It’s All Connected

Underground Too!

Habitat Enhancing Land Management (HELM)

“Essentially, all life depends upon the soil… There can be no life without soil and no soil without life; they have evolved together.” —Dr. Charles E Kellogg, USDA Soil Scientist

Christine Middleton

There is more life underground than above. A teaspoon of good forest soil can contain a billion bacteria, several yards of fungal threads, several thousand single-celled animals, and a few dozen ringworms. Such soil organisms perform a variety of functions that positively impact the health of plants. Some decompose organic matter, and others promote plant growth by releasing essential nutrients. Then there are organisms that suppress diseases, which can weaken or kill plants. Others help improve soil structure, enhancing water filtration. And that’s not all. The interaction between plants and soil biota is awe-inspiring. As renowned soil food web researcher Elaine Ingham puts it, “All the critically important things in soil happen because the biology is present.”

What is Soil?

Email from Chris 9/22 - The first one which shows the percentages of soil components Audrey Stewart tells me (see below) is from the Kiss the Ground. It’s a movie and here is the link to more information - https://kissthegroundmovie.com/. It’s not in the trailer so can’t verify that. Hopefully we can just credit them. If not we can redo a similar diagram with the %ages which are found in other sources.

Dirt is a mixture of sand, silt, and clay. However, plants require more than just these three elements to grow and flourish. In addition to sand, silt, and clay, soil contains air, water, and organic material. Soil is also teeming with organisms that are key to providing the nutrients plants need. It has been said that a handful of soil can contain more organisms than there are humans on Earth!

Less than half of healthy soil is sand, silt, or clay. The texture of the soil comes from the size of the rock and mineral particles it contains. The largest particles are sand, which are about 0.05 to 2.0 millimeters in diameter. Silt is much finer, between 0.002 and 0.05 millimeters. In clay soils, the particles are less than 0.002 millimeters—some so fine that it takes a microscope to distinguish them.

The spaces between the particles, called pores, are similar in size to the particles themselves. These pores store the water and air essential for plants and soil organisms. The larger the particles, the more quickly the soil drains. That’s why soils with a high clay content, like those found in the Texas Hill Country, often drain more slowly. When conditions are hot and dry, soils with a high clay content shrink and crack. Clay soils are easily compacted, leaving little room for air and water. Most plants, beneficial fungi, and bacteria prefer aerobic soil conditions, which means there is plenty of room for oxygenated air. If the pores are compacted or filled with water, the soil is considered anaerobic (i.e., lacking oxygen).

Although organic material is the smallest component proportionately, it drives soil biology. Without sufficient organic matter, most plants struggle. Soil contains two kinds of organic materials. First are things that break down easily—mainly leaves and roots. The second is woody material, which breaks down more slowly. As these materials, along with animal remains, decompose, humus forms, which gives soil its rich brown color. As microbial activity increases, so does soil structure and nutrient availability, resulting in improved water infiltration, storage, and higher plant survival rates.

Caption: Soil type can vary even on a small property, as illustrated in this web soil survey map of one of the properties visited by the HELM team.

The Soil Food Web

Who are these organisms that live all or part of their lives underground, and how do they support healthy plant life? Like the above-ground food web, the soil food web starts with the sun. Plants use photosynthesis to produce energy, and some of that sugar is transported to the roots. The roots combine a portion of that sugar with other substances to produce what are called exudates. The composition of the exudate can vary from species to species, but the purpose is the same—to begin the process of converting nutrients into a form that plants can use. The exudates “wake up” bacteria and fungi, which then absorb and store nutrients.

PLACEHOLDER FOR SOIL FOOD WEB GRAPHIC

Email from Chris 9/22 - - The second one, the Soil Food Web, is from USDA as credited on the slide. I’ve found it other places and it is similarly credited. Found it on the USDA site as I was writing the article and remember reading anyone could use it if credited to USDA. Might want to make it USDA Natural Resources Conservation Service rather than just USDA. If you want a better version of the diagram found it other places. If you have trouble finding that, I can help.

The third trophic level facilitates the conversion of the nutrients absorbed by the bacteria and fungi into a form plants can use. Protozoa, single-celled microscopic organisms, feed on bacteria, while certain nematodes (roundworms) feed on both fungi and bacteria. Arthropods, which include a wide range of creatures such as cockroaches, crabs, butterflies, beetles, centipedes, and scorpions, also play a role. A subset of arthropods, including springtails, mites, and silverfish, feed on fungi and, to some extent, on bacteria. Importantly, these organisms excrete nutrients that their prey have stored, releasing them directly into the root zone (rhizosphere), where plants can access them.

But There’s More to the Story!

This all sounds fairly simple, but beneath the surface, things are much more complex. Fungi, for example, interact with plants in numerous ways beyond exchanging carbon sugars for nutrients like nitrogen and phosphorus. They protect plants from disease, build soil structure, improve water absorption, and send signals to other plants. As Suzanne Simard observes in her bestselling book Finding the Mother Tree: Discovering the Wisdom of the Forest, “The mushroom is the visible tip of something deep and elaborate, like a thick lace tablecloth knitted into the forest floor.”

If you’ve done some gardening, you may have heard of mycorrhizae. The relationship between mycorrhizal fungi and plants was first recognized in 1855 by German botanist Albert B. Frank. Frank discovered that mycorrhizal fungi were widespread in the root systems of many woody plants across various ecosystems. Today, we know that most of the Earth’s vascular plants have associations with some type of mycorrhizal fungi.

There are two major categories of mycorrhizal fungi—ectomycorrhizae (EMF) and arbuscular mycorrhizae (AMF). One key difference is how the fungi interact with plant roots. The term "ecto" means outer, so it is no surprise that ectomycorrhizal fungi form a sheath that encases the outside of the root as well as the outer layer of cells. Arbuscular mycorrhizae, thought to be the first mycorrhizae to evolve, literally penetrate the root. The term “arbuscular” refers to the tree-like structures that form inside the root.

Soil organisms also influence the arrangement of sand, silt, and clay, thus improving soil structure. As mentioned earlier, larger pores allow better drainage. One of the substances that helps bind soil particles together is glomalin, a sticky glycoprotein produced by arbuscular mycorrhizal fungi. Discovered in 1996, glomalin helps create what farmers and gardeners call “tilth,” a condition that promotes seed germination and root formation.

And That’s Not All!

Soil organisms also play a significant role in improving soil conditions. Both bacteria and fungi are integral to the decomposition process, essential for creating soil organic matter. Generally, bacteria consume softer organic materials, like leaves and small sticks, while fungi break down tougher materials, such as large fallen branches or stumps. More surface area means more bacteria and fungi can colonize the organic material and further decompose it. Therefore, organisms such as earthworms, ants, mites, and springtails are crucial for breaking organic matter into smaller fragments.

Springtails (collembola), which are about the size of a grain of rice, are fascinating for their ability to escape predators. Underneath their body is a lever that functions like a jackknife. When released, the springtail catapults away from its enemy. According to E.O. Wilson, “Milligram for milligram, the springtail’s strike is one of the most powerful locomotory forces in the animal world. It carries the collembolan high into the air and forward as far as, for humans, would be the equivalent length of a football field.” Just imagine how much organic material is broken up in the process!

Not all bacteria, fungi, and nematodes are beneficial. Some are pathogenic bacteria, fungal diseases, and root-damaging nematodes that can stress and damage plants. However, these harmful microorganisms are present even in healthy soil and play important roles. For example, parasitic fungi can weaken trees, creating opportunities for surrounding plants to thrive. Saprophytic fungi often start as parasites, but later decompose dead tissue, performing a valuable ecological service.

Succession Happens Underground Too!

Ecological succession refers to changes in species composition within communities over time. Succession occurs because organisms alter their environment, creating opportunities for other species to colonize. This process starts when an area is disturbed, either by natural events (e.g., floods, wildfires, droughts) or by human activities. The first to colonize such areas are annual plants, often considered weeds. These are followed by low-growing perennial plants and grasses, and later by trees and shrubs. A "climax" community forms when things stabilize until another disturbance restarts the process.

Email from Chris 9/22 - - The third one, the succession diagram, Audrey suggests this could be the original source - https://permacultureapprentice.com/building-soil/. It appears there with no credit. But if you don’t feel comfortable with that we can come up with another succession diagram. But I like this one as it shows the underground part and the other ones I’ve found don’t.

It’s not just about what happens above ground—succession also occurs underground within microbial communities. While detailed studies on microbial succession are scarce, we do know that nitrogen-fixing plants are more common early in succession, helping prepare the soil for mid- and late-succession plant communities. As succession continues, the ratio of fungi to bacteria increases, creating conditions that allow later successional grasses and woody vegetation to thrive. In particular, fungi enhance drought tolerance and increase survival rates.

Mutualistic relationships with fungi are one reason native grasses like Little Bluestem (Schizachyrium scoparium) are so successful. However, bare soil combined with heat negatively impacts soil biology. In his book Armadillos to Ziziphus: A Naturalist in the Texas Hill Country, David Hillis notes that restoring native grasses is challenging once soil fungi are lost. He suggests that transplanting whole native grass plants into fields where their seeds have been sown may help.

Want help with your land restoration project? As part of the Hays County Master Naturalist project called HELM (Habitat Enhancing Land Management), we offer property visits to discuss land stewardship. If you or your neighbors would like the HELM team to visit, simply fill out the request form at BeautifulHaysCounty.org and help spread the word!

The HELM Network News is a periodic publication featured in our magazine, The Hays HUMM. The latest issue can be found at BeautifulHaysCounty.org. You can also sign up for our mailing list to receive HELM Network News articles directly in your inbox at BeautifulHaysCounty.org/subscribe-to-helm-news.