Make Friends with Clay Soil

Caltrans photo

Having used many choice four-letter words and thrown a few shovels across the yard, I know how difficult our clay soil can be to work with. Yes, it’s harder to dig and yes, it can cause drainage and aeration problems for some plants. But clay happens to have several beneficial properties for gardeners.

It’s a nutrient magnet. Clay tends to be the most nutrient-dense soil type. Its surface structure and chemical properties naturally attract and hold many of the essential elements of plant nutrition, such as calcium, magnesium, potassium, hydrogen and iron, among others. “Because clay minerals are so active in nutrient exchanges,” explains the California Master Gardener Handbook, “they are major determinants of the chemical and physical properties of a given soil, and they largely determine how well plants will grow in that soil.”

It’s great at retaining water. Imagine watering a plant that’s sitting in sand. The water runs right through, so plant roots don’t have time to take it up. It’s the opposite in clay soil, where water drains much more slowly, giving plants a chance to soak it in.

So, keeping clay-rich soil’s benefits in mind, here are some tips to overcoming its drawbacks:

Choose your plants well. First and foremost, select plants that either like or aren’t bothered by heavy soil. Not surprisingly, foothill natives are good candidates. So are a number of drought-tolerant prairie perennials like coneflower, yarrow, black-eyed Susan, Russian sage, baptista and switch grass.

Don’t aggravate the problem. Any soil type can be damaged by compaction or over-tilling, but especially clay, and especially if it’s too wet. Avoid walking in planted beds when the soil is wet. UC Cooperative Extensive-Sacramento County states, “Clay soils must be worked when moist enough that the tines of a spading fork can be pushed all the way into the ground, but not so wet that it sticks to the tools.”

Amend wisely. University of California Agriculture and Natural Resources’ (UCANR) research has found that dumping a little packaged garden soil into your planting hole to “solve” the clay problem is mostly a waste of effort and money. The more effective solution is to add high-quality organic matter on top of the soil annually, then let the earthworms, microbes and weather work it in. Sacramento’s Cooperative Extension offers this advice for amending clay soil: “Animal manure, green plant material and leaf mold all decay quickly so they are especially helpful in improving soil structure … Peat moss, straw, sawdust, shredded bark or leaves may also be used, but they decompose slowly … “

Consider a cover crop. Especially if you’re starting a new bed, a cover crop like legumes, clover or buckwheat can break up and improve clay soil. UCANR’s Sonoma Master Gardeners offer a good description of the benefits: “Besides adding compost, cover cropping increases weed and erosion control, nutrient retention, water permeability, soil pore spaces, attracts beneficial insects, and reduces soil-borne diseases.” Once your cover crop cycles through a season, you can turn it into the soil or add it to your compost bin.

Because of its benefits to plants, that hard clay you love to hate is actually a great foundation from which to build nutrient-dense garden soil.

Rachel Oppedahl is a University of California Cooperative Extension Master Gardener of Tuolumne County.