When you think of all the fun you’re going to have gardening, getting the ground ready is at the top of your list. Right? Well, perhaps it’s not, but that doesn’t diminish its importance.
That soil you prepare is as important to your landscape and garden as the foundation is to your house, so you need to pay special attention to the details and get the job done right the first time.
Almost 95 percent of the soils in North Central Texas are what is loosely called the Blackland Prairie. It’s a heavy clay type of soil that stretches up and down Interstate 35 from Sherman to San Antonio and beyond.
The curious thing about the sticky “gumbo” soil that most of us use as our gardening bases is that it sits atop some of the whitest rock in the world, most commonly called caliche shale.
Because the caliche is a limestone material, it’s highly alkaline. Our soils typically have a pH of 7.5 to 7.8. Most of our cultivated plants perform far better in neutral or slightly acidic soils, so that forces us to make serious decisions before we start choosing our plants.
We can improve soils for small plants (flowers, vegetables, small shrubs and groundcovers), but it’s cost-prohibitive to attempt changes for very large plants that will have expansive root systems as they mature.
Your first decisions in plant selections, then, should be directed at the best large shrubs and shade trees — the types that will do well in alkaline soils.
I’m not insistent that only native plants be used in the landscapes that I’m designing, my own included. I’d always prefer to have a plant species that adapts to what I have to offer. However, in the case of shade trees, our best choices are almost all long-proven North Texas natives.
My list includes live oak, Shumard red oak, chinquapin oak, bur oak, cedar elm and pecan (all native in and around the Metroplex). The one non-native species that performs at that same high level would be Chinese pistachio.
If you want a large evergreen, our native eastern redcedar is excellent. For large shrubs or small accent trees, choose from crape myrtles, yaupon, Warren’s Red possumhaw and Nellie R. Stevens hollies, Mexican plum, redbud, and dwarf Southern magnolias such as Little Gem and Teddy Bear. Trees that are extremely difficult (if not impossible) to sustain in our alkaline soils include water oak, willow oak, pin oak, loblolly and slash pines, and even bald cypress.
When it comes time to develop shrub beds, you have more leeway. For small to midsize types, you may be able to amend or even replace the native soil enough to satisfy their needs. For most types, you’ll want to incorporate 4 to 6 inches of a combination of types of organic matter (sphagnum peat moss, rotted manure, finely ground pine bark mulch, compost and others) along with 1 inch of expanded shale (a porous and relatively permanent soil amendment available from nurseries) into the top 12 to 15 inches of topsoil.
That kind of bed prep will get your plants off to a great start. It also will provide a raised planting bed, and that will ensure good drainage in times of prolonged rain.
And then there are azaleas. They’re in a class of shrubs that must have highly acidic soils to avoid iron-deficiency issues. Their needs are so extreme (pH of 4 to 4.5) that you’ll have to replace the black clay topsoil entirely. Change it out with a 20-inch-deep bed of half sphagnum peat moss and half finely ground pine bark mulch. Half of the 20 inches should be below grade, the other half above.
That mixture will have the proper pH for azaleas (also camellias, gardenias and fringe flower). However, over the years, the organic matter will decay and the pH of the bed will rise. After eight or 10 years, you’ll want to remove the old plants, rework the bed and start over with fresh transplants.
About that dirt
Since soils are the source of most of the nutrients your plants will need, it’s appropriate to include that in our discussions of bed preparation. Soil tests show that most North Texas garden soils are “low” or “very low” in nitrogen content.
In almost all cases, those tests also show that the soils are “very high” or “excessively high” in the amount of phosphorus they contain. Potassium, the third number of the fertilizer analysis, is a component of the black clay gumbo soils, so it’s already there.
Adding all of that data together, don’t be surprised if your soil test results come back with instructions to apply only an all-nitrogen fertilizer, one where half or more of the nitrogen is in high-quality, slow-release form — and to use it on all of your plants.
So it really wasn’t a boring topic after all. Right? Hello? Are you there? Hmm. I guess they’ve stepped out to start up the tiller.