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A Science Project: Soil Sampling for Microorganisms

Soil is actually alive with tiny organisms (microflora) such as fungi, bacteria, actinomycetes and algae, as well as arthropods, which include springtails, millipedes, spiders, mites and beetles. One-fifth of a teaspoon of soil can contain more than 100 million bacteria, 1 million actinomycetes and 100,000 fungi, and a single square yard of soil contains 500 to 200,000 arthropods. These organisms are responsible for natural events like decomposition, mineralization, and breaking down pollutants and toxins. Although some microorganisms are parasites, others play a role in fighting pests, diseases and weeds.

What You Will See
- Soil organisms can be divided into two groups: animals (fauna) and plants (flora). Macro-organisms are larger than 2.0 mm in diameter, meso-organisms are 0.2 to 2.0 mm in diameter and micro-organisms are less than 0.2 mm. Microorganisms' require the observer to use a magnifying glass or microscope. Scoop some garden soil with a trowel, and spread it out on a sheet of paper. Inspect the dirt for the presence of insects such as worms, beetles, ants, spiders, mites, centipedes and millipedes.
Under the Magnifying Glass
- Soil can be examined at various degrees of magnification. Take a petri dish and layer it with agar. Take a sample of soil, spread it thinly over a glass microscope plate, then spread the sample over the agar. At 10x magnification; macrofauna, such as earthworms, beetles and ants are visible along with the structure of the mineral content of the soil.
Under the Microscope
- At 50x magnification, mesofauna (collembola, mites) become visible, as do particles of silt or sand-sized grains. Decomposing plant material will become visible. At 100x--200x magnification, microfauna -- such as single-cell protozoa, mites, nematodes -- silvery worm-like creatures, insect larva, rotifers -- can be seen. At 1000x magnification, bacteria, actinomycetes, fungi and even viruses are visible, as is mycorrhizae fungus, which helps plants absorb nutrients through their roots.
Trapping Arthropods
- Make a pitfall trap to catch large arthropods, by sinking a container such as a yogurt cup into the ground. Keep the rim level with the soil surface. Cover the container, to keep the rain out and pour in 1/2 inch of non-hazardous antifreeze to preserve the creatures. After a week, you should have soil creatures to view in your trap. For smaller arthropods, place a 1/4-inch rigid wire screen in the bottom of a funnel, made from the bottom of a plastic soda bottle. Half fill the funnel with soil, and suspend it over a cup with a bit of anti-freeze or ethyl alcohol in the bottom as a preservative. Position a light source a few inches over the soil to corral the organisms from the soil, into the cup. Leave the light on for 3 days to dry out the soil. Then pour the alcohol into a shallow dish. Use a magnifying glass to examine the organisms.

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