what are living organisms found in soil

If the soil food web were a city, bacteria would be the infrastructure. They are the most numerous and in many ways the most essential organisms in healthy soil. In a single teaspoon of agricultural soil, there can be anywhere from 100 million to 1 billion bacteria — and research confirms that a single gram can host up to 10 billion microorganisms spanning thousands of species (Multiple authors, Frontiers in Microbiology, 2024). One teaspoon. Let that sink in.

Bacteria do several critical jobs. They are the primary decomposers of organic matter, they break down plant residues, dead organisms, and complex organic compounds into simpler forms that other organisms can use. They convert atmospheric nitrogen into forms that plant roots can absorb, a process called nitrogen fixation. Some bacteria, like the Rhizobium species, live in the root nodules of legumes and fix nitrogen directly for the plant in a direct partnership.

Bacteria also produce glue. They secrete sticky polysaccharides that bind soil particles together into aggregates, those small clumps you see when you pick up a handful of healthy soil. Those aggregates are the physical structure of good soil. Without the bacterial glue, the structure collapses. The soil compacts, loses its drainage, loses its aeration, loses its ability to support plant life.

If your soil has good crumb structure, if you can pick up a handful and it holds together in loose clumps but breaks apart easily, that's bacteria at work. That's the sign of a living, functioning soil.

If bacteria are the infrastructure, fungi are the communication network. Fungal hyphae, the long, thread-like filaments that fungi extend through the soil, form what scientists call the mycorrhizal network. Some people call it the "wood wide web," and that nickname captures something real about what it does.

Fungal threads extend from plant roots outward through the soil, sometimes for meters in every direction. In exchange for sugars from the plant, the fungi deliver water and nutrients, especially phosphorus, which plant roots struggle to access on their own, from distant parts of the soil. The fungi basically extend the reach of the plant's root system by an enormous factor (Smith & Read, Journal of Experimental Botany, 2008).

In a teaspoon of healthy soil, researchers have found several yards of fungal filaments. Multiple yards of network packed into a teaspoon. The density of that fungal web in healthy soil is extraordinary.

Fungi are also key decomposers of the tougher organic materials. Woody debris, lignin, the structural compounds in plant material, bacteria struggle with these, but fungi break them down over time. This is a big part of the long-term carbon cycling in forest soils and in healthy garden beds.

The problem with conventional farming, and with excessive tillage in gardens, is that fungal networks are physically destroyed by mechanical disturbance. You shred the hyphae every time you turn the soil over. In a no-till system, the fungal network can persist and deepen year over year, building an increasingly sophisticated infrastructure for plant support.

This is where it gets interesting. Protozoa, single-celled organisms like amoeba, flagellates, and ciliates, eat bacteria. A teaspoon of healthy soil can contain several thousand protozoa. Their job, from the soil's perspective, is to regulate the bacterial population and release nutrients locked up in bacterial biomass.

Here's how it works. Bacteria consume organic matter and build up nitrogen in their bodies. When protozoa eat bacteria, they release that nitrogen in a form that plant roots can absorb. This is called the microbial loop, and it's one of the primary mechanisms by which nutrients become plant-available in a biologically active soil. No protozoa, no loop.

Nematodes work similarly. They are microscopic roundworms, and they exist in enormous numbers, several thousand per teaspoon of soil in a healthy system. Some eat bacteria. Some eat fungi. Some eat other nematodes. The ones most people have heard of in a gardening context are the predatory nematodes that eat plant-destructive insects and larvae. Biological pest control that comes for free with a healthy soil ecosystem.

SARE, the Sustainable Agriculture Research and Education program, describes nematodes as critical participants in the nutrient cycling process, and their absence or suppression is one of the early warning signs of a damaged soil ecosystem.

Earthworms get more appreciation than most soil organisms, and it's deserved. They are arguably the most visible indicator of soil health available to a gardener. If you dig into your bed and pull up a handful of earthworms, you're doing something right.

Earthworms move through the soil eating organic matter and bacteria, and their castings, their waste, are one of the most nutrient-rich biological materials you can put in a garden. Worm castings are high in available nitrogen, phosphorus, and potassium, and they also contain beneficial microorganisms. Their movement through the soil creates channels that improve aeration and water infiltration. Their activity mixes organic matter down into the mineral soil layers.

A healthy garden soil should contain five to thirty earthworms per square foot. If you're finding fewer than that, your soil biology is probably suppressed, either by chemical inputs, compaction, or lack of organic matter.

Above the microscopic scale, soil hosts a remarkable variety of arthropods, beetles, mites, springtails, millipedes, centipedes, ants, and many others. These larger organisms are the first line of mechanical decomposition. They shred organic matter into smaller pieces, making it accessible to the bacteria and fungi that do the finer work of breaking it down into soil nutrients.

Researchers at Nature Scitable have documented that arthropods can number up to 100 per square foot in healthy soil. About 90% of forest leaf litter is processed by millipedes, earthworms, and woodlice. That physical processing is essential, without it, the finer decomposers don't have material to work with.

Ants in particular are fascinating soil engineers. They dig tunnels, move seeds, bring organic matter underground, and aerate large volumes of soil. A healthy ant population in your garden is a sign of biological vitality, not a problem to solve with pesticides.

Here's the payoff. All of these organisms, bacteria, fungi, protozoa, nematodes, earthworms, arthropods, are working together to do one thing: feed your plants. They are the natural nutrient delivery system that has been operating for millions of years before humans ever invented synthetic fertilizers.

Albert Howard, who studied traditional Indian farming practices in the early twentieth century, argued that this biological system was what he called the "chain of life", the interconnected biological community that moved fertility from the soil into the plant and back again. When you break the chain, you have to replace the natural function with synthetic inputs. And the synthetic inputs are never quite as good, because they can't replicate the complexity and responsiveness of the biological system.

Gabe Brown talks about the soil as a living organism that you either feed or you deplete. His approach, no tillage, cover crops, diverse rotations, compost, integration of livestock, is designed to feed and expand that biological community, not to work around it.

When I pick up a handful of soil from my garden beds and see it teeming with earthworms, when I smell that rich earthy smell from biological activity, when I see the crumb structure that tells me the aggregates are intact, I know the community is healthy. And when the community is healthy, the food it grows is healthy too.

Y'all, the most important thing in your garden is the thing you can't see. Take care of the organisms in your soil, and they'll take care of everything else.

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