Getting to know the soil sponge

On Sunday November 17, the neighbors gathered at  Rices Mills Community Center in Thetford were treated to a presentation about the soil sponge and climate resilience by Didi Pershouse. For those who do not know her, Didi practiced health care for many years in Thetford Center and is the author of "The Ecology of Care.” She has a long-standing interest and expertise in ecosystem health and its intersection with human health and now teaches internationally. She was one of 5 speakers at the UN Soil Day in 2017. She also serves on the board of the White River Natural Resources Conservation District that encompasses several towns, including Thetford, that share the common watershed of the Ompompanoosuc River.

Her presentation explained the link between climate and the earth's water cycle and how human-wrought changes to the land are affecting the cycling of water, creating flooding, drought, bigger storms, and increases in local temperatures.

In a nutshell, a healthy water cycle begins when moisture evaporates (from vegetation, soils, lakes, and oceans), rises, cools, and forms clouds with the help of airborne biology including bacteria, pollen, spores, and biogenic VOCs (volatile organic compounds) released from bacteria in surface ocean waters. Prevailing winds carry the clouds across land and sea until they have collected enough water and biological precipitation nuclei to form actual raindrops (or snow). Rain is absorbed by soils and slowly refills groundwater, wells, springs, and streams, and some of it is then taken up by plants. Whenever they are photosynthesizing, plants undergo transpiration, a process of evaporating water from their leaves (akin to sweating) which drives the transport of nutrients through the plant. The evaporation of water from liquid to vapor uses heat in the air and dramatically cools the air around the plant. (One recent study showed that urban botanical gardens are, on average, nine degrees F cooler than surrounding streets.) This cooling over forests and other biodiverse landscapes creates cool low-pressure zones that draw moist air in from the coast and across the continent, so that rain spreads evenly across the land.

The role of forests and plants in creating weather is vastly under-appreciated. The widespread destruction of coastal forests interrupts the water cycle, reduces inland rainfall, and promotes drought. Any rainwater that is not absorbed into vegetation or soils makes its way to culverts, storm drains, streams, and rivers, ultimately flowing back to larger bodies of water and the ocean, without creating any cooling through transpiration and without helping to move moisture across land. 

The role of soil in this cycle, as Didi explained, is crucial. Not all soils absorb water readily, it turns out. There is a spectrum of different conditions of soils, ranging from soils that have been over-tilled, over- fertilized and depleted of organic matter, bacteria and fungi to soils that are teeming with bacteria and fungal threads and have far more structural and functional integrity because they have been managed more like a natural landscape with diverse vegetation such as mixed species cover crops or pasture plants year-round and without tillage or chemicals. This provides the conditions for the underground workforce to build a sponge-like matrix out of the mineral particles that form the substrate of soils.

Bacteria and fungi associate together as complex communities or biofilms on soil particles. They also exude slimes and glue-like substances, known as extracellular polymeric substances (EPS). A magnified photo in Didi's presentation showed soil particles stuck together in clumps by a combination of EPS and fungal threads, with spaces, or soil pores, in between the clumps. This structure is what constitutes the soil sponge.

In a simple and elegant demonstration of these two extremes, Didi prepared two dishes. One held a small pile of flour, representing the depleted soil, the other, a pile of three slices of bread (which is flour after its interaction with yeast/microbes) representing the microbe-rich soil sponge. When she invited participants to "rain" water on the flour, the water ran off, carrying some flour with it, and did not infiltrate. This illustrates what happens when rain falls on depleted, over-tilled soil. It cannot soak in to be received by the roots of plants. Instead it runs off, carrying soil with it in the process of soil erosion. (Each year, an estimated 24 billion metric tons (tonnes) of fertile soil are lost due to erosion. That’s 3.4 tonnes lost every year for every person on the planet (Soil Fertility and Erosion). When the gathered neighbors poured "rain" from perforated paper cups onto the bread, the water was immediately absorbed, demonstrating the action of the soil sponge in accepting and holding water so it is available to plant roots and can refill wells, springs, and groundwater. In another photo, Didi showed how clumps of actual healthy, microbe-rich soil sponge do not disintegrate readily, even when soaked in water for 24 hours.

The soil sponge provides many services. The pores in the sponge allow oxygen to reach plant roots and soil microbes and allows a healthy microbiome as well as good plant growth. Because it retains water by capillary action, it functions as a reservoir so plants have a consistent supply of water. As water moves through the soil sponge, sediment and impurities are filtered out creating delicious, clean spring water for all of life. When floodwaters soak into the soil sponge of naturally vegetated or forested floodplains, it reduces the intensity of the flood. Vermont and many other states are bearing the consequences of floodplain development — which destroyed and paved the floodplain forest or converted it to overtilled agricultural fields — as well as our history of extreme deforestation in the 1800s. These practices have degraded our region's original spongy soils and increased the severity of flooding. 

Another photograph showed a "rainfall simulator" that compares the ability of differently managed soils from the same farm to retain and filter water. The healthy soil with year-round diverse vegetation and no tillage or chemicals infiltrated all the water with no runoff, and it came through the soil clean and clear. At the other end of the scale, a soil degraded by conventional farming was impossible to infiltrate, and all of the water ran off. This runoff water was full of sediment, which we see in our own landscape when rivers turn brown after a heavy rain. 

At this point Didi turned to the audience for questions. One resident asked how we can restore the soil sponge structure. Plants — and a diversity of them — was the answer. Growing a minimum of 4 plant families encompassing 16 different species (4 from each family) would accomplish that over a number of years. Plants are microbe farmers; they put as much as 40% of their photosynthetic products into root exudates that are mixtures of sugars, amino acids, organic acids, proteins, and fatty acids, etc.

The plants do this for a reason: exudates attract and feed bacteria, fungi, and protozoa in the soil around the roots. The microbial community, or microbiome, is instrumental in greatly expanding nutrient absorption by roots, which makes the food grown in that system much more nutrient-dense for all of us — bees, cows, birds, and humans. The same microbiome also secretes the slimes and glues that bind soil sponges together.

Adding well-composted organic material can also help feed the soil microbiome, as well as providing nutrients to plants. Another important practice is to avoid tillage, which breaks apart the structural integrity of the soil sponge so that it erodes more quickly. Tillage and heavy machinery also compact the soil, further reducing water infiltration. Leaving soil bare is another harmful practice. Bare soil heats up rapidly in the sun, accelerating decomposition and loss of organic material. The heat can also directly kill microbes.

Didi asked the audience to think of how nature grows food. In nature everything is recycled, including animal excrement and urine. But in nature those excessively rich wastes are applied in scattered small quantities. Plants are not grown in monocultures, but as an intermingled community of many species. Thus pest outbreaks are limited, although the word "pest" is a human-centric term applied to anything humans perceive to be harmful to outcomes that humans want. In nature those "pests" become food for other, predatory species that, in turn, feed other species. The natural system breaks down when humans kill off all life forms they deem "undesirable." Applying biocides (pesticides and herbicides) can actually increase pest problems by eliminating everything, including beneficial predators. For example, when we use insecticides, it kills thousands of predatory (carnivorous) insects that would have eaten the very few vegetarian "pests" while the vegetarians still have food: your tomatoes and brussels sprouts. 

It's a little-known fact that the commonly used herbicide Roundup (glyphosate) was patented as a broad-spectrum microbicide. It is very harmful to the soil microbiome that supports the sponge structure. Glyphosate also damages the gut microbiome of humans and other animals that promotes brain development, creates our brain chemicals, and modulates our metabolism. Yet glyphosate spares disease-causing microbes.

Agriculture without tillage preserves the soil sponge and frequently employs cover crops combined with animals. The soil is protected under a cover of diverse vegetation that is grazed down by livestock prior to planting a crop. The animals add their manure to the soil and help the leftover vegetation decompose by trampling it down. The crops are planted directly through the remains of the cover crop. 

In summing up, Didi informed the group that the White River Natural Resources Conservation District received a large award this year for the Ompompanoosuc River watershed to help farmers and landowners to adopt better soil management and other practices.  and likely will do the same next year. The USDA has approved the opportunity for $1 million in federal funds to be divided across two Local Funding Pools for the White River and Ottauquechee Natural Resources Conservation Districts in 2025. Anyone engaged in community-scale agriculture, agroforestry, wildlife habitat management, and riparian plantings is encouraged to apply.

To learn more about Didi's work, Check out "The Wisdom Underground" podcast, and articles on Substack. Find courses, community, and resources at The Land & Leadership Initiative. Download her free manual used in 95 countries: Understanding Soil Health and Watershed Function. Or read her book about her transition to this work in Thetford: The Ecology of Care: Medicine, Agriculture, Money, and the Quiet Power of Human and Microbial Communities 

Photo credit Li Shen