Working with clay soil

We have very high clay content in our soil, especially in developed neighborhoods where the ground was excavated at some point in the past. You'll notice clay because it sticks to your shoes. You can clump it in your hand and it will mold to your fingers and stay that way. We have so much clay in our area, there are several local businesses that even make pottery out of the clay from our local creek beds.

Unfortunately, clay can challenging to work with when it comes to new plants. Clay is dense and hard, and plants struggle to create roots through it. Clay prevents water from draining, and will drown the roots of new plants, depriving them of oxygen and even causing root rot. This is especially important for trees that develop deeper root systems (as opposed to smaller plants with shallower roots). Tree roots are often completely contained in an improperly prepared wet clay hole, stunting their growth and ultimately leading to their decline.

There are 2 types of clay - reddish/brown and grey. Reddish/brown clay is most common closer to the surface, drains slightly better, and can be amended more easily. Grey clay is grey because of poor aeration leading to an anaerobic environment that nothing will survive in. If you find grey clay, consider removing it completely from the planting area.

If you find clay when digging a hole for your next plant, think of the clay hole like a clay pot. Try filling the hole with water and observing the rate at which the water gets absorbed into the ground. If the water stagnates for hours, the hole must be improved if you want your plant to thrive.

For plants: Consider raising the planting area above the clay soil with organic matter. Use a shovel to break up the first few inches of clay, and mix in compost and topsoil at about 50% ratio until the planting area is a few inches over the original clay line. Even better if the soil can be prepared a year in advance so that the organic matter has a chance to break down before planting.

For trees: Consider making the hole twice as wide and deep (4 times as wide and deep as the root ball). Increasing the size of the hole will increase the rate of drainage proportionally, and the larger hole will allow the plant or tree more space to expand its root system before hitting a clay wall. Mix in topsoil and organic matter, 50% ratio with the existing clay should be sufficient. You can also raise the bed slightly for trees, but to less of an effect than with smaller plants because eventually the larger root system will have to go somewhere.

For all things: Composting the area with a couple inches of leaf litter or mulch is a wise annual investment. As organic matter breaks down, it deprives the topmost layer of soil of nitrogen which will deter weeds from growing (but not affect the slightly shallower roots of medium size plants and trees). Over time the organic matter will increase the carbon content of the soil and be a rich source of nutrients for our plants. Clay is not carbon rich, so the addition of carbon (both mixed in, and at the surface) is important if you want your plants to grow big and strong.