded row crop farm in the Anoka sand plain, to a fertile and abundant landscape.
As a lifelong student, he spent decades studying soil and sustainable farming systems. In the early 2000’s, he teamed up with some academia PhD’s to research the soils on his farm of forty years. It was when genomic technology was really starting to pick up speed, and Bruce and his unlikely team of soil scientists were on the forefront of using this technology to study soil microbial communities. Though the price of genomic testing had dropped dramatically in the decade before, he spent thousands of dollars doing microbial soil tests. And they found Garden Farme was rich in abundance and diversity.
I was fortunate to be looped into this band of soil geeks. Bruce and others would forward research documents to the email group on soil mass assay genomics and other topics that were way above my paygrade, so I would read the abstracts and try to comprehend as much as I could. For several years Garden Farme would hold annual field days where the soils team would share and explain the results of the testing.
Eventually the PhD candidate finished her dissertation, and the money dried up. The emails still came, just less often, and most always from Bruce. But I believe Bruce had learned what he needed to know: that his life's work was well-spent. He’d restored the soil health at Garden Farme. He had restored the vital living ecosystem that is healthy soil, and the science had proven that.
Concept of Soil as Energy
Bruce passed away a few years later. In the year or so before he passed, he would call me occasionally just to catch up. During one of those calls, he shared his concept of soil as energy. He explained that soil fertility is, at the very basis, the exchange of ions as energy, and that this can happen through natural systems or by adding fertilizer.
In a natural system, he explained, there are four “pools” of energy, which are all related to living soil. These pools align within the soil food web ecosystem; plant exudate, biology, carbon and chemical energy.
Plant exudate is the base of the soil food web. Plants exude as much as 50% of the photosynthate they produce into the soil through their roots. The energy contained in plant exudate is largely created through the photosynthesis process, captured by the sun and combined with soil nutrients to create photosynthate. It is consumed by the smallest organisms in the soil food web as the base of the food chain.
Soil organisms, or biology, is the next pool of energy in a properly functioning soil ecosystem. Bacteria and fungi are at the base of the food chain, feeding primarily on plant exudate and carbon. These are eaten by larger predatory organisms such as protozoa and nematodes, and these in turn are consumed by arthropods and other larger organisms. These are the “living” energy in the soil, and need good soil structure and living roots to thrive.
Soil carbon is the next pool of energy Bruce spoke of. This is all the material in the soil that was once living. Soil carbon is extremely complex, some of which is short term-carbon made up of partially decomposed organic matter. Organic matter serves as another vital food source for soil organisms. Longer lived soil carbon is composed of completely degraded plants and animals, typically in the form of humus. While not a particularly valuable source of food for soil organisms, humus plays an important role in the ability of soil to hold water in the soil profile and to hold soil nutrients in the soil profile in a plant-available form.
Soil chemistry is the last pool of energy, and the one most understood. It includes both the chemical makeup of the soil parent material and the nutrients that are in a form that is available to the plant. It is what most agriculture is based on– feeding plants the needed nutrients based on a soil test. Reliance on this pool of energy in the soil without consideration to the other pools leads to soil degradation and greater reliance on purchased inputs. By building the other pools of energy, more nutrients in the soil profile become available for plant use.
After 45 minutes of the soil energy phone lecture, he began speaking about the potential of cosmic energy and interstellar dust as topics for further research. As typical of conversations with Bruce, most of this discussion was over my head, but I listened intently to glean what I could.
For me, the concept of pools of energy helped me understand the workings of the soil food web. These pools of energy don’t live in isolation, but are in fact interdependent of each other. They are always changing, transfering, and evolving. They are the foundation of the soil food web. Like the microscopic National Geographic like video opened my eyes to the life in healthy soil, the pools of energy concept helped me better understand the workings of the soil food web; plant exudate consumed by smaller soil organisms which are eaten by larger ones up the food chain, all the while defecating and excreting, until they die and turn into organic matter…back to carbon, to stabilize and store nutrients and water.
The last time I saw Bruce was when he visited the farm I and my wife Audra own, Island Lake Farm, for a keyline design workshop we were hosting. He had a smile on his face all day. I can only speculate that he was pleased with the work of his mentee and the work we’ve done to build the pools of soil energy on our small piece of land.
Learn More
For in-person learning about sustainable growing, including soil health, permaculture and irrigation, check out the Root Your Knowledge workshop series happening March-May 2024. To delve more deeply into the topic of soil health, check out Happy Dancing Turtle’s resources.