Before you start planting this spring, take some steps to improve the soil your plants will live in. “Early spring is a good time to make your soil a good home for your plants’ roots,” said Julie Janoski, Plant Clinic manager at The Morton Arboretum in Lisle. “The time to add soil amendments is before you plant.”
Here are some tips for spring soil improvement.
Learn about your soil’s texture. If you are new to your garden, figure out the soil’s texture. Pick up a handful in your fist and squeeze. If the ball falls apart easily, it is a medium texture called loam or sandy loam, with relatively large particles. If it sticks together, the soil has a lot of clay, with many very fine particles. “Clay soil tends to hold water and drain poorly,” Janoski said. “Plants’ roots can struggle to get the air and water they need in clay soil.” It is common in the Chicago area.
Add organic matter. Clay soil can be improved by adding organic matter — the remains of dead plants. A common source of organic matter is compost, which is plant matter that is partially decayed. Over time, organic matter will break down further, releasing nutrients and improving the soil’s texture. If you add organic matter such as compost to the surface of the soil — a process called top-dressing — the beneficial organisms that live in the soil will break it down and incorporate it. “Top-dressing is usually better than digging compost into the soil,” Janoski said. “When you disturb the soil, you will damage its structure and bring weed seeds to the surface where can germinate.”
Top-dress existing beds. Spread organic matter such as compost over existing beds and in raised vegetable beds in a layer about 1 inch deep. You can also add some compost to individual planting holes. Remember that the mulch you spread over your beds after planting will also break down and improve the soil.
Dig compost in if necessary. “The only time it is worth disturbing the soil to mix in compost is when you are renovating an entire area with extremely poor soil,” Janoski said. A major addition of compost may improve clay soil. To amend an entire bed, spread compost several inches deep over the soil and mix it into a depth of 6 or 8 inches deep. This is easiest with a digging fork or cultivator. In future years, top-dress the bed instead of disturbing it again. A layer of mulch over the bed after planting will help reduce the germination of the weed seeds exposed by digging.
Let trees stand on their own. The Arboretum generally does not recommend adding compost or other soil amendments to the hole when planting trees. “If the soil environment in the hole is richer than the surrounding soil, it will encourage tree roots to circle inside the hole instead of spreading out,” she said. Instead, dig a wide hole that isn’t too deep. After the tree is in the hole, refill it with the soil you dug out. Then spread mulch over the roots of your new tree.
Avoid compacting the soil. When soil particles are squeezed together, air and water will not be able to penetrate to the roots. Clay soil is easily compacted. To avoid this problem, don’t dig when the soil is wet. Your own weight can compact soil, so work from the sidewalk or pathway as much as possible rather than walking on soil.
Get a soil test. A professional soil test can provide valuable information about the chemical properties, nutrients and physical properties of the soil in your garden. It involves collecting samples from several areas of the yard and sending them to a laboratory. Learn more at mortonarb.org/soil-test.
“Preparing your soil before planting will get your garden off to a good start,” Janoski said.
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or [email protected]). Beth Botts is a staff writer at the Arboretum.