Why and How to Improve Clay Soil

Clay soil is a headache for most of the gardeners who experience it.  Clay soils have the tendency to be compacted, smothering plant roots and slowing root growth. Clay soil both drains and absorbs water slowly, causing standing water and runoff.  Clay is not all bad though: clay in soil holds critical plant nutrients like calcium, magnesium and potassium well, and holds water reserves deep in the soil. 

How do you determine if you have clay soil? There are lots of multi-step tests, including the percolation test and the jar test, but there are nearly instant observations you can make that will determine if your soil has high levels of clay.  If your soil sticks to your shoes and coats your tools, and dries to rock hard clods after being disturbed, or if you can form it into a ball in your open palm and it holds its shape without crumbling, you have clay soil. 

Why should you improve your clay soil, especially for newly planted perennials, vegetables and annuals?  Improving the quality with soil amendments—usually organic material—allows better penetration of air and water to the roots.  Organic material provides a home for the microorganisms that further improve soil structure and nutrient capacity.  The looser soil texture of amended soil versus heavy clay soil allows roots to grow more freely, and a larger root system makes for stronger plants.  Take all these factors together and well-amended soil results in better nutrition, better drought tolerance, less water use and stronger growth for whatever you plant.

(Most woody plants with tough, vigorous root systems are capable of tolerating and growing into higher levels of clay. While you should still amend the soil in the planting hole, they will eventually develop a much larger root system.  With a few exceptions, the strong root system will soon be spreading through the surrounding unamended soil.)

So how do you amend your soil? It’s easiest and gives the best results when you are creating a new planting bed. After loosening the underlying soil in your new bed by hand or with a tiller, add at least 6 inches of organic material—compost, shredded leaves or grass clippings (no herbicide on those please!), rotted manure, or bagged soil conditioner.  (A combination of several different organic materials is fine.) Then hand turn or till in to a depth of 6-12 inches.  Yes, it is labor intensive, but the difference in how your newly planted plants perform will be noticeable.  They will establish better, grow better, bloom better, and vegetables will produce better.  Keep your new bed mulched and over time the soil will improve even more.  For vegetable gardens, using winter cover crops and tilling them under in spring does wonders for the soil also.

So what if you have established beds, and can’t or don’t want to till in organic material throughout? Amending the soil in the largest planting hole you can manage with soil conditioner helps.  It doesn’t work quite as well as preparing a full bed, but sometimes you’re stuck with what you have.  You can gradually improve your soil with organic mulch that breaks down relatively quickly; preferably one that is less coarse, like compost, soils conditioner, or mini nuggets.  Coarser mulches like large bark nuggets break down more slowly, as does pine needle mulch.  Scratch the decomposing mulch into the soil every year before topdressing with new, and in time, you will have better soil. 

So you have clay soil, and you don't have the time or energy to amend it.  Fear not-here are some plants that don’t mind clay soil:

Aster
Bee Balm (Monarda)
Black Eyed Susan (Rudbeckia)
Blazing Star (Liatris)
Bluestar (Amsonia)
Canna Lily
Coral Bells (Heuchera)
Coreopsis
Daylily
False Indigo (Baptisia)
False Sunflower (Heliopsis)
Fountain Grass (Pennisetum)
Hosta
Miscanthus
Purple Coneflower (Echinacea)
Russian Sage (Perovskia)
Sedums
Switchgrass (Panicum)
Yarrow (Achillea)
Yucca