Want to drive a gardener crazy? Call that stuff plants grow in “dirt” instead of “soil.” What’s the difference? Dirt is what you track into the house on your shoes. Soil is an amazing and complex ecosystem that is one of our planet’s most valuable natural resources. It’s a mix of inorganic minerals, water, air, organic matter from dead and decaying plants and animals, and an incredible array of living organisms, ranging in size from microscopic bacteria and fungi to earthworms, moles, and shrews.

Show students a cup of soil taken from an undisturbed area of native soil and ask them to estimate the number of organisms living in it. Let them dig around in the soil to see if they can see any creatures in it. Unless they find a stray earthworm or possibly some tiny mites or springtails if they look closely, it’s likely they’ll say that there aren’t any living creatures present. But they’ll be off on the order of billions! While there may not be any that you can see with the naked eye, if you looked through a microscope at the same soil sample, you’d be overwhelmed. There could be as many as 200 billion bacteria, 20 million protozoa, 100,000 nematodes, and 100,000 meters of fungal hyphae in that cup of soil!

Many of those microscopic organisms benefit plants, either directly or indirectly. (Of course, some soil microbes cause plant diseases, while others have no effect on plants, for good or bad.) For example, a special type of bacteria, called rhizobia, inhabits nodules on the roots of legumes (plants like peas and beans). In return for some food from the plant, the rhizobia take up or “fix” nitrogen (an essential plant nutrient) from the air, where it is not in a form available to plants, and change it into one that plants can take up and use. This is a good example of a symbiotic or mutually beneficial relationship between plants and soil microbes.

Mycorrhizae (my-cor-rye-zay) on plant roots display another symbiotic relationship. Mycorrhizae are fungi that form an association with the roots of specific plants (their name translates as “fungus root”).  The fungi receive nutrients from the plant; in return, they enlarge the surface area of the roots, allowing them to take up water and nutrients for the plant more effectively. 

There are also untold numbers of soil microbes that help plants less directly by breaking down organic matter in the soil and changing the nutrients it contains into forms available to plants. As these decomposing microbes break down organic matter, they also produce natural “glues” that bind soil particles into aggregates, enhancing soil structure and improving soil drainage and aeration.

What do all these millions and billions of garden helpers need to keep them thriving? Plenty to eat, in the form of organic matter that supplies them with the energy they need for growth and reproduction. Adding organic matter like compost to the soil keeps these beneficial microbes thriving. Remind your students of all the creatures they’re supporting as they spread compost on the soil in your school garden this spring!

Blog by: Susan Littlefield

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