How to improve clay soil

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Soil is made up of sand, silt and clay. Its texture depends on the size of the particles it’s made of.

Sandy soil is coarse with lots of space between particles, water runs right through it. Nutrients, too. Plant roots can’t hold onto sand. Those relatively few plants that survive in sandy soil do so by sending their roots down deep, below the sand into more hospitable subsoil.

Silt is smaller and finer than sand. It consists of tiny rock particles, so it feels a little gritty. It easily blows away or washes away into rivers. It retains moisture and nutrients, so most plants will grow in it.

Clay is the smallest soil particle of all. It is so tiny that one particle of clay compared to one 1/16-inch piece of gravel would be invisible to us. They pack so tightly together that air and water can hardly penetrate. When clay does get wet it’s a sticky muck; when it’s dry, it’s hard as concrete. Plant roots can’t worm through it and they can’t breathe in it — it’s no wonder they struggle to survive. Most don’t.

The perfect soil, the kind all gardeners yearn for, is “loam.” Loam contains about equal parts sand and silt, along with a little clay to hold it together. Loam allows air, water and plant roots to move through it. The challenge is to make clay into something more loamy, something your plants will settle into and then thrive. Sand and silt can also be made loamy by applying variations of the following.

Turning clay into something better
Ideally, you should mix into clay equal parts of coarse sand and coarse organic matter. The easiest way to do this is to spread 3-4 inches of organic matter and 3-4 inches of sand on top of the area to be amended. Dig down about 8 inches (about the length of a shovel blade), turn the clay, chop it as finely as you can and mix in the amendments. A tiller makes the job easier (they can be rented if you don’t have one.) Clay is easiest to work with when slightly wet.

Organic matter can be any kind of decayed material, such as molded leaves, ground bark and compost. Shop for compost that has no weed seeds. If you’re using compost from your own bin, be sure it’s “mature” — dark-colored and “earthy”-smelling. If you can tell what it used to be it isn’t ready to use.

To make the soil richer, add composted manure. Bags of this can be purchased from garden centers, home improvement stores, even discount stores. Apply according to the directions on the bag. Composted manure provides such nutrients as nitrogen, phosphorous, potassium and trace elements, such as iron.

When you’re struggling with clay, it’s tempting to improve the soil for only each little spot where you intend to place a plant, but you can test for yourself why this isn’t a good idea: Dig a hole 2 feet deep and a foot wide. Fill it with water. Is water still standing in it the next day? And the next? Most plants won’t survive this. Furthermore, the roots of those that do will eventually begin to bump into the border of the clay-walled “flowerpot” you planted them in. They’ll turn back and go where the soil is friendlier. They’ll wind and wind, become root-bound and stunted, just as though they were overgrowing a real flowerpot.

Regarding trees, the general consensus among experts is to NOT amend the soil for trees.

*Heavy clay soil from a quarry in Estonia. (Siim Sepp / www.sandatlas.org)

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