University of Vermont Extension 
Department of Plant and Soil Science

Anytime News Article


Dr. Leonard Perry, Extension Professor
University of Vermont
Many gardeners are stuck with clay soils that are hard to cultivate, and in which many plants don't grow.  How do you know if your soil is clay?  How can you improve clay soils?  What plants grow best in clay?
If your soil dries like a brick with cracks when it's dry, or in clods, and is like putty when wet sticking to shoes and tools, you have clay soil.  It is hard for most roots to penetrate such soil. Take some soil, add a little water, and form a ball in your hand.  Then squeeze the ball into a flat ribbon.  If the ribbon reaches two inches long or more before breaking, this is a sign you have clay soil. 
Soil particles come in different sizes, which contribute to soil texture.  Sand particles are the largest, clay particles the smallest.  Being so small, clay particles pack together not allowing the necessary spaces between them for air and water to flow, especially air that most roots need to function.   Contributing to this is the fact that clay particles are plate-like, stacked like a deck of playing cards, with little space between them. This traps water for long periods. One positive side to clay is that its particles hold onto nutrients, making them more fertile.
So how do you increase the space between these particles?  Some recommend adding sand, but unless it is coarse sand, and you add about 3 parts to each part clay soil, the soil structure will likely just get worse.  When planting, some recommend putting gravel in the bottom of the planting hole.  This is bad as it just raises the water level, creating what is termed a "perched water table".  Another recommendation is to add gypsum, similar to lime, but this may increase calcium and pH levels too high (clay soils tend to be alkaline).
Best is to add organic matter, particularly compost.  Be sure and add when your clay soil is dry, as working in wet clay is not only quite messy but will compact it even further. Peat moss is sometimes recommended as an addition, but this breaks down quickly in wet and clay soils, and can create an undesirable (for most plants) bog.  Compost is good as it not only helps with the soil structure, but a compound (glomalin) the microorganisms (mychorrhizal fungi) in compost create binds the small clay particles together into aggregates with a waxy coating, thus creating more space between them for air and water to flow. As an aside, glomalin also benefits the soil, and earth in general, by storing carbon.
You can't really overdo the amount of compost added to clay soils.  For a lawn and landscape that is a quarter acre, just increasing the soil organic matter from 2 to 3 percent would take 5000 pounds of an amendment!  Don't get discouraged by this, as adding some is better than none.  You can add organic matter over time, and you can deal with small areas or beds at a time. Adding 3 to 4 inches at minimum is recommended.  If you have a local compost facility, check to see if you can have a bulk load delivered.  This is cheaper overall and avoids having dozens of plastic bags to dispose of.  
Best is to work organic matter deeply into the soil as roots will eventually end up there, and do so prior to planting.  You can till as deep as a tiller will go.  Or, dig sections of a bed at a time to a foot deep, work in compost, then replace the soil and do the next section.  In future years, just work the surface, as organic matter you've worked in deeply will decompose quickly when brought up to the surface.
Another method to increase organic matter in clay soils is with cover crops.  These are crops planted as you prepare beds, for a season or year prior, or in fallow periods between annual crops such as flowers and vegetables.  They consist of small grains and grasses like buckwheat, ryegrass, and oats.  Legumes, such as clover, also benefit by "fixing" nitrogen from the air for use in the soil.  Cover crops have additional benefits such as suppression of many weeds.
If all this sounds like too much trouble, till or break up with a spading fork the area you'll plant.  Then build a berm, or raised bed with sides 6 to 12 inches high or more, over it and fill with a good loam topsoil. You can use drainage tiles, or pipes, on the bottom if a wet area.
If you're planting an individual tree or shrub, it is especially important to chose ones that will tolerate clay soils.  Their root systems are so extensive that you can't amend the soil well enough over a large enough area, especially for trees.  Dig the hole only as deep as the pot or rootball.  Otherwise it will sink over time, causing the plants to end up too low.  This is a major cause of woody plant demise after a few years.
Don't make flat sides to the planting hole that wont let water drain.  You'll only be creating a bathtub for the roots, without a drain.  And only amend the backfill soil with no more than one third of a better soil as the plants were growing in.        
If planting large plants, break up an area around where you'll plant (out to as wide as the plants will eventually grow) with a long spade or fork.  Add organic matter on top of this area which, over time, will work into these cracks you made. 
For low, groundcover perennials that tolerate heavy clay soils, consider the carpet bugle or dead-nettle (Lamium), both of which can spread vigorously.  Low to medium height perennials, mainly attractive for their foliage and good for massing, include the lady's mantle, sea thrift, pigsqueak, daylily, Japanese iris, Japanese painted fern, Ostrich and Cinnamon ferns, and the late-spring flowering globeflower. Most spring-flowering bulbs need well-drained soil, however, checquered lily (Fritillaria meleagris) will tolerate clay  and periodic wet soils.
Taller perennials for clay soils include aster, Helen's flower (Helenium), goldenrod, goatsbeard, foxglove, coneflower (Echinacea), false sunflower (Heliopsis), blazing star, black-eyed daisy (Rudbeckia), meadow rue, Joe-pye, and the compass plant (Silphium).  If you have a contained area you might also consider these aggressive spreaders:  white or yellow loosestrife, plume poppy, and bee balm.  Several grasses will live in clay soils including the switchgrasses and eulalia grass.
There are actually many shrubs that will tolerate clay soils, some better suited to them.  These include the chokeberry, Siberian peashrub, shrub dogwoods, forsythia, common ninebark, potentilla, currants, shrub willows, and viburnums. 
Best choices for smaller trees include European alder, river birch, hawthorns, crabapples, and ornamental pear.  For large trees consider hickories, hackberries (a native plant and vase-shaped replacement for elms), black and green ash, common honeylocust, Kentucky coffeetree, butternut, larch, amur corktree, cottonwood or aspen, oaks, willow, linden, and elm.

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