Q: The soil in my community garden plot is heavy clay. A neighboring gardener suggested adding leaf mulch and sand to improve its texture. How much sand and leaves should I add to attain a plot that is suitable for growing healthy plants?
A: Before adding anything to your garden plot, if you’ve not done so already, decide what you wish to grow and then submit a soil sample for testing to determine existing composition and nutrient levels. Knowing precisely the current conditions will better inform your choices for amendments. Sand should not be one of them.
It may seem logical to add something quite porous like sand to heavy and dense clay soil to improve its porosity and drainage, but very often the end result is the opposite — a garden plot resembling something akin to concrete. If you wanted to turn a clay soil to a sandy loam by adding sand, you’d have to add at least three parts sand for every one part clay. That’s just not practical. You’re much better off adding organic material like compost, peat moss, composted manure, and yes, leaves.
Whether to till this material in or pile it on top is controversial. Tilling it in will give you a better soil texture more quickly, but tilling can also compact the clay and destroy the soil structure. If you were just growing perennials, you could simply add a layer of mulch every year, and eventually your soil would improve without tilling or digging.
But since you are growing vegetables, you want faster results, so you’ll need to dig or till the material in. Do this on a day when the existing soil is not too wet, to try to avoid compacting it as much as possible. Using organic mulch or straw at a depth of 2 inches will also help. It will not only deter weeds and reduce moisture loss, but will eventually break down and contribute to healthy soil composition.
Another option is a raised bed filled with a mix of soil and compost.
Think of your soil as a living organism that needs to be, like your plants, fed and cared for. Healthy planting mediums need to promote air circulation, maintain moisture, have the optimal balance of nutrients and micronutrients so your plants are able to absorb them, and provide an environment rich in microbes, fungi, and nematodes.
Clay isn’t all bad, by the way. As long as it’s balanced with organic matter, SOME clay in your soil is a good thing. It contains nutrients and it holds onto moisture well. But you will need to stay on top of this. Adding the organic matter isn’t a one-time thing. You’ll want to get a new soil test and amend the soil again year after year.
Written by U of M Extension Master Gardeners in St. Louis County. Send questions to firstname.lastname@example.org.