By Janet Macunovich / Photos by Steven Nikkila
Ah, spring! The season of new life, warm earthy scents, and fresh starts on gardens!
Unfortunately, it’s also a season of miracle products—powders and potions claiming to “break up heavy soil,” “dissolve clay,” “ionize damaged soil,” etc.
Twenty-five years of gardening in soils throughout southeastern Michigan, around homes new and old, has taught me that only one ingredient can be added to hard soil to make any real difference in its garden potential. Air is that magic ingredient.
“Wait!” you may say. “I bought this stuff, mixed it in and it worked wonders…”
You’re right, for the wrong reasons. You feel that stuff made the difference, but I know that mixing was what worked wonders. The digging allowed air to flow through the soil.
Air brings water with it. Water doesn’t fall into soil, it’s pulled in by capillary action, a draw that exists only if there’s air moving freely in pore spaces in the soil.
Abundant, moist air fuels an explosion of life between the soil particles. Microorganisms and soil animals increase geometrically in aerated soil. These creatures’ chewings, manhandlings, regurgitations, excrements, and other life processes transform solid soil minerals into forms which can be imbibed by species restricted to liquid diets—plants.
Clay in soil is not terrible. When the tiny bits of mineral called clay are numerous, that soil offers far more nutrients and holds moisture far better than one that’s mainly sand. Compaction is what’s terrible – the state soil takes after pressure has been applied, usually by earthmoving and grading equipment. Even a farm’s best sandy loam can be packed down so hard that the average gardener will cry “clay!” and begin seeking wonder products.
In a compacted soil, air spaces have been squeezed closed. Crumbs of combined clay, sand and humus have been pulverized – crumbs that once gave the soil an airy, root-friendly structure. Separated by crumb-busting, the component particles settle into dense layers. They will not re-form into crumbs until treated with that mix of worm spit, fungal strands and bacterial residues called microbial glue.
Pressed down and airless, compacted soil can’t attract or support many worms, fungi or bacteria. Thirty years after being graded with heavy equipment, such a soil will still be dense and lifeless unless physically broken and aerated.
So can anything be done with the miserable leavings called soil on your property? Certainly. However, there is no immediate fix unless you can afford wholesale excavation and replacement.
Here’s a much less expensive option that trades time for money.
This spring, start some soil-loosening by wetting the soil. Since pore size in hard-packed soil is small to non-existent, it takes a long time for water to infiltrate. So cover the soil with porous mulch to encourage water to “sit” and “stay.” Wood chip mulch is fine, but pine bark is best for reasons explained later. Layer the mulch with grass clippings if you can—more on that later, too.
Keep the area well watered throughout summer.
In late summer or early fall rake off the mulch. Use a garden fork to loosen the bed. Insert the tines as far as you can, lean back on the handle and pop a chunk loose. No need to lift the chunk out of the bed, just pop it far enough that it doesn’t settle back level with the undisturbed soil.
Move over one fork’s width, insert the fork and pop again. Continue doing this row by row through the garden until the whole surface is lumpy.
Add one of those miracle products now if you’d like. Scatter or sprinkle it over the area and water it in well. Me? I’d rather use that money to add compost.
Re-cover the area with mulch. Add more grass clippings or leaves that are small or shredded. Water. Wait some more.
What’s going on while you wait is, well, life. Worms move into the cool, moist mulch and dine on the leafier parts. Between meals they burrow into the moist clay, dragging organic matter with them and depositing worm manure (casts) along the way. Other soil animals follow these trails, which is why worm burrows often contain a soil’s greatest diversity of species. After the first waiting period you were able to loosen previously-impenetrable soil to a depth of 8 or 9 inches because the worms led the way. Now the worms will go even deeper.
During the waiting periods, some of the organic matter that is dragged into or falling down worm burrows is decaying bark. That’s good, especially if it’s pine bark with its high lignin content. Lignin, partially rotted, lasts a particularly long time in the soil and each bit becomes a nucleus for soil crumb formation.
After a year, in spring, the bed is ready for planting. Don’t worry about planting among still-whole chunks of clay. Don’t remove them, either. Just plant alongside them or break them in half and fit them, plus decaying, mulch around the new root balls as backfill. Roots will follow the crevices between clods, making fibrous nets over every moist, rich clay surface—nets which hasten clay’s crumb-ling.
Maintain a layer of leaf compost on this bed. In 4 to 5 years your visitors will exclaim “aren’t you lucky to have such good soil?!”
Here are three ways to take some of the labor out of this process.
One, drill rather than dig. Use a soil auger or rent a power posthole digger. Punch holes in the clay every 12 to 18 inches rather than loosen with a fork. Let drilled soil fall back in and around each hole. No need to backfill the holes with “good” soil because the drill adds the only necessary magic—air.
Two, if you have a heavy-duty lawn tractor or can hire a farm-grade tractor and operator, knife the soil rather than fork or drill it. A soil knife attachment slices the soil vertically, cutting about 18 inches deep. The soil doesn’t turn over, as with a plow, it just parts. Knife the soil in rows 18 inches apart. Go over the area twice, first in rows parallel to any slope, then up and down the slope.
Sorry—don’t try drilling or knifing if you have buried utility wires, pipes, or sprinkler lines in the area.
Your third out is to moisten the soil with watering and mulch, fork it lightly, then build a raised bed of imported soil on top of it. Don’t, however, skip the forking. Many raised beds over clay fail because there is no transition area between the two soil layers. Water pools there, unable to penetrate the clay as quickly as it ran through the top layer. Roots rot.
Finally, if your soil really is clay, be happy with that. Clay soil is the choice of many fine plants. Common species like roses, lilacs, iris and crabapples plus rarer beauties like Rodgersia and Ligularia thrive in the loose, clay-based soil.
Janet Macunovich is a professional gardener and author of the books “Designing Your Gardens and Landscape” and “Caring for Perennials.” Read more from Janet on her website www.gardenatoz.com.