Planting in Clay Soil

by Rod Smith
Oregon Certified Nursery Professional

© 2000-2016 Rodney A. Smith
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The soil here in Washington County, Oregon, is very heavy clay, like it is in many parts of the world. Clay needs extra preparation to help plants grow. Here are some tips to help newly planted plants grow better.

Never work clay soil when it is soggy or bone dry. If the soil is too wet, it will pack into hard clods. If it is bone dry, it will shatter into dust which will turn to mud, then brick when it dries. Bone dry soil should be watered with at least an inch of water, then allowed to soak in for a day or two. Test the soil by squeezing a handful into a lump, then push your thumb into the lump. If it dents like modeling clay, it is too wet. If it crumbles, then it is perfect to work.

Always dig a hole three to five times as wide as the root ball of the plant and about the same depth as the root ball. Digging a deep hole usually causes the plant to settle too deep, which leads to crown rot disease if water sits around the plant. Avoid digging a hole with smooth sides which encourages roots to circle the hole. Chop the sides with the point of the shovel to create slots which force roots to grow into the surrounding soil. Pour a couple inches of potting soil or planting compost and sand into the hole. Add 1/8 to 1/4 cup, depending on the size of the hole, of a complete fertilizer that has the middle number, phosphorus, at least as high as the first number, nitrogen. Bone meal is an excellent choice, because it will not burn roots. Chop it into the bottom so the potting soil and existing soil are mixed. Pour more potting soil or compost onto the pile of soil and add 1/4 to 1 cup of fertilizer. Mix it together so it is about 1 part potting soil or compost and 3 or 4 parts existing soil. The potting soil or compost will improve both aeration and drainage. I always shovel the soil into large plastic pots or a wheelbarrow as I dig the hole to make it easier to mix the soil, and the soil is not lost into the grass or mulch.

It is better to improve the existing soil than to replace it with a rich soil. This is worth repeating. Filling the hole with rich soil is likely to cause root rot. Rich soil will absorb water quickly, but it is very slow to drain away through heavy clay soil. The rich soil will usually be even wetter than heavy clay and root rot is likely. When you improve the existing soil, it is easier for water and roots to move from the improved soil to the existing soil. The only exception is if you hit blue clay. Roots will not grow in blue clay because there is no oxygen in it. Replace blue clay with sandy topsoil mixed with the top layer of soil.

Place the plant in the hole and adjust its height so the crown of the plant, the line between the stem and the root, is 1 to 3 inches higher than the original soil level. Shovel the mixed soil from the pile into the hole and use the shovel blade to cut the soil into the hole. This breaks up the big chunks and works the soil down so there are no large air pockets. If there is burlap or twine around the trunk, loosen it so there is at least an inch of room to grow while the twine and burlap rot away. Completely remove any plastic twine around the trunk. Otherwise the twine will cut into the trunk as it grows.

Level the soil off so the soil is at the crown of the plant. Avoid burying the stem or low branches. You can build up a ridge around the plant to hold water while it soaks in. During wet weather, level the ridge so water does not stand around the plant. Watch out that the plant does not settle and create a puddle at the base of the plant. That would encourage crown rot disease.

When you water, give it enough water each time so it wets the entire ball of soil in the hole. Then let the surface dry out between waterings. During hot weather planting, you may need to water every day, but it is important to let the soil surface dry out between waterings so soil diseases do not become a problem. After a couple of weeks, the plant should need watering less often as the root system grows wider.

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