How to Plant in Clay Based Soil

Welcome to southeastern Virginia, specifically the Tidewater region! We love our sandy soil near the coast, but a little further inland, the easy digging can come to an abrupt end when you hit clay or a clay blend. Planting in the clay soil that Hampton Roads has to offer is difficult, and establishing plants is even harder. I hear many friends say they have a black thumb and then give up. In reality, they are just dealt a poor hand.

What is clay?

All soil is comprised of different combinations of minerals, organic material, moisture, living organisms, and chemical nutrients. Clay is mostly made up of fine particles, creating a sticky, dense, soil. Clay also happens to be extremely absorbent. You may think quite the opposite, because you’ve probably frequently seen water collecting in a clay hole. When you see this, it’s mostly due to the clay reaching its saturation point. 

Try this little experiment to see how much moisture the clay in your yard is retaining: 
Take a shovel full of clay. Place it in an area where it can dry out. You may struggle to get it off the shovel at first. While you do, it’ll stick to your hands and make a mess. As it sits in this dry area, you’ll notice that it becomes stiff, less pliable, and eventually cracks. After a day or so, you’ll actually be able to crumble the clay into a fine powder. After it’s completely dry, add a little water back into the soil and see how much it takes to rehydrate it to its original condition.

Because clay is so absorbent, you’ll need to have a plan for dealing with the additional moisture retention. Here are 7 ways you can improve your clay soil and get those green thumbs you’ve always wanted.

1. Gravel Under the Plant

We will often check the density of the soil when we plant. This doesn’t always mean that there’s clay, but it can be a helpful indicator of how our plants will do. We will dig a hole 12-18″ deep, dump a gallon of water into this hole, and then set a timer to see long it takes for the water to dissipate. Ideally, this would be under 10 minutes. If the water doesn’t seep into the soil within this rough timeframe, we know we have an issue. We will often handle this by digging an additional 12-18″ past the bottom of the planting hole with a pair of posthole diggers. We will then fill the hole with clean gravel, allowing the water to wick away from the plant roots, and buying more time for the moisture to dissipate.

2. Use Compost

We will often amend and entire garden bed with compost, especially if we are planting smaller material like perennials or annuals. However, if the area is large and the plants/trees are spread out, we will stick to just amending the soil in the direct vicinity of each plant. Using a rich compost like McGill’s SoilBuilder will introduce rich organic material to help break up the clay. This has the additional benefit of ensuring the establishment of necessary nutrients and healthy fungi, which are beneficial to future growth.

3. Blend Clay into Plant Hole

It’s tempting to dig a hole, remove all the clay, refill the area with back rich topsoil around the new plants. However, this in effect creates a smooth sided bowl, causing the roots of our plant to avoid spreading into the edges of the “bowl”. We have removed many a sick shrub that was planted 2 or 3 years earlier in clay. The plant may have done well for a while, but the roots eventually grew in circles around the smooth-sided hole, depleting all the available nutrients and choking the plant. At this point, we usually have to replace the plant because it is not salvageable. Best practice is to take some good topsoil or compost and blend it with the clay (keeping it clay-heavy) then install the plant. Now, the roots will be familiar with the resistance of the heavy clay, and will break out of the original hole in the end.

4. Add Gypsum, Lime, and/or Fertilizer

Gypsum can be extremely useful in the right application – breaking up heavy clay and loosening compacted soil by changing the particle size. Gypsum will improve heavy clay while also removing sodium from saline rich soil, exchanging sodium for calcium. Gypsum is not ideal for every situation though, such as clay layered with coarse soil types. Make sure to take note of the clay type before adding gypsum. Gypsum must be tilled in or mixed with soil when added to the planting hole; a topical application will have little effect. Gypsum can also be harmful to mycorrhizae fungi. It is highly recommended to add a fertilizer or amendment with beneficial mycorrhizal 3-6 months after using gypsum to repair any damage. 

Lime can be used in a similar fashion as gypsum. Unlike gypsum however, lime will adjust the pH, lowering the acidity of the soil. Similar to gypsum, it will encourage the clay to form clumps. Because clay is largely made up of very small particles, the slightly larger clay clumps help organic material to filter further down into the soil structure. Lime can be applied over the top, but is best tilled into the soil. While it is hard to go wrong with lime, over-application is still possible. Simple test tools are available at most garden centers to rapid test the pH of your soil.

I will always try to use a plant fertilizer tablet over a granular fertilizer. Either way though, most plantings in clay soil require the use a slow-release fertilizer instead of a liquid or foliar application. If you do use a granular fertilizer, it is typically more effective when tilled into the whole planting bed instead of just the plant hole. Many fertilizers focus on nitrogen which will encourage leaf growth. However, too much nitrogen can be harmful in clay soil, especially heading into the winter months. Make sure your fertilizer has a healthy dose of phosphorus to encourage root and stem growth. 

5. Plant High

Keep the surface roots slightly above the base grade of the soil. Often plantings will settle slightly, so we will leave our plants about an 1″ higher than the base grade to ensure they aren’t drowned in the planting hole. This will also help any surface moisture to drain away.

6. Do Not Add Sand

It may be tempting to add sand into the planting hole, or even till it into the soil. The intent is to increase the overall particle size of the soil, but sand does not help with drainage in clay and can actually makes the issue far worse. The sand becomes packed and filled with sediment, creating an even tighter soil structure in the end. ​

7. Lots and Lots of Mulch

Mulch is the easiest way to ensure success. Mulch takes years to break down, but over time the organic material filters through the soil, breaking up clay particles and creating a blended soil with lots of organic material. Make sure not to choke the crown of the plants- a mulch application of 3″-4″ over the whole planting bed is ideal.

If you live in Hampton Roads, it’s likely you’re dealing with clay soil – but that doesn’t have to end your gardening dreams! Follow these seven tips to condition the ground, plant correctly, and grow the lawn you’ve always wanted, no matter what dirt you lies underneath your plants!