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Beneficial Soil Organisms
Part 3: Protozoa

Paulette Mouchet

Originally published in the Rose Garden newsletter, October, 2003.

Bugs, bugs, and more bugs. You thought you were finished with soil critters after learning about beneficial bacteria and fungi. Nope. Now comes protozoa. In terms of your plant's health, they might be more important than bacteria.

Protozoa are single-celled animals that eat bacteria. If you remember, bacteria are nutrient warehouses, storing stuff like nitrogen as biomass. When protozoa come to the bacterial dinner table, they release the nitrogen and other nutrients making them available to plants. Yum! The way this works is the protozoa eat the nitrogen-rich bacteria but they can't handle that much nitrogen in their bodies. They have to get rid of most of the nitrogen they ate so they release it as ammonium (NH4+). This usually occurs near the roots of a plant. Some of the ammonium goes into the plant roots to nourish the plant and some is taken up by bacteria in the vicinity.

Organic matter such as grass clippings, leaves, or alfalfa hay mulch contains nitrogen and other nutrients. These nutrients are not readily available to growing plants because they are held in complex forms within the organic matter. Most bacteria are decomposers—they eat fresh organic matter. In doing so, they convert the complex nutrients into bacterial biomass. When a protozoa comes along and eats a bacteria, the nutrients are released to the growing plants in a form that is readily available! Pretty cool, huh?

Another interesting role for protozoa is in regulating bacterial populations. When protozoa eat a bunch of bacteria, they actually stimulate bacterial populations. This works sort of like pruning a plant to stimulate new growth.

Protozoa need water to move in so you'll find them in greatest numbers where moisture is present in the soil. They are particularly active in the rhizosphere next to the roots. Highly fertile soil contains a million or so protozoa in a teaspoon of soil!

You can encourage protozoa in your soil by applying a mulch of oat straw (not alfalfa hay) and spreading vermicompost. Fish hydrolysate (not emulsion) as a soil drench will also encourage protozoa.

Read Part 4: Arthropods and Earthworms

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