A World In Every Spoonful Of Soil
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I was recently asked to explain what the soil food web might be. A decade or so ago, only a handful of people in the country had even heard the term. These days, it’s more commonly used, yet many gardeners still wonder what on earth it means. The general concept of a food chain is fairly familiar, but not surprisingly, the real picture as applied to soil is more complex that the linear construct we often see in kid’s science books.
Instead of a straight line of critters feeding on the next lower rung on the environmental ladder, reality looks more like an interconnected web of life forms. More familiar models might show mammals and birds, but rarely include insects and plants. In real life, everything that can be eaten is involved in a cyclical food web, from algae to whales. It’s a web and a cycle because what goes around comes around: If few animals prey directly on Kodiak bears, a host of humble creatures from mice to microbes may dine on a bear’s carcass, assisting in a natural decay process that ends up nourishing new life.
The Amazing Life Of The Soil
The same kind of web is found in soil, where minute creatures feed on each other and plants eagerly lap up the residues (particularly bacterial excreta), leaving organic residues of their own (humus) which in turn feed microbial biota. Much the same kind of cycle can be found in water, and some folks even use the term “air food web” to describe the relationship between plants and insects.
When we add compost and other amendments to the soil, we are nourishing this host of tiny creatures, including bacteria, protozoa, and mycorrhizae. Soil biota are the microscopic beings that bring life to the mix of humus (rotting organic materials) and minerals that make up the non-living portion of soil. When we feed these minuscule arthropods and other beings, we feed the living soil. When the soil is rich and well fed, our plants flourish as well. Here’s a brief introduction to the cast of critters that create the soil food web and make soil come alive.
Beneficials and Pathogens
Soil fauna and flora come in many forms, from bacteria to nematodes and fungi. Some biota help build healthy soil and support healthy plants, and these are considered to be beneficials. Others can cause many problems for gardeners, from root rots to blights, molds, and mildews. These critters are considered to be pathogens. Both have a legitimate and important place in the growth and decay cycles of the natural world. In garden settings and on farms (places where we particularly enjoy the illusion of control), we prefer to boost the growth of our beneficials and suppress as many pathogens as we can.
Basically, when we improve soil tilth, texture, aeration, drainage, and nutritional content, we improve the •balance between helpful and harmful soil biota. All soils contain both bacteria and other kinds of biota, notably fungi, in varying proportions. Soils that are bacterially dominated are best suited for growing lawns, most annuals and perennials, and most vegetables. Soils that are fungally dominated are best for woody plants (trees and shrubs).
Bacteria
Our native soils are full of bacteria, both beneficial and pathogenic. A spoonful of ordinary backyard soil may contain billions of bacteria of thousands of different kinds, many of them specific to a region. Nitrogen-fixing bacteria dine off particles of humus, creating waste products (bacteria manure) that add new forms of organic content to soil.
Many plants absorb nutrients through this bacterial waste product, so the better the bacterial balance, the better the soil quality for plants. Bacteria (and bacteria’s waste products) are eaten in turn by fellow soil dwellers of many kinds. Thus, beneficial bacteria help retain the nutrients and organic matter they eat in the soil. Bacterially dominated soils are most favorable for lawns, flower beds, and most vegetables.
Earthworms
Charles Darwin wrote whole thick tomes about worms and their actions in the soil. In fact, he figured out that worms, not people, buried the dwellings of our ancestors. Worms are among the most beneficial of soil dwellers. Sadly they are harmed or killed by exposure to many pesticides and herbicides, including some common weed-and-feed products. Most gardens cherish their earthworms, knowing that these hard workers are the soil builder’s best friend. Worms do the mixing for us when we layer amendments onto garden soil. Worm tunnels open heavy soil to let air get down to plant roots. Worm castings promote sturdy root growth and feed many soil dwellers. It would be hard to have too many worms, but soils suffer quickly when worms are in short supply.
Fungi
Most gardeners assume that soil fungus most be bad but this is farâ from the truth. Fungi are vitally important to soil health and beneficial forms are found in virtually every kind of soil on earth. Like bacteria, fungal hyphae break down organic matter by digesting and excreting humus and recycle nutrients through the soil food chain. Healthy woodland soils are fungally dominated, meaning that there are more fungal creatures than bacteria.
To keep woodland soils healthy, we need to maintain the fungal balance by restoring the nutrients stored in fallen foliage that is often removed for the sale of visual tidiness. To restore the lost nutrients easily, simply shred the leaves and replace them as light mulch. Since woody plants grow best in a fungally dominated soil, “parking out” woodlands by replacing understory shrubs with grass usually results in stressed trees.
Microarthropods & Protozoa
These tiny recyclers feed on bacteria and fungi as well as plant particles, making nitrogen and other nutrients more readily available to plants and other soil biota. Soil dwelling protozoa eat bacteria and produce a manure rich in available nitrogen. Protozoa are a favored food for nematodes and other soil fauna, which release nitrogen and other nutrients back into the soil as they excrete in turn.
Nematodes
Nematodes, like fungi, are usually assumed to be pathogens but beneficial nematodes abound. Good garden soil contains an ample supply of beneficial nematodes which feed on many other creatures, from bacteria and protozoa to other nematodes (including the pathogens). These support root growth, passing vital nutrients along to plants through their manure. Pathogenic nematodes eat live plant tissue, harming roots rather than promoting healthy root growth. In healthy soil, beneficial nematodes help keep their pathogen cousins under control.
Pretty amazing stuff, and it seems likely that we barely know the half of it. Bottom line: love your soil and treat it well!