(updated March, 2017)
Soil is more than sand, silt, and clay with some small amount of organic matter. You may have 3 inches of clay over caliche, or 12 inches of blackland clay, or 5 feet of sand over hardpan. Whatever you have, soil is teaming with life.
Experts estimate that a tablespoon of forest soil contains 6 billion microorganisms consisting of 75,000 species of bacteria, 25,000 species of fungi, 1,000 species of protozoa, and 100 species of nematodes. And that is not a typo; that is number of species.
James Urban in Up By Roots uses a cup of soil (that’s a handful for me) and has numbers in the billions; 200 billion bacteria, 100,000 meters of fungi (that’s about 60 miles!), 20 million protozoa, 100,000 nematodes, 50 micro arthropods. That’s not much soil to contain such mind boggling numbers. The numbers are so large, so extreme; I do not think they register with most of us. It’s hard to imagine how small you have to be to have 200 billion in a handful of soil.
Under good conditions bacteria have the ability to multiply or divide every 20 minutes. They could quickly overwhelm the soil unless billions of them were consumed every hour. A protozoa can consume 10,000 bacteria a day and there are 20 million protozoa in a handful of soil. Nematodes eat bacteria, some protozoa, and a lot of fungi. Arthropods eat all of the above. Fungi consume nematodes, decompose the bodies of whatever dies, decompose organic matter, and release nutrients from soil particles and rock. Bacteria also decompose organic matter and release nutrients from soil and rock. All are completely dependent upon carbon which can only be supplied by plants.
This is the soil food web. The entire process protects nutrients from leaching out of the root zone of plants by securing nutrients within the bodies of microbes. Plants and trees are very particular about the nutrients they absorb and prefer nutrients in a form that has been manipulated by microorganisms. The nutrient needs to be consumed by a bacteria that is then consumed by a protozoa or nematode and then pooped out in the rhizosphere in ion form. The rhizosphere is a small zone of intense biological activity about 2mm wide (1/10th inch) around a root. Only then is the nutrient in a form that the plant root can absorb. So bacteria put out enzymes that release nutrients that they absorb. A protozoa consumes the bacteria, uses what it needs and releases the leftovers which have been converted into a form plants can use. If the waste is released in the rhizosphere, the expanding root is able to pick it up thru diffusion.
In exchange, the plant provides carbon sugars it has produced during photosynthesis. About 40% of what a tree produces in its leaves is leaked out thru roots to nourish the bacteria and fungi around those roots. That is a lot of product being leaked, an indication of how much our trees depend upon the micro soil food web. The bacteria depend upon their sugar daddy and crowd in on the expanding root like pigs at a feeding trough, forming a physical barrier that excludes bad bacteria. When things are healthy, the bad guys do not have access to the root or to the sugars. When things are right, the good guys out compete the bad guys, limiting the numbers that could cause trouble.
Fungi can also form a physical barrier around a root, so thick that nematodes and other bad guys are excluded. Many of these fungi are mycorrhizae fungi that are attached to the root or have structures inside root cell outer walls. The tree provides carbon sugars to the mycorrhizae in exchange for water and nutrients, especially phosphorus. Fungal hyphae can be 1/60th the size of an expanding root and have the ability to get water and nutrients from very small pore spaces in the soil. Mycorrhizae fungi are able to deposit water and nutrients directly into the plant root where the water and ions move through a cell membrane into root cells.
Fungi eat or consume nematodes. Some form snares with their hyphae, put out a nematode attractant, and close the snare when a nematode enters the trap. They then grow a special structure that penetrates the sightless worm to consume the groceries. Other fungi produce a poison that kills the microscopic worm when the toxin touches its mouth. Some use glue to glue trap the worm.
And then there are the arthropods (mites). They are the big guys in this micro world, eating fungi and nematodes and protozoa. And all these guys are pooping nutrients in forms that plants can use. Many of these waste products get picked up and consumed enough times in this soil food web to become fairly stable compounds which are how humates are formed.
Trees and plants are completely in charge of this system. They have the ability to change the sugars they provide to stimulate certain bacteria or fungi according to their needs. They change the sugars they produce by season, by temperature, and by moisture levels. If they need iron, they produce a sugar to stimulate the fungus or bacteria that can provide that need. Low on water and they stimulate the mycorrhizae fungi to bring in more water.
Bacteria could easily be washed away so they glue themselves to soil particles. They use organic glues produced from the sugar compounds supplied by plants. These glues are similar to the ones produced by the bacteria in your mouth that cause morning mouth and tartar. Clay particles tend to be rod shaped and bacteria can glue these end to end forming odd shaped structures that can resemble a snow flake. This is the smallest soil aggregate and it protects the bacteria and provides space for the slow movement of air and water. In sandy soils bacteria form cup like aggregates which hold water.
Fungi need to protect their reproductive structures from grazing arthropods, so they weave or glue several of these bacterial soil aggregates together and hide their fruiting structures inside. The glue they use is called glomalin and it is responsible for about 33% of the carbon found in soil when mycorrhizae fungi are present. This forms a larger soil aggregate, still too small to see with anything other than a high powered microscope. Millions of these are formed within a handful of soil and they are an important, critical component of soil structure. The movement of nematodes and arthropods create small passages that also provide for the slow movement of air and water. These are the aggregates we destroy with compaction, plowing, tilling, and double spading our gardens. Once these small aggregates are eliminated, water or air will not pass through. If you want your soil to be healthy, you should not see it very often. Keep your soil covered with plants and undisturbed.
Trees and their roots can get lazy. When we provide nutrients to plants in forms they can use with very little biological activity, the tree will reduce the exudates it releases, which reduces the population of the good guys and can give the bad guys access to our plants. It makes plants dependent upon chemical fertilization. Fertilizers with high salt content (quick release and water soluble) can desiccate bacteria and fungi and can irritate worms causing them to leave the area.
We need to regard soil as a complex system of living organisms and seriously think about becoming microbe farmers, like the trees in a climax forest. With the exception of pH, the physical properties of soil are not nearly as important to a gardener or arborist as the invisible living microbes occupying the soil. Your top priority in caring for soil should be to protect and preserve the soil food web.
This article was based on the incredible work of Dr. Seuss in his landmark soils manual, Horton Hears a Who with help from Up By Roots by James Urban, The Soil Will Save Us by Kristin Ohlson, Teaming With Microbes by Jeff Lowenfels and Wayne Lewis, the Certified Arborist Study Guide, Teaming With Nutrients by Jeff Lowenfels, and Teaming With Fungi by Jeff Lowenfels.
David M. Vaughan
Certified Arborist TX 0118
Member American Society of Consulting Arborists
Organic Certified, Texas Organic Research Center