Important facts about North Texas soils and how to prepare them for your landscape
Stick with me, friend, and I’ll give you important facts you need to know about the soils in which you garden. I can do it in a 4-minute read, and it might change your level of success as a North Texas plant person.
For starters, take a closer look at the native soil in your neighborhood. It’s going to tell you a lot about itself just from a quick glance.
▪ Color is most obvious. Most of us in the Metroplex garden in Blackland Prairie soils. Dark soils like we have are usually high in organic matter. That stands to reason, because the Blackland Prairie was the grassland of the old cattle country of Texas, and it extended down Interstate 35 clear to San Antonio. Grass blades and tree leaves turn soils dark brown as they decay. Dark soils are usually good soils.
Parts of the Mid-Cities and most of East Texas have red and orange soils. That’s because they are high in content of iron, and it often suggests they are acidic as well.
Yellow soils are often poorly draining, sometimes even stagnant and foul smelling from lack of oxygen. The only way of gardening in them is to improve the drainage.
White and light gray soils are usually infertile, often sandy. They generally lack organic matter. Success in growing in them will require addition of organic matter and regular feedings with all nutrients including trace elements.
▪ Soil depth is critical. Even the best garden soil won’t ensure a great crop if it’s only an inch or two deep. If you’ve dug a hole to plant a tree in your landscape, think back to how far down you went before you hit bedrock. Or use a sharpshooter spade to dig a test post hole. That will tell you a lot about depth. And look at creekbeds and washes around you. See how much topsoil is in place before the bedrock is exposed as soil erodes.
Most shade trees, particularly large species, need 2 or 3 feet of topsoil to succeed. Shrubs can do well with 1 to 2 feet of good soil. Groundcovers, low shrubs, and annual and perennial flowers and vegetable gardens will need 12 to 18 inches for best growth.
If you have less soil than that in your part of town you can still garden successfully. You’ll just need to choose your plants more carefully and care for them more regularly. Raised beds are an option for smaller types of plants. You can either mound soil up in berms, or you can construct actual enclosures with stones or boards to hold the soil in place.
▪ Texture matters. This aspect of your soil probably isn’t something you think about often, but it’s a critical feature. It refers to the size of the individual particles of soil, and for one small moment I’ll take you back to my old soil sciences class at the ag school (Texas A&M). Consider a triangle. At one point you have sand and very fine gravel. Those are the coarsest of the soil particles. At another of the points you have clay. Those are the finest of the particles — in fact, they are so small they’re visible only with an electron microscope. And the third point is occupied by silt – the soil particles that fall in between.
Now we’re going to simulate erosion. To do so we’ll take a large graduated cylinder out of the chemistry lab. We’ll put 1 cup of topsoil into the cylinder, then we’ll fill it with water. We’ll shake it vigorously for a couple of minutes (put your hand over the open end so you don’t make a mess). Put it on a stable countertop with a bright light behind it and leave it in place for 24 hours.
After one hour you’ll see that the big particles have settled to the bottom of the cylinder already. That’s the sand and fine gravel component. After a few more hours you’ll begin to see that the silt is settling to the bottom. At the end of the 24 hours, most of the silt will have settled, but the tiny clay particles will still be suspended — they’ll be “staining” the water, and they’ll continue to do so almost forever.
Translate that back into the real world, that’s why streams are lined with rocks and sand, as silt fills up the lake beds, while the lake water retains the color of the soil that’s been carried by the stream.
The ideal garden soil will have some of each of these particles. Sand for its porosity and aeration, clay for its stability and water and nutrient-holding capacity, and silt just because it’s there. Such a soil — from the middle of that triangle — is known as a “loam.”
▪ Of pores, pH and fertility. Now for some less tangible aspects. The perfect soil should be 50% solid matter, 25% water, and 25% pore space (air/oxygen). After a heavy rain or irrigation, the pore space will fill up with water and the soil would become 50/50. But because it drains properly, it will quickly return to the desired 50/25/25 combination.
The soil’s pH refers to its acidity and alkalinity. Almost all of our cultivated plants prefer soils that are slightly acidic (pH 6 to 7 on a 0 to 14 scale). Our native soils locally are mostly pH 7.5 and higher, which means we should avoid large trees (East Texas pines, pin oaks, sweet gums, etc.) that require copious amounts of iron. Smaller plants with high iron needs (azaleas, loropetalums, wisterias, even dogwoods) can be maintained in less soil, and so iron can be added successfully.
Your soil’s fertility will be critical. In nutrient-holding clay soils it’s very likely that you’ll get an accumulation of phosphorus (middle number of the fertilizer analysis) to a harmful level. Let the Texas A&M Soil Testing Laboratory run tests every three to four years and make recommendations. Follow their guidelines.
