6 Principles of Soil Health: Armor on the Soil Surface
Armor
The word armor is most often associated with battle or war gear used as protection for the warrior. The first image that comes to my mind is a medieval knight dressed head-to-toe in shiny armor, charging full-speed ahead on his steed. Other quick examples include armored tanks, body armor used by police and military members, and our Christian brothers and sisters might even think of the Armor of God discussed in Ephesians chapter 6. Each type of armor protects the one wearing it from attacks by an enemy. This war analogy illucidates why bare soil is to be avoided. Wind, sun and rain are harsh elements that can quickly turn into the enemy of the soil when we don’t provide the proper armor.
Another helpful analogy is to think of plant cover as the “skin of the soil”. It seems preposterous at first to compare plants and soil to living human body parts, but the similarities between the two are quite striking when laid side-by-side. First, the skin covers nearly every surface of the human body, just as plant material covered nearly every surface of the land on earth prior to urbanization. In both cases, the barrier is semi-permeable, allowing substances to enter and exit. Also, the skin plays a large role in temperature regulation. This fosters a comfortable environment for our internal organs and microbes to do their jobs efficiently. Plant cover provides the exact same benefit to the soil. Cross-sectional views of human skin and soil help us realize the similar layer structures they share. Shrek would be proud. Doctors call them the layers of the skin, but soil scientists decided to call them horizons. Also interesting to observe is the similarity between roots/shoots and follicle/hair. I believe the two images would bear even more resemblance if the soil image included all of the critters living in the soil that are creating channels and tunnels similar to the blood vessels of our skin.
Temperature Regulation
Extending the skin analogy further, think about why lizards and turtles sun themselves on chilly days. Reptiles are cold-blooded, which means their internal temperature is determined largely by the temperature of their environment. They don’t possess skin like ours with the capability to keep heat in on cold days and release heat on hot days. Bare soil is like a cold-blooded animal in that temperature is determined largely by the environment and can fluctuate wildly. In fact, the surface of desert soils can experience temperature swings over 75 degrees Fahrenheit in a matter of hours.1 Dr. Allen Williams of Understand Ag, LLC, details differences in agricultural soil temperatures due to variability in cover in his article The Heat is On. He consistently found soil temperatures reaching over 150 degrees Fahrenheit on days where the air temperature was “only” 97-102 Fahrenheit. These temperature cook soil biology and transfer a lot of the water out of the soil into the air through evaporation. No wonder so many farmers and ranchers say they’re only “2 weeks from a drought” after a rain event.
One reason why dead or growing plants help keep the soil cool is by physically providing shade. We, and our livestock, know all this intuitively because we go an stand under shade on a hot, summer day. In addition, growing plants absorb heat radiated from the sun as part of their growth cycle before it can reach the soil surface, turning it into “latent heat” held in water, as opposed to “sensible heat” that you can actually feel in the air. AND if that’s not enough to make you go out and shake the hands of a few plants in your yard, living plants actively release that latent heat back into the air through the process of evapotranspiration where it can radiate back out into space, cooling the planet just like our body cools us with sweat.2 Bare soil bereft of cover cannot do any of these cooling mechanisms.
Judith Schwartz’s Water in Plain Sight and Cows Save the Planet detail this amazing transformation of solar energy using plants and water. Water for the Recovery of the Climate – a New Water Paradigm is also a great read for those that would like to learn more about these processes in a more technical, textbook format.
Alternatively, cover keeps the soil warmer on cold days. Physical blanketing of the soil allows heat to stay in the system and the cold to stay out. Heat in the soil comes largely from biological activity, as chemical reactions give off heat as a natural byproduct of cellular activity. Think of how hot we get during a bout of exercise. Although small individually, unfathomable populations of microbes are the reason why compost piles can heat up to 175 degrees Fahrenheit, proving once again that there is mighty strength in numbers. Cover on the surface traps a lot of this heat in the soil system, keeping temperatures higher during the colder months.
Water is also a key factor in the stabilization of soil temperature as water has a higher specific heat value (the energy required to heat 1 kilogram of a material 1 degree Celsius) compared to soil minerals.3 Water is held in the soil by the adhesion of water molecules to clay particles and organic matter due to opposite electrical charges on their outer surfaces, like two opposite ends of a magnet. And because clay content remains constant, we can say that raising organic matter content in a soil is how to increase a soil’s ability to hold and store water. In fact, a soil can hold roughly 20,000 additional gallons of water per acre with each additional one percent increase in organic matter!4 More water means it takes more energy to heat or cool the soil. Therefore, it can be said that systems designed to increase organic matter content in the soil over time will benefit temperature regulation. The benefits of temperature stabilization to producers are numerous with the greatest benefit, in my mind, being that the growing season window is extended by having more consistent temperatures in the spring and fall.
Erosion Control
One unfortunate characteristic of bare soil is that it can break off from itself and hitch a ride on the wind or inside water to a new location, taking its invaluable organic matter and nutrients with it. We call this “erosion”. It’s now a well-established fact, even in academia5, that cover on the soil surface greatly reduces soil erosion. There are a few contributing factors to this, all very straight-forward. The first reason is that living roots anchor the soil in place. Likewise, soil biological glues, like glomalin, cement soil particles together, making them bigger, bulkier and harder to breakdown. Bigger and bulkier means they are less likely to be carried away.
Living and dead plant material on the surface also protects soil from direct contact with high winds and rain events. This is especially important to think about as the majority of soil erosion occurs in short bouts of intense weather during the rainy season. Research from Minnesota and Wisconsin found that 85% of total soil runoff happened during 10% of runoff events, mostly from strong storms in the spring, precisely when conventional fields are most likely to be tilled, bare, or barely covered with tiny plants.6
If you still need further evidence, check out the most powerful tool we have to illustrate how covers and no-till eliminate harmful soil erosion: The Rainfall simulator experiment.
Biological Improvements
Temperature regulation, increased water holding capacity and improved erosion control are all nice benefits that result from armoring the soil. However, I believe the most important benefit from covering the soil is that they all work together to promote the growth and flourishing of soil biology. Hard-working soil inhabitants require the proper temperature, water content and living quarters to do what they do and it just so happens that what they do is extremely beneficial for you and I.
Temperature affects the rate of reactions, plain and simple. We refrigerate and cook foods to place microbes in an environment where their activity is either slowed to a crawl (refrigerator) or they are killed (cooking). The same effects are observed with soil biology. Most middle-ground soil bacteria (“mesophiles”) operate within a range of roughly 65 degrees F to 113 degrees F. Inside of that range, microbial populations can double every 10 degree Fahrenheit, with rates decreasing as they get closer to 65 or 113 degrees F.7 Outside of that range, soil microbial activity either slows to an eventual crawl (<65 degrees F) or they are cooked and killed on the spot (>113 degree F). Services, like nutrient cycling and soil building, are severely hindered in both cases.
Moisture level is also important for soil biology as microscopic organsims are typically aquatic or subaquatic, meaning that they live on thin films of water to move around and find food. All organisms require water to survive, even if they do not live inside water, so water’s importance can’t be understated. Too much free floating water will exhaust oxygen levels and create an anaerobic environment that promotes pathogenic microbes. Now, you may be saying to yourself, “wait a second… First you tell me more organic matter means more water holding capacity and that’s a good thing. Now you’re saying more water is a bad thing?!” Gold star for you, dear reader. The difference is that organic matter holds water in a way that maintains pore space where oxygen can diffuse in and carbon dioxide can diffuse out. Imagine the same amount of water poured onto a sponge compared to pouring that water directly on the countertop. Same water, very different results. Now you know why people call organic matter and soil organic carbon the “soil sponge”! On the flip side, too little water leaves no home for subaquatic creatures and no drinking water for others. Cover on the soil reduces evaporation and builds organic matter, keeping soil moisture levels at more moderate levels throughout the year.
Lastly, reduced erosion is beneficial for soil biology for a couple of reasons. First, the uppermost layer of soil contains the most food for soil biology, so we lose fuel for microbial growth anytime topsoil washes or blows away. We want to keep as many of the resources on the land as we can. Second, populations of soil biology are up to 100 times higher in the uppermost layers of soil compared to lower levels, so we’re losing a significant number of our soil-dwelling employees with every erosion event.9 For those that think this is not an issue and we will never run out of rich topsoil to erode, I would encourage you to look at a study from 2021 showing that 35% +/- 11% of the U.S. Corn Belt’s topsoil has completely eroded away.10 The biological implications of this mass exodus of soil is hard to comprehend, but we can start to reverse the trend by making sure our soils are covered as many days of the year as possible.
Summary
Covering the soil with living or dead plants is a winning strategy to moderate temperature levels, increase water-holding capacity, and reduce erosion, all of which create an environment conducive to efficient biological activity in the soil. These are not just “feel-good” benefits. They each bring with them economic and ecological gain to the producer and the community, saving everyone time, money and effort over the long-term.
References
1https://www.livescience.com/why-do-deserts-get-cold-at-night.html
2http://waterparadigm.org/indexen.php?web=./home/homeen.html
4https://www.nrdc.org/bio/lara-bryant/organic-matter-can-improve-your-soils-water-holding-capacity
5https://www.canr.msu.edu/news/controlling_soil_erosion_with_cover_crops
6https://agwaterexchange.com/wp-content/uploads/2016/12/2015-Understanding-Soil-Loss.pdf
8https://ohioline.osu.edu/factsheet/sag-16