The soil is a garden in its own way. Good soil, the kind that makes for healthy plants, churns with life: bacteria by the million in every teaspoonful. Fungi that send slender filaments snaking between the very particles. Burrowing earthworms. Insects and arthropods. Tiny carnivorous worms called nematodes. Chipmunks and moles.James Nardi, an entomologist at the University of Illinois at Urbana-Champaign, is so fascinated by that part of the garden that he wrote a field guide to it: “Life in the Soil: A Guide for Naturalists and Gardeners” ( University of Chicago Press, 293 pages, $25).
Some of the organisms he describes break down dead plant matter and release its nutrients to be absorbed by the roots of living plants. Others help make roots more efficient. Some improve the texture of the soil so plant roots can get air and water. And some eat others, maintaining the population balance that keeps the whole underground society — what scientists call the soil food web — humming along.
“Feed the soil, feed the plants” is an ancient maxim. Of course, when you feed the soil, you are really feeding the organisms that live in it. But what do you feed them? What nature has fed them for millions of years: dead plants, full of energy-rich sugars and carbon harvested from the air, the food for all creation.
Any plant matter that lands on the soil eventually will be consumed by the many organisms that rustle or creep through the layer of leaf litter that falls all over the planet. Every living leaf is covered with bacteria, Nardi says, and when the leaf dies bacteria start consuming it before it even falls.
But we can help the process along considerably by spreading plant matter that is already partially decomposed and full of soil dwellers. We call it compost.
Compost does more than feed the critters that feed our plants. It also improves soil texture. Texture is about how air and water flow. Plants are mostly made of water and water carries their nutrients, so they need soil that will let water flow freely. But they need air around their roots too.
The ground-up minerals that make up the bulk of any soil can have particles of different sizes. Smaller particles have less space between them for water and air.
The finest particles make sticky clay. It clumps together to block air and water and easily packs down to the consistency of a brick. Sand particles are bigger, allowing water to drain easily — so easily that water and nutrients don’t stay around plants’ roots long enough to be absorbed. Silt particles are in between.
The best soil is loam, with at least two sizes of mineral particles. Different proportions give different types: clay loam, sandy loam, silty clay loam. You can add sand to clay soil to get more large grains. But it’s better to add compost, which has many particle sizes. Organic matter also contains soil microbes that can bind small particles together into larger ones.
Adding organic matter makes many other improvements too: Because it is less than half as dense as the minerals, it lightens the soil and reduces compaction. It holds moisture. It changes the soil chemistry, making alkaline soil more neutral, which helps deliver nutrients to plants. “It’s hard to pick out one effect because they’re all connected,” according to Bryant Scharenbroch, who does research on urban soils at The Morton Arboretum in Lisle.
Compost offers food for worms, insects and other creatures that burrow through, making tunnels that circulate air and water. It supports fungi that ferry water and nutrients to roots.
Where do they come from? Soil with enough organic matter contains the organisms it needs. “If the soil is treated well, these creatures will come,” Nardi says.
But once you’ve got them, don’t kill them. Too much fertilizer can be deadly for microorganisms and other soil friends. And pesticides destroy not only the kinds of bacteria, fungi and insects that cause diseases, but the many other kinds that do such essential work for us.
So for healthy soil and healthy plants, it’s best to avoid pesticides. Dig in plenty of compost when making a new bed and replenish the supply by spreading more at least once a year. Mulch over plants’ root zones with organic matter such as shredded leaves or bark.
And when you find earthworms or even earwigs in the soil or under a leaf, congratulate yourself on your underground garden.
What’s in your soil?
If you are a new gardener or have just moved into a new landscape, there are several ways to investigate the quality of your soil.
Get a professional soil test. The report will tell you about the texture, pH, available nutrients and (if the sample is fresh) microbial activity in your soil. You will need to get a kit from a soil test laboratory and collect several samples. For a list of labs, see urbanext.uiuc.edu/soiltest .
Do some snooping on your own. Dig up a shovel full of soil from the top 6 inches or so in several spots in your yard. In each place, pick up a handful of soil and feel it. Does it feel sticky? When you squeeze it, does it hold its shape? If so, it’s got a lot of clay. Does it feel rough and sandy and not hold a shape at all? If so, it’s got a lot of sand. Does it feel light, with lots of chunks of different sizes and a clean, earthy smell, and partly hold together when you squeeze it? If so, it’s probably got lots of organic matter. That’s good stuff.
You can’t judge by color, says Bryant Scharenbroch, a soil researcher at The Morton Arboretum in Lisle. Good soil tends to have a deep color from decomposed organic matter. But because of the mineral content in the Chicago area, soil that looks black can be solid clay.
Test the organic matter content yourself. Put a cup or so of soil in a large glass jar, fill it with water, add a teaspoon of liquid dishwashing detergent and stir. Let it stand for several days. The different mineral particles will separate into layers, giving you a rough idea of the proportions you’ve got. If the water is dark, like tea, with lots of floating debris, be happy: the murk is organic matter.
Do a ribbon test. Screen out the organic matter and test the mineral remainder for particle size. For details, see chicago tribune.com/soiltests.
Add organic matter to your soil. If you don’t have homemade compost, you can buy various composted materials bagged or in bulk at garden centers.
One thing you can do
Your assignment this week: Start a compost pile. It doesn’t take much. Begin by gathering non-woody plant waste — leaves, weeds, dead houseplants — in a heap in an inconspicuous place in your yard. Once the pile is big enough, bacteria, fungi and other industrious critters will go to work to convert it to black gold. The process goes faster in a bin (required by law in Chicago), which holds in heat and moisture and repels foraging animals. You also can add plant matter from the kitchen, such as apple cores and vegetable peelings. Want to know more? See http://www.urba next.uiuc.edu/homecomposting/basics.html.
