I didn’t think it was going to be an exciting class when I signed up for “Soil Science” my sophomore year back at A&M.
That name just doesn’t ooze of high drama and intrigue. But it ended up being useful for a guy about to enter a lifelong career in horticulture, and I’m going to share a few highlights. I hope you enjoy them.
Soils are the foundation of everything that we grow, whether it’s for food, fiber or flowers. Your gardening endeavors will be no more successful than the soils you prepare. So don’t just take them for granted.
Color. You can learn a great deal just by looking at the color of the soil. Our Blackland Prairie soils here in Fort Worth/Dallas and southward along I-35 are rich with organic matter. That’s typical of dark-colored soils.
Red and orange soils such as you find in East Texas and along the Red River are usually high in iron content. That’s important to remember when you’re trying to grow plants that require more iron than average: azaleas, gardenias, loropetalums, dogwoods and most types of pines.
Yellow soils are usually poorly drained and even foul-smelling. Few plants will succeed in those kinds of conditions.
White and light gray soils are almost always low in organic matter and typically very low in nutrients, too. They generally require a great deal of help prior to planting, even to the point of bringing in new soil to add to what’s already there.
Depth. There are places in our area where the white caliche bedrock extends to the surface of the landscape. It’s really tough to grow plants in that kind of setting. Compare that to river bottom locations where floods may have deposited many feet of rich soil over hundreds of years.
At a minimum, you’ll need 12 to 14 inches of good soil for turfgrass to thrive. If you have less than that, you’ll have to water and fertilize more often, but you’ll want to use less with each application.
Groundcovers and small shrubs will need those same 12 to 14 inches of soil (or more!). Larger shrubs and small trees will survive with 2 feet of good soil. Large shade trees will need 4 to 6 feet of soil to perform to their full potential. If you have less, they can still survive, but expect slower growth and smaller final sizes.
If you decide to bring topsoil in to raise the grade of your gardens, buy from a reputable source. Your favorite local independent retail garden center operator will know the best suppliers. Specify that you want “sandy loam topsoil and that it have absolutely no nutsedge (nutgrass).” Inspect the soil closely before it gets dumped to be sure it’s free of that noxious weed.
Texture. This is where things get a little bit technical. This refers to the size of the soil particles.
Think about a flowing river. The coarsest soil, the sand particles, wash up on the riverbanks first – they’re too heavy to be carried very far downstream.
Silt particles are carried until the river stops flowing – until it reaches a lake or a reservoir. Then they are deposited on the bottom of the still water. Eventually they begin to fill the body and must be dredged and removed.
Clay particles are microscopic in size. They stay suspended almost forever, “staining” the water in the process.
If you ever want to see this happen in real life, put 1 cup of topsoil in a graduated cylinder. Fill the cylinder with water and hold your hand over the top as you shake it vigorously until all of the soil goes into suspension. Over the next 24 hours you will be able to watch the different soil types settle at their own rates.
Think of a triangle where each of the three points is represented by 100 percent sand, silt or clay. That’s called the “textural triangle.” The ideal soil would fall exactly in the middle of such a triangle, and it would be called a “sandy clay loam.”
Fertility. As you learn to “read” your soils, you also need to find out how to read your plants. Nitrogen produces deep green growth. Plants that need more nitrogen will be lighter green all over – old leaves and new leaves, uniformly across all leaves.
Phosphorus is almost never deficient in our Texas soils, but if it were it would result in purplish, stunted growth and poor flower and fruit production. But never add phosphorus without having a soil test run first. Most of our soils have excessive amounts of phosphorus already.
Iron deficiency results in yellowed leaves with dark green veins, most prominently displayed on the newest growth (ends of the branches) first. It is a problem for acid-loving plants in highly alkaline soils such as we have in the Metroplex.
Acidity/Alkalinity. This ties in with iron deficiency. If you have a soil with a pH greater than 7.0, iron will be converted to an insoluble form and plants that need more iron will start turning yellow as described.
Have your soil tested every 2-3 years, and be sure a pH test is included. The Texas A&M Soil Testing Laboratory is extremely dependable. Get instructions on sampling and mailing online.