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 nematodes. 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. There are also organisms that help suppress diseases capable of weakening plants. Others help improve soil structure, thus 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?

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. Photo courtesy Christine Middleton

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). Some plants, such as these Dwarf Palmettos (Sabal minor) at Palmetto State Park, have evolved mechanisms to survive under anaerobic conditions.

Dwarf Palmettos (*Sabal minor*) Photo courtesy Christine Middleton

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.

**Web Soil Survey:** *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.* Graphics courtesy HELM team member Dick McBride

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.

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 include a wide range of creatures, such as cockroaches, crabs, butterflies, beetles, centipedes, and scorpions. A subset of arthropods, including springtails, silverfish, and some mites, play a role in the soil food web by feeding 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 such as nitrogen and phosphorus. They protect plants from disease, build soil structure, enhance the ability of plants to absorb water, 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) are about the size of a grain of rice. Perhaps the most fascinating thing about these creatures is their ability to escape predators. Under their body is a lever that opens and closes like a jackknife. When the free end is 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, sometimes dual, 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 (e.g., construction, deep plowing, overgrazing).

Above ground, the first to colonize disturbed areas are annual plants, which are often considered weeds because they tend to invade our farms and gardens. These are followed by low-growing perennial plants and grasses. Later trees and shrubs tend to predominate. A "climax" community forms, and things remain fairly stable until another disturbance restarts the process.

Artistic rendering courtesy Melinda Seib

It’s not just about what happens above ground—succession also occurs underground within microbial communities. While detailed studies on microbial succession are scarce and also inconclusive, 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 soil conditions that allow later successional grasses and woody vegetation to thrive. In particular, fungi enhance drought tolerance and increase plant 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 goes on to suggest, “One trick that seems to help is to transplant a few whole plants of native grass into fields that have been sown with their seeds.” Sounds like great advice!

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 feature in The Hays HUMM, an online magazine of the Hays County Chapter of Texas Master Naturalist™.

The latest issue of the magazine and additional articles you may find of interest can be found at www.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.