Add organic matter (dead plants) to improve your soil

To improve their soil, gardeners often are told to add organic matter. What’s that?

“Organic matter is anything that used to be alive,” says Meghan Midgley, soil scientist in the Center for Tree Science at The Morton Arboretum in Lisle. “In nature, everything that dies falls onto the soil, and over time, living organisms in the soil break it down. That’s what makes topsoil.”

Most of it is dead plants. Trees and other plants naturally live in a layer of discarded leaves, stalks and branches from last year and all the years before. Bacteria, fungi, insects and other organisms continually break those discards down, mixing them into the soil and releasing their nutrients for plants’ roots.

So when we add organic matter to our garden soil, we’re imitating nature, according to Sharon Yiesla, plant knowledge specialist in the Arboretum’s Plant Clinic.

In autumn, organic matter is abundant, she said. Soon, most of our trees will drop their leaves. Meanwhile, other plants in our gardens are dying back, leaving behind stems, leaves and branches that can enrich the soil.

How does organic matter help our garden plants? Large particles of partly decayed plants create spaces in the soil through which air and water can flow to plants’ roots. This is especially helpful in the sticky, close-grained clay soil that is typical of Chicago-area yards.

Bits of organic matter act like little sponges, soaking up water, so there is a reservoir of moisture in the soil for plants’ roots.

The decaying plants also are a food supply for soil organisms. Organic matter supports an entire food chain, from bacteria and fungi to minuscule insects and arachnids to earthworms and centipedes. As these organisms consume the decaying plant matter and then are consumed or decay themselves, they release nutrients.

Plant materials, like fallen leaves, decay and make soil better. However, gardeners usually add compost — plants that have been partially digested by soil organisms.

Shred fallen leaves with the lawnmower or with a leaf shredder to make the best use of them. Smaller pieces will break down faster in the compost pile and will be less likely to mat down or be blown around when used as mulch. The organic matter used at the Arboretum as a soil amendment and garden mulch consists mostly of composted leaves.

If you don’t have your own compost, or don’t have enough, home centers and garden centers sell compost in bags or in bulk. “Read the label carefully to make sure that all you’re buying is partially decayed plants,” Yiesla said.

For new beds, dig in lots of organic matter several inches deep to improve the soil and give new plants a good start, Yiesla says. For established trees, shrubs and garden beds, spread compost or leaves right on the surface of the soil.

“That’s nature’s way,” Yiesla says. The living things in the soil will take it from there.

For tree and plant advice, contact the Arboretum’s Plant Clinic (630-719-2424 or plantclinic@mortonarb.org).

MORTON ARBORETUM Organic matter is made of anything that used to be alive — mainly dead plants. In nature, leaves and other litter fall to the ground and are broken down to enrich the soil.