Clay soil: A mixed blessing, but easily corrected

“Extension on the Go” podcast by Debbie Johnson. Episode 64: Amending Clay Soil

KIRKSVILLE, Mo. – You dig your first spadeful of soil in your new garden and discover you have a shovel full of clay.

But if your garden is more clay than loam, not all is lost. Jennifer Schutter, horticulture specialist for University of Missouri Extension, says you can improve clay soil.

“Clay soils present several obstacles that need to be overcome,” Schutter said. “It tends to compact easily and form a hard, dense layer on the surface that prevents water, air and fertilizers from moving downward.”

This tendency toward compaction reduces air space in the soil. This can lead to plants developing shallow root systems, which makes them more prone to drought stress, Schutter said.

There is just one practical way to improve clay soil: working in plenty of organic matter.

Common sources of organic matter include peat moss, decomposed animal manures and compost made from leaves, plant refuse or grass clippings, Schutter said.

Another good source of organic matter is green manure. “This is a crop, such as annual ryegrass, that is grown and then turned under when it’s one-third to one-half grown and is still green and lush,” Schutter said.

Clay soil improvement is as easy as digging organic material into the soil once a year, in either spring or fall, she said.

“Spread anywhere from a 1- to 4-inch layer of organic matter on the soil and work it into the top 6-10 inches,” Schutter said. “Remember, organic matter breaks down over time, so it’s best to replace it once a year.”

However, Schutter doesn’t recommend annual application of organic matter for bulb gardens or perennial flowers. “Instead, work organic material in when you plant new bulbs or flowers and then mulch in the fall,” she said.

When planting new trees and shrubs, don’t put organic material into the planting hole, Schutter said. “Work it into the circle of soil skirting the tree or shrub. Be sure to go out 2-5 feet in all directions from the trunk to improve the soil where the roots will eventually grow.”

Clay soils aren’t all bad, Schutter said. They hold water and nutrients very well. Clay particles hold onto ammonium and trace elements like potassium, calcium and magnesium. This makes it a fertile soil. Schutter says all you need to do is break it up a bit so water and air can flow freely.

For more information, see the MU Extension publication “Improving Lawn and Landscape Soils” (G6955), available for free download at www.extension.missouri.edu/p/G6955.