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Living life in the dirt

There are many intriguing relationships between what we grow in our gardens, farms and lawns and the complex web of simple life forms that live essentially entirely underground.

One teaspoon of healthy soil contains 100 million to 1 billion individual bacteria, although the quantity is likely to be in the lower limits in soils that are primarily sand or clay.

In addition to the bacteria, there are algae, yeast, protozoa, yeasts, nematodes, microscopic insects, earthworms, beetles, ants, mites and fungi. This complex web has worked over millions of years to provide us with the soil that lets us grow so many plants.

The most abundant varieties of single cell bacteria are so small that they are difficult to clearly see in basic microscopes. Some of the bacteria forms symbiotic relationship with plants like clover, taking carbon compounds from the plant, and then supplying the plants with a usable form of nitrogen (very important for plant growth). Some bacteria convert ammonium into nitrite, which is a preferred form for many plants to use. There are a few forms of bacteria found in some soils that can be detrimental to either plants or humans (such as E. Coli), but they are not real common in most healthy soils.

One fairly common and beneficial fungus is Arbuscular mycorrhizal, which has a symbiotic relationship with many plants. They attach themselves to plant roots, get carbohydrates which have been transplanted from the plant leaves, and then have their own filaments grow out from the roots and bring water and soil nutrients back to the plant. There is also a fungus that feeds on harmful insects. There are nematode-trapping fungi that are parasites to one of the types of nematodes that cause a plant disease. There are, of course, some fungi that are detrimental to plants. These include fungi pathogens and parasites that can cause significant crop losses.

Nematodes might be characterized as very small, non-segmented worms, about 1/20th of an inch in length. There are a few that have a negative impact on our plants, but most are beneficial to either our soils or plants. Some types of nematodes feed on bacteria, some on algae, some on fungi and a few on other nematodes. The body wastes of nematodes include a version of nitrogen.

There is also a wide variety of arthropods. Arthropods range in size from very small up to several inches in size. They include ants, beetles, sowbugs (related to crabs), spiders and mites. Arthropods very greatly in their lifestyles; they break down decaying material, eat other soil life forms (some beneficial and some harmful to our plants), mineralize nutrients from what they consume to plant friendly nutrients. Again, a few are detrimental to our chosen plants, but many more provide benefits to our plants and keep the number of smaller life forms in balance.

The natural biological processes in the soil are responsible for about 60 percent of the available nitrogen and about 50 percent of the available phosphorus in the soil. This is one of the reasons that we should be cautious when we use chemicals to kill one variety of fungus, bacteria or nematode that is causing problems. Many chemicals that kill one variety will also kill others that are beneficial in some way to soil life. This is one of the reasons that when chemical controls are used, they should be applied only where needed, and directions should be carefully followed. Other options can include planting varieties that are naturally resistant to common pests or using a native biologic control.

Much of the information here is sourced from the U.S. Department of Agriculture/ Natural Resources Conservation Service, primarily from their online Soil Biology Primer.

Refer to the revamped and updated University Of Minnesota Extension Service website, for more information on horticultural topics.

Local Master Gardeners will again answer your questions on home horticulture. Call (218) 444-7916, leave your name, number and question and you will receive a return call.