The Practical Gardener

The Eskimo are said to have many words for snow. It is such a vital part of their life that they have invented words to describe its different states and various natures. You would think that in 10,000 years of gardening, we would have come up with a similar variety of words to describe soil. Of course there is sandy soil, loamy soil and clay. But that’s about it. People without much regard for soil refer to it as “dirt.” And you call it dirt when it doesn’t have any life in it. Some people who don’t garden don’t see much difference between soil and dirt; it’s all the same stuff to them.

It’s pretty easy to turn productive soil into dirt. Fact of the matter is, the dominant industrial-agricultural paradigm since World War II has viewed soil as dirt — something to be used simply as a holding medium for crops while they grow. In a typical commercial-scale, mono-crop farming operation, crops get planted, and then they get fertilized with chemicals; when insects and disease attack, they are repelled by chemicals. The rains wash the chemicals into the soil and the soil dies. Or more accurately, the life within the soil dies. When that happens, all you are left with is dirt.

In much of Europe, organic agriculture is referred to as “biological,” rather than “organic.” Biological is actually a much better term than “organic” because it more accurately describes what this form of agriculture is all about (whether it be large scale or garden scale.

If you look at any healthy, diverse garden, you are likely to see lots of biological activity taking place. But this activity isn’t what really makes the soil “biological”; it’s what you can’t see that makes it so.

What’s remarkable is the large number of microscopic populations thriving in healthy soil — all of them busy with their own tasks, all of them interacting on some level with the plants growing in the garden. Decaying organic matter feeds soil organisms, and they in turn nourish plants by turning the organic matter into a form that can be reused by the plants. In addition, microorganisms enhance the structure of the soil and render nutrients available to other organisms.

Here is a breakdown of the types of microorganisms found in a typical healthy garden, with a brief description of what they do.


Though smaller than most other microorganisms, bacteria are typically numerous in healthy soil. They fall into four basic groups.

1. Decomposers make up the greatest share of bacteria. They consume carbon compounds found in fresh organic matter and turn it into forms of nutrients that can be absorbed by living plants. Some members of this family of bacteria are even able to break down pollutants in the soil.

2. Mutualists form alliances with plants. The best known of these are nitrogen-fixing bacteria, which live on the roots of legumes. Pull the roots of a pea plant or a bean plant out of the ground and chances are pretty good you will see on them tiny white balls, called nodules, where these critters live. Their job is to process nitrogen from the atmosphere, changing it into a form the legume can use to feed itself. In return, the plant supplies simple carbon compounds to the bacteria, made via photosynthesis, for the bacteria to feed on.

3. Pathogens are a third group that cause gall formations on plants. While this type of bacteria perform important functions in nature, you don’t want this type in your garden.

4. Lithotrophs, the fourth group, are important because they cycle nitrogen into useable forms for plants, and because they capture and degrade pollutants as water flows through the soil.

Bacteria are responsible for the “earthy” smell of freshly-turned soil. These single-celled creatures help keep nutrients in the root zone of plants, and they enhance soil structure by binding soil particles together into small aggregates. The improved soil structure, created by stabilized aggregates, improves soil’s water-holding ability.


Yeast and mushrooms are two of the better known fungi, but there is a whole universe of fungi operating on the microscopic level beneath your feet. Most often, beneficial soil fungi grow in long threads called hyphae, which can form into large masses that are very effective in reducing the leaching of nutrients from around plant-root structures. They decompose complex carbon compounds and compete with plant pathogens.

Like bacteria, one can divide fungi into groups of decomposers, mutualists and pathogens. But fungi are doing their jobs with different materials. Fungal decomposers are good at reducing cellulose and lignin in wood. Fungal mutualists colonize plant roots and utilize carbon from the plant, as do nitrogen-fixing bacteria, but their job is to bring phosphorus and micro-nutrients from the surrounding soil to the plants to use.


These single-celled critters are several times larger than bacteria. They feed on bacteria and then release bound-up nitrogen that plants can use. They are able to do this because protozoa are relatively high in nitrogen and low in carbon (typically the ratio is 10:1), whereas bacteria have a much lower ratio (3:1); consequently, after protozoa eat bacteria, they release the excess nitrogen — which becomes a byproduct for plants to use. By “grazing” on populations of bacteria, protozoa regulate bacteria populations. This regulating function has been likened to pruning a tree to enhance its health. Protozoa are also important food sources for other microorganisms, and they feed on pathogens that attack your plants.

These are some of the primary microscopic creatures working in your garden, though there are many other critters and innumerable associations and interrelationships among all of them. The important point is that healthy soil is dependent on a great diversity of microorganisms. The failure of chemical agriculture is due to the leaching of pesticides, herbicides and fertilizers into the soil, which disrupts the soil ecology and kills the beneficial microorganisms in the soil. When the beneficial microorganisms are gone, the soil becomes dirt.

The very cool thing is, with a little bit of effort, dirt can be turned back into soil. All it takes is for you to foster an environment where soil can become alive again. As a gardener, you can create the needed environment by adding organic matter to the soil. Compost, bags of composted cow manure, composted leaves from the municipal leaf dump, straw, kitchen scraps — all of these and more are what is needed to keep beneficial microorganisms happy in your garden. Dry conditions force microorganisms deep into the ground. It is for this reason that you want to utilize mulch and keep your plants watered. By the way, an organic mulch (rather than a plastic mulch) gives them that much more organic matter to feed on.

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