The Great Soil Food Web

Posted: February 6, 2015

Soils are the fundamental food factory on our planet. Pay attention to the health of soil in your garden and landscape, and you will reap the benefits of healthier and more productive plants.

Healthy soil is full of microorganisms that can assist plants by breaking the chemical bonds of soil minerals making them water soluble and thus available to plant roots. There are some cardinal rules of soil management. Incorporate these best practices to improve the soil health in your garden.

Never leave your soil bare.

Remember the dust bowl in the early 20th century? Wind and water will act on bare soil to erode the topsoil and leach away the nutrients. When the nutrients are depleted, microbial life in the soil dies. The exposed surface forms a hard crust that sheds rain water, and runoff begins to dig channels through the garden. Cover your soil in the winter by leaving your dead plants standing until spring cleanup. Then once your garden is prepared in spring, cover bare soil with a layer of wood chips. This will help keep weeds from getting a start, as well as keeping your soil protected from sun, wind and rain.

Churn up your soil as little as possible.

The less you disturb your soil by tilling and pulling out weeds, the better for soil health. Left to itself, soil forms a profile of what are called soil horizons. The top horizon, called the O horizon is where the organic activity is happening. Above the soil line, the green plants are actively growing.

Directly under the plant is a layer of organic debris being broken down by bacteria and fungi. When you mulch with wood chips, you actually contribute fuel for these voracious decomposers. Since wood chips decay very slowly over the growing season, they continue to protect your soil from sun, wind, and rain.

Under the O horizon is the broad area of topsoil called the A horizon. This is the zone where a great deal of biological activity takes place. It's composed of fully decayed organic matter called humus bonded to soil minerals to create aggregates that hold water and nutrients needed by the plants. These aggregates are the crumbly dark rich topsoil we all love.

Make sure your A horizon is sufficiently deep.

Plants thrive when they can stretch their roots out in healthy topsoil because there they have everything they need to carry on the nutrient cycle. Topsoil holds many of the soil animals that aid plant growth: the microorganisms, the earthworms and beneficial nematodes to name a few. This is where the underground life of plants plays out.

I would like to share with you a delightful article written by Mark Smallwood, a soil scientist at the Rodale Institute in Pennsylvania.

Smallwood begins by saying "it takes a village to grow a plant, but not the kind of village you might think. It's the micro-village that lives in the top couple of inches of soil. That is where you should be focused on if you want a healthy garden." As the plant puts down roots, the roots release simple sugars into the soil to attract bacteria and fungi. Smallwood calls it "releasing cookies and cakes" into the area around the roots. Bacteria and fungi sense these "cookies" by chemical means and move in to feed. However, since the microorganisms need more than sugar in their diet, they simultaneously release enzymes to break down mineral compounds in the grains of soil so they can uptake them.

The plant needs those minerals as well, but the roots can't get them directly because they lack the necessary enzymes to dissolve them in water. However, as the bacteria and fungi process the minerals, they excrete the excess in a form that brings another set of critters to the party. Protozoa, nematodes, microarthropods and earthworms gobble up the excess nutrients expelled by the bacteria and fungi and excrete the excess in a form the plant roots can uptake.

This completes the nutrient cycle. When the soil is alive, the plant feeds and is fed by the community of organisms in the soil.

The role of compost

One very important principle in the nutrient cycle is this. When the decomposers run out of food, they either leave or die, and the nutrient cycle is impaired. The O horizon must be fed to be able to keep up with the withdrawal of nutrients by the flowers we cut, the fruit we pick, the vegetables we take for our dinner. When we add compost to the top layer of the soil, we add organic material that is still actively decomposing. It contains undecomposed plant matter, and it contains the bacteria, algae and fungi that are beneficial to our plants. Chemical fertilizers feed the plant directly, but they do nothing for the life in the soil. Also, chemical fertilizers, herbicides, and pesticides can be hard on or even fatal to some soil life. Compost buffers that effect and harbors bacteria that can break down chemical compounds in the soil and render them harmless.

I like to think about it like this. When I'm sick, I may have to take an antibiotic. The antibiotic kills the pathogen, but it also takes a toll on my beneficial gut bacteria. If I have a good healthy gut, the bacterial will re-colonize the areas that get wiped out. However, if I have an unhealthy gut with little of the beneficial bacteria in residence, the outcome may be much worse.

Soil science has moved forward at an amazing pace since the discipline of microbiology has given new tools to soil scientists. Some of our old practices have been affirmed while others have been called into doubt. When we didn't know about all the good microbes in the soil, we unthinkingly plowed them under. Now we know that even good old fashioned weed pulling can disrupt the top two inches of soil and bury the good bacteria and fungi. As one farmer said, scratching his head, "when I look at my field, it's the darn thistles that have earthworms and crumbly soil, not the corn rows."