Hands in the soil

Grab a handful of soil. Go ahead, step outside and scoop up a small amount in your hand. It’s hard to imagine all that’s going on in there. You’re holding trillions of creatures, though that’s just an estimate. Nobody has done the actual counting.

Unless you happened to grab an earthworm or beetle in your handful, you’re not likely to see much of anything without the assistance of an optical device. But with a simple hand lens or magnifying glass, you’ll see some tiny animals, including mites, springtails, and potworms. Some are these animals are predators, but many are detritivores (eaters of dead things). They’re largely responsible for tearing holes in the tissues of the dead things and for spreading them around -- the first step of decomposition.

With a microscope, you can see even more minute animals. You’re likely to see tiny worms called nematodes and rotifers, as well as amoebae and miniscule, eight-legged animals called water bears. These organisms eat everything: other animals, dead things, bacteria, fungi. Some nematodes can infest plant roots by stabbing into them with hypodermic-needle-like mouth parts. (When gardeners plant marigolds because they are "good for pests," they are taking advantage of marigold roots’ nematicidal properties.)

You can also see root hairs, algae, cyanobacteria, fungi, actinomycetes, and bacteria.

They may not be very noticeable, but they are the source of most of the diversity in the soil, accounting for the bulk of the soil’s biomass and most of its metabolism.

Root hairs are tiny, single-celled extensions of a root’s epidermal layer. They anchor the roots, which are actively pushing through the soil, and increase the surface area with which to absorb water and nutrients. Algae need light to photosynthesize, so they are generally found near the surface of the soil. Green algae are most often found in moist (though not flooded) acidic soils; diatoms, another kind of alga, are often plentiful in garden soil that is rich in organic material. Cyanobacteria are similar and are often found in wetland soils.

Over 700 species of fungi have been found in the soil so far. They include single-celled yeasts, filamentous molds, mildews, and rusts. These various kinds of fungi are responsible for much of the humus formation, soil stabilization, and the breakdown of organic matter. Although we may think of mushrooms as above-ground organisms, this visible fruiting body is only a small portion of the whole organism; fungal strands called hyphae extend throughout the soil. Some fungi are even predators. One uses a lasso-like device to trap nematodes.

Fungi are the source of many plant diseases, including late blight, the disease that plagued tomato and potato plants in the region in 2009, but one fungus (a mold) is also the source of penicillin. Fungi are particularly important in bogs and forest soils, where they are able to withstand acidic conditions. Perhaps the most important fungi, however, are those that form mutually beneficial associations with plant roots, called mycorrhizae ("fungus root"). Most plants depend on mycorrhizae to survive.

Actinomycetes have some fungus-like characteristics and some bacteria-like ones. They are detritvores. Many produce antibiotics to kill other microorganisms. The drugs actinomycin and streptomycin are derived from actinomycetes. In forest soils, the genus Frankia (one type of actinomycete) is responsible for much of the nitrogen fixation that makes atmospheric nitrogen available to higher plants. Actinomycetes also produce volatile compounds that make the "good earth" smell you think of when you think of dirt. There are hundreds of millions of actinomycetes per gram of soil, weighing up to 5,000 kilograms per hectare.