Bartlett on Gardening: Soil: It’s not [Just] Dirt

July 7, 2018


 A chart from the U.S. Department of Agriculture website.


About 15 years ago I read an article about a unique research study conducted in France. An agriculture professor and his students (think minions) farmed parallel fields for 20 years. One field was farmed using conventional agricultural methods. The other was an organic farm. At the end of the study, there was no significant difference in the proverbial bottom line. Although the conventionally farmed field was more productive, the cost of the inputs offset that at the end of the day. What grabbed my attention was that the conventional field soil was practically sterile while that of the organic field was teeming with microbes.


So what? Well, in a good and perfect world, soil should be about 48 percent mineral and organic material, 25 percent water, 25 percent pore space and 2 percent microorganisms. These little critters are critical to the wellbeing of plants.


This relationship is called the soil food web. Let’s take a closer look at the world beneath our feet.


At the base of the food chain are bacteria, single-celled organisms that break down organic matter, recycling elements such as nitrogen and carbon so that plants can use them again. The bacteria also feed on sugars excreted by plant roots. The bacteria in turn are eaten by protozoa, bigger single-celled organisms.


Also living in the root zone are two types of fungi. The first group, called saprophytic, are number one in breaking down organic material. They are able to consume dead matter lying on the surface of the ground as well as in the soil. Their excretions return elements to the soil as well as improving soil structure.


The second group are mycorrhizae. These live among the plant roots where they feed off root exudate and transfer nutrients and water to the roots. Clearly plants cannot live without them. Sometimes the gardener may see a white web around roots. This is made of mycorrhizal hyphae. Hyphae are branching filaments that make up the mycelium of a fungus.


Nematodes, unsegmented worms, are members of the community, too. There are about 40 of them in a teaspoon of soil. They feed on bacteria and fungi, releasing nitrogen into the root zone.


All manner of insects, spiders and “bugs” live on plants and in the soil. They shred organic matter, making little-bitty pieces which are easier for the microorganisms to use. They also eat each other, helping keep plants safe from pests.


Earthworms eat bacteria, fungi, protozoa and nematodes as well as organic matter. Veterans of high school biology know that worms have digestive systems which process this material, the end product being vermicastings. Gardeners pay good money for this worm poop, which is 50% higher in organic matter than soil that has not been through a worm’s system. Earthworms are hardly an endangered group. Birds, moles and voles eat a few of them. They in turn enrich the soil with their excretions.


Clearly the wellbeing of our plants depends on the health of the soil food web. Repeated use of chemical fertilizers results in a buildup of salts which kill the microorganisms vital to nutrient recycling.


Because most of the web members cannot be seen with the naked eye, we may judge the state of our soil by its abundance of earthworms.


Master Gardener Ann Bartlett never lets lack of familiarity with a plant stop her from trying it in the ornamental beds around her home. Contact her at

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