Soils and the Edwards Plateau

This talk focuses on the soils of the Edwards Plateau—how these soils developed and how they control the native vegetation and animals. I am speaking from the perspective of a geologist, so I have the peculiar view that it is the underlying bedrock that controls much of the landscape, from the soils to the plants that grow in these soils.  First I would like to discuss what the Edwards Plateau is, both with regard to its geography and geology. Then I will review the different factors that influence the character of various soils and apply these concepts to the soils of central Texas. I will conclude with a discussion of the endangerment of soils and their intrinsic value.

The Edwards Plateau, located in central Texas, is a “physiographic region,” meaning that it has distinct characteristics in terms of landscape, flora, and fauna. The eastern margin of the Edwards Plateau is very abrupt and is manifested as a steep escarpment, a feature visible from space. The Edwards Plateau is indeed a plateau with a relatively flat topography, situated at a higher elevation than regions to the southeast. However, on the southern and eastern margins of the Edwards Plateau, many rivers have eroded downward and cut steep valleys into the plateau, creating valleys and adjacent hills. The erosion by these rivers creates the landscape that we call the Texas Hill Country.

The rocks underlying the Edwards Plateau are limestones that were deposited in a warm, shallow sea during the Cretaceous Period in earth history, or about 100 million years ago. At that time, the land of the Edwards Plateau was located closer to the equator, and the climate of the earth was generally warmer than today’s climate. In this warm, shallow sea limestone formed as a chemical precipitate from the seawater by the reaction (Ca2+ + 2HCO3- à CaCO3 (limestone) + H2O + CO2). On a geologic map of Texas that shows the different rocks exposed at the earth’s surface, the Cretaceous limestones are usually portrayed in green, and they have a fairly direct correspondence to the region of the Edwards Plateau. The abrupt eastern margin of the Edwards Plateau goes through Austin and is controlled by a series of northeast-southwest trending faults, along which the southeast blocks are dropped down relative the northeast block. The soils of the Edwards Plateau are thin and rocky, where there is any soil present at all, because many areas on the plateau have exposed bedrock rather than soil. The nature of the bedrock of the Edwards Plateau controls the soils and thus the native life and the utility of this land for agriculture.

Hans Jenny, the preeminent soil scientist of the twentieth century, provided a comprehensive framework to describe those characteristics of a place that control the nature of its soils. He defined these factors of soil formation as bedrock, climate, time, biota, slope and aspect, and human perturbation. These six factors combine at a particular site to influence the characteristics of its soils. Let’s compare the state soils of Texas—the Houston Black soil series—with the soils of the Edwards Plateau. The Houston Black, found to the east of the Edwards Plateau, is thick, black, and amenable to growing grains and cotton. The distinct character of the Houston Black soil series reflects the formation factor of bedrock; the bedrock beneath the Houston Black consists primarily of shales and sandstones. When shales and sandstones undergo chemical weathering, they produce insoluble minerals like clays; in comparison, when limestones like those on the Edwards Plateau undergo chemical weathering, the rock nearly completely dissolves, leaving behind almost no insoluble residue in the soil. Thus the nature of the bedrock beneath the Houston Black is what makes these soils thick and fertile.

Two other important factors of soil formation are climate and time. The cold, moist climates of northern forests produces spodosols, or soils with rich topsoil overlying a white, leached zone, overlying colorful black, tan, and orange soil beneath. In contrast the hot, moist climates of the tropics produces vibrant red, clay-rich soils. A landscape must be stable for a long period of time to develop these colorful patterns (1,000 to 1,000,000 years), and the length of time over which a soil develops can have a profound influence on its character and thus the life found on the landscape. An interesting example of the importance of time on soil development is the state soil of California, the San Joaquin soil series, which is found on ancient surfaces and is the product of about 3 million years of rain and earthworms, plants, and burrowing animals. The landscape underlain by the San Joaquin soil series is commonly covered by a bumpy pattern called Mima mounds, which are several meters across and about a meter high. The soils have a thick horizon cemented by silica, developed over millions of years, that prevents rainwater from draining through the soil; instead the valleys between the Mima mound are internally drained and accumulate water during the rainy season. These tiny ponds are called vernal pools and support a wide variety of endemic species. These soils create a unique habitat for many threatened species and have been spared much of the disturbance of much of the agriculture of the San Joaquin Valley because the hard silica horizon makes farming difficult. The unusual hydrology and biology is the direct result of the antiquity of the soils of the San Joaquin soil series, the product of 3 million years of weathering. Yet a large intact area of these unusual soils and vernal pools are directly under the proposed University of California campus at Merced.

The thin, rocky soils of the Edwards Plateau are produced by the weathering of limestone, as I mentioned earlier, but they are also the product of a more complicated history of erosion over the past 15,000 years. I am investigating the erosion of soils from the Edwards Plateau with my colleague Jay Banner and our student M. Jenny Cooke. We are using an extremely well-dated sequence of sediments that have accumulated in a shallow cave in Kerr County over the past 20,000 years as a record of the environment of the Edwards Plateau through time. Rickard Toomey found no prairie dogs in this sedimentary sequence after about 11,000 years ago, whereas they had been abundant prior to this time. Because prairie dogs require a least a meter of unconsolidated soil in which to burrow, their local disappearance is consistent with thinning soils.

To test this idea that the soils of the Edwards Plateau underwent substantial erosion, we are applying a novel technique to quantify the timing of soil erosion from the Edwards Plateau. Specifically we are using the natural abundance of strontium isotopes in fossils of this sedimentary sequence as a record of soil thickness. The limestone bedrock and the clays in the soils have very different strontium isotope compositions: as the soils thinned, more strontium, which is chemically similar to calcium, would be derived from the limestone bedrock rather than the clay minerals in the soils. We have measured the strontium isotope composition of the calcium carbonate in fossil hackberry seed coats as well as fossil mammal tooth enamel from cave deposits. The pattern in strontium isotope composition in these fossils is consistent with thinning soils on the Edwards Plateau from about 15,000 to 9,000 years ago, with little change after this time interval.

Further support for the theory that thick, clay-rich soils once covered the Edwards Plateau region in central Texas comes from small patches of thick, red soils found at the highest elevations on the Edwards Plateau, in contrast to the thin, rocky soils more commonly found in the region. How could thick sediment be deposited at the tops of hills, forming these soils?  These soils are not developed on a sedimentary deposit; rather they are relicts of the thick, ancient soils that blanketed the region about 10,000 years ago, and only on flat surfaces at the tops of hills have these soils escaped erosion. There are distinct plants that grow on these thick soils, including the combination of post oak and mesquite, and distinct animals that live in these soils, such as gophers. Thus the bedrock of the Edwards Plateau controls the soils, the geologic history of this region controls where there are thick and thin soils, and the soils control the animals and plants that live in particular habitats.

I was discussing a profound natural erosion event that occurred about 10,000 years ago, but humans are affecting soils in many ways—from promoting erosion to paving over prime agricultural lands. Soils provide an estimated $17 trillion of services annually, through ecosystem services such as flood prevention and as a forestry and agricultural resource. There are economic as well as ecological reasons to value soils. The character of each soil reflects the complex geologic and climatic history of the landscape, and these soils control the ecosystem. Some of these soils, such as the thick, red, clay-rich soils at the tops of hills in the Edwards Plateau, were formed over perhaps hundreds of thousands of years, and we can irreversibly change that soil in an instant by plowing or paving.