I didn’t think this would be an exciting course when I enrolled in “Soil Science” my second year at A&M.
This name doesn’t ooze drama and intrigue. But it ended up coming in handy for a guy about to start a career in horticulture, and I’ll share some highlights. Hope you enjoy them.
Soils are the basis of everything we grow, whether for food, fiber or flowers. Your gardening efforts will not be more fruitful than the soil you prepare. So don’t take them for granted.
Color. You can learn a lot just by looking at the color of the soil. Our Blackland Prairie soils here in Fort Worth / Dallas and south along I-35 are rich in organic matter. This is typical of dark colored soils.
Red and orange soils like those found in east Texas and along the Red River are generally high in iron. This is important to remember when trying to grow plants that require more iron than average: azaleas, gardenias, loropetalums, dogwoods, and most types of pines.
Yellow soils are generally poorly drained and even foul-smelling. Few plants will succeed in these kinds of conditions.
White and light gray soils are almost always poor in organic matter and generally very poor in nutrients. They usually need a lot of help before planting, even to the point of bringing in new soil to add to what is already there.
Depth. There are places in our region where the white caliche bedrock extends to the surface of the landscape. It’s really hard to grow plants in this kind of setting. Compare that to places at the bottom of rivers where flooding may have deposited several feet of rich soil over hundreds of years.
At a minimum, you will need 12-14 inches of good soil for the turf to thrive. If you have less, you’ll need to water and fertilize more often, but you’ll want to use less with each application.
Ground covers and small shrubs will need that same 12 to 14 inches of soil (or more!). Large shrubs and small trees will survive with 2 feet of good soil. Large shade trees will need 4 to 6 feet of soil to function 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 in topsoil to enhance the quality of your gardens, buy it from a reputable source. Your favorite local independent garden center operator will know the best suppliers. Specify that you want “topsoil of sandy loam and that it contains absolutely no nutgrass”. Inspect the soil closely before it is spilled to make sure it is free of this harmful weed.
Texture. This is where things get a little technical. This is the size of the soil particles.
Think of a flowing river. The coarsest soils, the sand particles, wash up on the banks first – they are too heavy to be transported very far downstream.
The silt particles are carried until the river stops flowing – until it reaches a lake or reservoir. Then they are deposited at the bottom of still water. Eventually, they begin to fill the body and need to be dredged up and removed.
The clay particles are microscopic in size. They stay hanging 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 it while shaking it vigorously until all the soil is suspended. Over the next 24 hours, you will be able to watch the different types of soil settle at their own pace.
Think of a triangle where each of the three points is represented by 100 percent sand, silt, or clay. This is called the “texture 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 should also learn to read your plants. Nitrogen produces deep green growth. Plants that need more nitrogen will be a lighter green all over – old leaves and new leaves, evenly across all leaves.
Phosphorus is hardly ever deficient in our Texas soils, but if it were, it would lead to purplish, stunted growth and poor flower and fruit production. But never add phosphorus without first performing a soil test. Most of our soils already contain excessive amounts of phosphorus.
Iron deficiency results in yellowed leaves with dark green veins, appearing first on the newer shoots (branch tips). This is a problem for acid-loving plants in very alkaline soils like the one we have in the Metroplex.
Acidity / Alkalinity. This is linked to iron deficiency. If you have soil with a pH above 7.0, the iron will be converted to an insoluble form and plants that need more iron will start to turn yellow as described.
Have your soil tested every 2-3 years and make sure a pH test is included. The Texas A&M soil testing laboratory is extremely reliable. Get sampling and shipping instructions online.
You can hear Neil Sperry on KLIF 570AM on Saturday afternoons from 1 p.m. to 3 p.m. and on WBAP 820 p.m. on Sunday mornings from 8 a.m. to 10 a.m. Join him on www.neilsperry.com and follow him on Facebook.