The distribution and characteristics of the major soil regions of Texas are determined primarily by regional differences in climatic conditions characteristic of the various sections of the state (see NATURAL REGIONS). Areal variations within the various major soil regions are determined largely by geologic and physiographic conditions, which vary from the average expression of these elements in the region. The light-colored timbered soils of East Texas occupy a humid region. Westward and southward from these leached soils with their light color and sandy nature are the dark-colored, heavy-textured soils of the prairies, primarily characterized by their calcareous nature. The prairie soils occupy a zone transitional between humid, forested conditions at the east and subhumid climatic grasslands to the west and south. The black earth soils extend onto the High Plains, and to the west of them are the brown grassland soils. The Trans-Pecos region has a mixture of soil types. Alluvial soils are found on the Coastal Plain.
East Texas soils. Forested East Texas soils belong in the red and yellow soils group characteristic of the southeastern United States. These soils occur in a humid climate, their outstanding characteristic being that they are strongly leached. Furthermore, where maturely developed, the surface soil is sandy and gray in color, the thickness of the surface layer varying according to the sand content of the parent geologic material and according to topography. The corresponding subsoil is a sandy clay, of good water-retaining capacity, practically always low in calcium. Exceptional to the general picture of the light-colored soils are those of the redlands hilly country, characterized by their deep red color. These red soils occupy dissected areas and are underlain by deposits of greensands. On both the northern and the southern margins of timbered East Texas, exceptional conditions give rise to dark-colored soils. At the south, just interior from the Gulf Coast, are the low, flat lands of the Coastal Prairies, where the soils have a high calcareous content, are dark to black in color, and support a dense growth of tall grasses. The soils of this region without a calcareous content are light in color and are of two types-those with a heavy subsoil, which support a vegetation of tall grasses and those with a well-drained subsoil, which support growths of pine forests. At the north of timbered East Texas are discontinuous areas of dark-colored soils representing the fraying out of the Black Prairies in extreme northeastern Texas. A westward extension of East Texas timbered soils comprises the Eastern Cross Timbers-an elongated, relatively narrow, sandy strip lying between the Black and Grand Prairies. The soils of the Eastern Cross Timbers are sandy, being derived from the weathering of the Woodbine sand formation. These soils are light colored, thoroughly leached, and support a woodland growth in sharp contrast with the dark colored, heavy textured soils that support a tall grass vegetation in lands on either side of the Cross Timbers strip. The factors that have caused the formation of the red and yellow soils are climatic-heavy rainfall occurring throughout the year and high temperatures, with mild winters during which the ground seldom freezes. Associated with the climatic factors are the geologic materials, prevailingly sandy and non-limy, and the general thoroughly dissected nature of the landscape. Because of their leached condition, the red and yellow soils have a low content of the chemical compounds essential to plant life, and as a consequence all crops grown on the uplands have to be heavily fertilized. The natural vegetation of these soils is the tree, of which the southern pines are typical. The roots of trees are able to penetrate the deeper subsurface materials, which are less thoroughly leached than the soil layers, and as a consequence, native trees make excellent growth without the use of fertilizers.
Texas prairie soils. The outstanding factor in the genesis of prairie soils is the presence of a high calcareous content in the geologic materials from which they are derived. The prairies have a rolling to undulating or flattish topography and heavily textured soil materials suitable for the growth of tall grasses, but inimical to the growth of trees characteristic of the humid middle latitudes. The dark color of the prairie soils is caused by the presence of considerable organic matter derived from the decay of grass roots. The prairies occupy a humid climate, which causes the soils to be subject to continuous leaching but the roots of the tall grasses are ordinarily deep enough to penetrate into the unleached subsurface and carry calcium compounds and other basic materials upward to the fibrous root system near the surface of the ground. Where leaching has removed lime, profound changes have taken place in the prairie soils; they lose their mellow structure as they become more and more deflocculated, later the dark color is lost, and in time the grasses give way to a tree vegetation. The typical prairie regions in Texas (see TEXAS PRAIRIES, and GRAND PRAIRIES AND LAMPASAS CUT PLAIN) are the Black and Grand prairies of the north central part of the state, characterized by an undulating to rolling topography and the presence of black soils of high productivity; and the Coastal Prairies, interior from the Gulf of Mexico, characterized by low, flat lands in which the limy areas have deep soils dark to black in color, and the non-limy areas have light-colored soils, usually underlain by an abnormally heavy subsoil. Interiorward from the Coastal Prairies in the Coastal Plain are the Brenham-Schulenburg-Yorktown Prairies, underlain by calcareous clayey materials and characterized by a topography more thoroughly and intricately dissected than is generally the case of the Black Prairies. The Black Prairies may be considered as the typical prairie area of the state. The region is underlain by the various calcareous formations of the Upper Cretaceous. Physiographically, the region is an inner lowland-or rather a series of elongated inner lowlands, each of which is underlain by one of the distinctive limy formations of the Upper Cretaceous. The continual lowering of the surface by physiographic reduction in the case of the marly formations, makes available a continuous replenishment of soil with calcareous materials. The prairie soils of Texas are the premier cotton soils of the state, alluvial soils alone excepted. These soils were brought under cultivation largely during the 1870s and 1880s, paralleling the extension of railroads through the prairie lands of the state.
Black earth soils of West and South Texas. West and south of the prairie soils zone is the black earth, or Chernosem, belt, occupying a broad subhumid zone in which the rainfall is not sufficient to maintain a percolation of moisture in the subsurface between the soil and the geologic water table. As a consequence a permanently dry layer of varying thickness underlies the top soil. Typical soils of the black earth type occupy plains areas with a characteristically constructional rather than an erosional surface. Because they occur in a subhumid climate, these soils are not leached, at least not the subsurface soils. Instead, the subsoil of all typically developed black earth soils possesses a characteristic layer of accumulated calcium carbonate, the depth of which approximately marks the maximum penetration of meteoric water. Where exceptionally well developed, this layer of calcium carbonate becomes indurated, forming the caliche so common in West and South Texas. Where maturely developed, the black earth soils are fertile because of their high content of chemical compounds essential to plant growth. The presence of calcium makes the soils thoroughly flocculated and highly workable. Despite their fertility the black earth soils are limited in productivity by the amount of rainfall in the regions in which they occur. The natural vegetation of black earth soils is short grass; mesquite shrubs also occur. The areas of typically developed black earth soils are not continuous but occupy constructional surfaces in the High Plains, the Lower Plains, the Edwards Plateau, and the South Texas Plains. Maturely developed black earth soils are characterized by a highly developed agriculture with machine production of such crops as cotton, wheat, and grain sorghums. Eroded areas in the black earth soils zone are not utilized agriculturally but constitute range lands that support large numbers of cattle and, in special environmental areas, considerable numbers of sheep and goats.
Brown grassland soils. Brown grassland soils occupy the western portion of the High Plains and extend well into eastern New Mexico. Even where typically developed, the brown grassland soils are utilized for farming only with considerable risk, except where water for irrigation is available. Maturely developed soils of the brown grassland zone are practically as fertile as those of the black earth, except that they have a lower nitrogen content. It is the rainfall factor which so markedly reduces their agricultural possibilities. Other than areas of irrigation agriculture, these lands, where farmed, are utilized only through the application of dry farming practices.
Trans-Pecos soil. Except for the summits of its high mountains, the Trans-Pecos is a region of reduced rainfall, a factor which is reflected in both its soils and its natural vegetation. The floors of the Trans-Pecos lowlands, characterized by a constructional topography and fine textured soil materials, have soils not greatly unlike those of the High Plains. In places the lowland floors are characterized by thick deposits of sands; in other cases they have thick layers of gravel with alternating beds of clayey materials, which, in cases of deep dissection, give a mesa-like topography. In general the steeper slopes of the mountain ranges are strongly erosional with shallow soils or bare rock surfaces. In the Davis Mountains, however, the dissected plateau area is characterized by soils deep enough to support an excellent cover of range grasses, except on the steepest slopes. The low rainfall of the Trans-Pecos region forbids agriculture except where water for irrigation is available. The smooth, alluvial floor of the great trench of the Rio Grande below El Paso is an important agricultural region and smaller irrigation areas occur at Balmorhea and in the vicinity of Pecos in the Pecos Valley.
Alluvial soils. The larger streams flowing across the Coastal Plain lands of Texas are generally characterized by wide valley lowlands with thick deposits of fine to moderately fine textured alluvium. The soil materials of alluvium bear little relation to the soils of the adjacent uplands in which they occur because the alluvium has often been carried in from distant areas. The alluvium in the valley lowlands of the Coastal Plain in Texas has usually been borne down from the western part of the state, particularly from the Lower Plains. Alluvial soils are rich as a rule; where well drained and not too severely flooded, they are highly productive. They are used for the growing of cotton and in the lower Rio Grande valley produce citrus and other subtropical fruits as well as numerous kinds of vegetables.