From a Correspondent:
What needs to be done to amend very dense clay-type soil? I have a small backyard with no plant life and would like to add suitable landscape plants to this full-sun environment.
A., in Mesa, Ariz.
Good morning A.,
A very dense clay soil can seem a bit daunting in planning your landscape, but the good news is that our desert plant varieties will thrive in it once they get in it and get growing. Consider yourself lucky: clay soils have excellent water and nutrient holding capacities and are much easier in the long run than are sand or sand/rock soils.
First, let me encourage you to not try replacing the soil with "custom-made" stuff. There are a number of reasons, including expense and impracticability, but mostly because it won't work. You'll never be able to"replace" it deeper than a foot and keep in mind that even the humblest wildflower has roots a foot deep, while shrubs and trees have roots that go, respectively, 2 and 3 feet deep, or more.
When we replace soils, lay a layer of soil on top of existing soil, or very heavily amend our soil, we create what's called a "soil interface," a zone that roots and water have difficulty crossing. If you replace your soil, one of two things will happen: either the roots and water will not cross the interface and you end up with shallow-rooted and drought-vulnerable plants, or the roots and water do cross the interface and happily grow in the native soil — which means it was waste of time to put the new soil there in the first place.
The biggest challenge with dense clay soils is opening up the surface to initially improve the water infiltration. Once you do that and then get your plants growing, the plants will take care of the rest. I would recommend you try this: water your soil slowly to a depth of about 1 foot — slowly so the water has time to penetrate without running off. Then wait a couple of days until the soil has dried enough to be diggable but not wet (never dig wet clay). Look for a crumbly texture when you put your spading fork in it.
Next, spread a 3-inch layer of organic mulch on the surface, preferably quarter-inch bark chips or wood chips 1/2 inch or smaller. You can get the bark chips bagged at home stores, or wood chips free from some power companies or arborists. You want something small, but with some substance to it.
Either with a spading fork if you're feeling muscular, or a tiller if you're not, work those chips well into the first few inches of the soil. You can add more and work them in deeper if you want. What those chips are going to do is open the top layer of your soil for water infiltration.
Next, start planting your desert-adapted plants, and be sure to plant them without adding organic material to the back fill. Dig your holes 2-3 times as wide as the plant's root-ball but no deeper. And, again, back-fill the holes with only the soil you took out of the hole.
After you've planted, top-mulch your soil 3 inches deep with an organic mulch: more of the bark or wood, or something else organic. Don't use gravel on this soil. Over time, your top organic mulch will slowly break down and help to keep your soil open. Add a little more each year as needed.
Do be careful to not walk on your newly fluffy clay soil when it's wet, as that will re-compress it. And, lastly, water your new plants deeply, slowly, and infrequently: 1 foot deep for ground covers and wildflowers, 2 feet down for shrubs, and 3 feet down for trees.
I think you'll find this will help you and your plants get the most out of your clay soil, and with minimal effort and expense.
I hope this helps,