Caliche, sandy loam, and what is under a Laredo yard

Ask anybody who has dug a post hole in Laredo what the ground is like and you will get a one-word answer. Caliche is the layer of calcium carbonate that accumulates and cements within soils in dry climates, where rainfall is not sufficient to leach carbonates down out of the root zone. Across the Rio Grande Plain it sits close enough to the surface to be the first thing a spade finds, and it is the single most important physical fact about landscaping here.

What the survey records

Two USDA soil series are worth knowing by name because they are described directly from this ground.

The Brennan series was established in Webb County in 1906 in the Laredo area. It is a very deep, well drained fine sandy loam with an argillic horizon of sandy clay loam beneath, and its official description records secondary calcium carbonate through the profile, with carbonate content in the Bk horizon between 9 and 40 percent. That last part is the one that matters: secondary carbonate is carbonate that has moved down through the soil and been redeposited, and it is the material that cements into the layer people call caliche.

The Webb series, named for this county, is classified as a fine, smectitic, hyperthermic Aridic Paleustalf. Unpacked, that means a very fine sandy loam surface over a sandy clay subsoil, a clay fraction with shrink and swell behavior, and a soil temperature regime formally described as hot. The survey lists trichloris grasses, tanglehead and buffalograss as its climax vegetation, which is a direct statement about what this ground grows without any help at all.

Why how far down it sits matters more than what it is called

Soil names are useful shorthand. What changes a landscape design is where the cemented layer falls, because that is as far as roots go without help, and how far roots go decides how big a plant the ground will carry.

A low shrub is asking for very little. A live oak intended to shade a house is asking for a great deal. Where the layer sits high, a large tree either gets an excavation that breaks past it, which is a real line item on a quote, or it gets swapped for something smaller. Guessing about it is what produces the ten-year-old tree that leans over in the first serious windstorm, because its roots never left the hole it was planted in.

1. 1 Topsoil and subsoil. Sandy loam over a heavier sandy clay loam. This is the part a spade moves.
2. 2 The cemented carbonate layer, the caliche. Calcium carbonate that has moved down through the soil and hardened into a crust. It is where a shovel stops and where a trencher starts costing money.
3. 3 Hole stopped on the layer. Roots reach the wall of the pit, find nothing they can enter, and turn back on themselves. The tree stays alive and stops getting bigger.
4. 4 Water in a closed hole. Rain and irrigation run through the loose backfill, meet the crust, and stand there. The pit behaves as a container with no drain, and the roots sit in it.
5. 5 Hole taken out past the layer. Roots leave the backfill and enter undisturbed soil, which is the only way a tree ever anchors itself.
6. 6 Water with somewhere to go. Once the crust is broken the pit drains, and the difference between a tree that establishes and one that sulks for a decade is usually this and nothing else.

The second effect is drainage. A cemented layer under a planting hole perches water. Rain and irrigation collect in the pit, the root ball sits wet, and a plant that would happily survive a drought drowns instead. This is a genuinely counterintuitive failure in a place averaging 19.68 inches of rain a year, and it is common enough to be worth checking for directly.

Testing your own lot

You need a spade or a hand auger and a bucket of water, and an hour of a Saturday at the outside.

1. Pick three or four spots. One where a tree might go, one in the middle of a proposed bed, one near the house, one at the far corner. Variation between them is the point of the exercise.
2. Dig until the resistance changes. Mark where the tool stops moving easily, not where you gave up. Those are different points and only the first one is information.
3. Test what stopped you. Scrape it. Soft nodules that crumble under a spade are one thing. A continuous slab that rings when you hit it with a bar is another. Both are carbonate; only one is a barrier.
4. Fill a hole with water. If it is still holding water several hours later, the layer is perching it, and that changes both the planting method and the drainage plan.
5. Write down what each hole did. Hand it to whoever prices the work. It is the difference between a quote with a realistic excavation line and one with a revision in its future.

What to do when the shovel stops

There are three reasonable responses and one bad one.

• Break through it. An excavation that carries past the layer, or spreads out beyond it, gives roots somewhere to go that is not back into the hole. This is the expensive option, the ground rather than the crew decides which machine it needs, and on some lots it is the largest single line in a planting quote.
• Build up instead of down. Raised beds, berms and seating walls that double as planters give root depth without excavation. On a lot with a shallow continuous layer this is frequently cheaper and more reliable than fighting the ground.
• Choose plants that do not care. The native brush flora grows on this ground without help. Mesquite, huisache, guajillo, cenizo, prickly pear and the native grasses are all adapted to shallow, calcareous soil. This is the cheapest option and it is the one the xeriscape approach is built on.

The bad response is putting a plant that wants deep ground into shallow ground and then irrigating harder to make up the difference. That produces a plant permanently on life support, and life support in this climate is expensive to run and easy to interrupt.

Amending soil, and how much is worth doing

Working compost into a planting bed is worth doing, particularly on a sandy loam where water holding capacity is the limiting factor. Amending an entire yard is not, for two reasons. It is expensive, and a plant set in a pocket of rich soil surrounded by native ground tends to keep its roots in the pocket, which produces exactly the shallow root system you were trying to avoid.

The more durable approach is to prepare the beds properly, choose plants adapted to the surrounding soil, and put the money saved into mulch. Mulch cuts evaporation, moderates soil temperature and suppresses weeds, and on a hyperthermic soil under a July sun it does more for plant survival per dollar than almost anything else.

Builder lots are a special case

A large share of Laredo housing sits on ground that was scraped flat and then finished with a thin layer of imported topsoil. What that produces is a shallow working layer over undisturbed and often compacted subgrade, with a sharp interface between the two that behaves much like a cemented layer: roots reach it, stop, and turn sideways.

On those lots the dig test is even more useful, because what you find has nothing to do with the mapped soil series. What is under the grass is whatever the builder left. The drainage guide covers the other consequence, which is that a scraped lot rarely has the fall it needs to shed a heavy storm.