Bill Swantner, Bexar County Master Gardener
(All photos were taken by the author, Bill Swantner)
Soil consists of four elements: air, water, “weathered” rock (i.e. minerals), and organic material. Weathered rock means that the weather (sun, cold, rain, wind) has converted the rock into smaller particles; sand, silt, or clay. Clay is a microscopic particle that, when mixed with sand, silt and organic material, holds various minerals in the soil. Silt is another by-product of weathered rock that is slightly larger than clay and it functions to hold water in the soil. Sand is the largest of a weathered rock’s by-products. In soil, sand allows for moisture and air movement. The material composition of the weathered rock determines if the soil will be sandy, silty, or heavy in clay. Organic material is decomposed plant and/or animal life.
The geologies of Bexar County can be grouped into three main types which also means the soils that developed from them can also be divided into three distinct soil types. These soils were formed over millions of years as the earth took shape. On the far northside of Bexar County, the native soils primarily developed from limestone & dolomite—they are the Mollisol (soft) soils of the Edwards Plateau. On the far southside of Bexar County, the soil is a sandy soil on top of clay soil. The great swath of land in between the far northside and far south side of Bexar County is a group of soils called Vertisols and the most dominant is the Houston Black clay soil.
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Areas of Bexar County and Their Associated Soil Types | |
| Far Northside (Edwards Plateau) | Mollisols (soft soils) from limestone and dolomite |
| Mid-County (Blacklands Prairie) | Vertisols (Latin for a “turning” soil that expands and shrinks because of moisture) mainly the Houston Black Clay |
| Far Southside (South Texas Plains) | Sandy soils on top of clay |
EDWARDS PLATEAU
The limestone soil of the Edwards Plateau that formed during the Mesozoic Era is a mollisol, Latin for, soft soil. During the Cretaceous period of the Mesozoic Era, the Edwards Plateau was a large, shallow body of water. The limestone bedrock of the Edwards Plateau was formed by millions of skeletons of sea life being deposited on the bottom of the sea. The soil is not very deep for two primary reasons. One reason is the millions of years of erosion as water flowed from the rising Rocky Mountains to the Gulf of Mexico. Another reason is that the dolomitic limestone resists physical and chemical weathering because of its density and composition. (Thin soil horizon of Edwards plateau).
MID-COUNTY
The great majority of the soil in Bexar County is a thick, clay soil that is part of the Blackland Prairie region of Texas. The Blackland Prairie extends from south of Bexar County to Dallas, following I-35, along the line of the Balcones Fault. Millions of years of weathering caused the shales, marls, and chalks along and to the east of the Balcones fault line to form a think clay soil mixture. The clay soil in the Blackland Prairie expands greatly when exposed to moisture and shrinks greatly when dried out. This expanding-shrinking characteristic of Houston Black Clay is the primary reason for foundation issues in Bexar County. The native Blackland Prairie soil was rich in organic material, and while European farming methods depleted some of the organic material, the Europeans settlers learned how to farm the soil of the Blackland Prairie.
SOUTH BEXAR COUNTY
The formation of the sandy-clay soil of far south Bexar County was influenced by a few factors. One is that the Rocky Mountains began to form and started rising which forced water southeast, through the Edwards Plateau and over the Balcones Escarpment; washing away gravel and sand. Another factor was the South American continent pulling away from the North American continent causing a basin to catch the gravel and sand coming in from the Rockies. As the water receded it created marshy areas and beaches during times of stability creating these geologic formations. And lastly, there were ancient volcanos in the Trans-Pecos regions whose ash blew east and settled in the Gulf Coast Basin.
The area in far south Bexar County is referred to as the Gulf Coast Plain or the South Texas Plains.
For more information for children see, Rockin’ and Rollin’.