Soils from Five Geological Regions Of

Mineralogical studies for the <~2 ~ fractions are presented for seventeen soil profiles and two surface horizon samples, which represent eight soil types and five geological regions in the state. There was no evidence of clay mineral formation in the upper 6 feet or more of soil. X-ray diffraction data from the Lufkin, Ruston anr Katy soils suggested some decomposition of montmorillonite and kaolinite in the upper horizon. However, the overall picture is one of little change in clay mineral composition between the soils and parent sediments. I N T R O D U C T I O N A soil from the geological viewpoint may be defined as that part of the mantle which has been sufficiently decomposed and otherwise modified that it supports rooted plants. On the basis of this definition, the larger portion of the earth's surface is covered by soil which may be credited as the major contributor to recent sediments. By the nature of its position, the soil is continuously exposed to a variety of intense weathering forces, which may modify its mineral composition with relative rapidity. Knowledge of the mineral composition of soils is of importance in determining the agricultural potentials of soils. Such information is also helpful in understanding the nature of the soil forming processes. The objective of this paper is to present the clay mineralogy for some 17 soil profiles representing the dominant and more agriculturally important soils f rom five geological regions of Texas. ! ~ I A T E R I A L S A N D M E T H O D S The five geological regions of the state are represented by 17 soi ! profiles and two surface samples. Soil types, numbers of profiles investigated and locations with respect to geological regions and counties are given in Table I. The organic matter was removed from ten grams of soil with hydrogen peroxide. The sample was transferred to a 250 ml. centrifuge tube and washed twice with 50 ml. of distilled water to remove any soluble mineral matter. I t was then transferred to a 250 ml. square bottle, 50 ml. of a 0.1 N solution of sodium hexametaphosphate added, the total volume * Contribution from the Agronomy Department of the Texas Agr. Exp. Sta., College Station, Texas, as Technical Article No. 2113. 373 374 REPRESENTATIVE SOILS FROM FIVE GEOLOGICAL REGIONS OF TEXAS TABLE I.GEOLOGICAL REGION, SOIL TYPE, NUMBER OF PROFILES INVESTIGATED AND LOCATION OF PROFILES WITH RESPECT TO COUNTIES Number of Location profiles of Geological investiprofiles region Soll type gated (county) Southern High Plains Amarillo fine 11 Lynn sandy loam Amarillo loam 1 Lynn Blackland Prairies Houston Black 5 clay 1 ~ Fayette 1 Bell 1 x McLennan 1 Ellis 11 Tannin Forested Coastal Plain Lufkin fine 2 sandy loam 1 t Fayette 1 Brazos Ruston fine 2 sandy loam 1 ~ Cass 1 Cherokee Gulf Coast Prairie Lake Charles 2 clay s 11 Fort Bend 1 Jefferson Katy sandy 11 Waller loam Rio Grande Delta Harlingen clay 3 11 Hidalgo 2 Cameron Willaey fine 2 sandy loam 11 Hidalgo (Engelmann Gardens Farm) Hidalgo (Rio Farms) 1 Denotes profiles for which X-ray diffraction patterns are shown in subsequent figures. Only samples for the 0-7" depth were available. brought up to 150 ml. and then stirred for 30 minutes with a malt-mixing machine. Following dispersion the sample was t ransferred to a 2,000 ml. beaker, the volume made up to approximately 1,200 ml. with distilled water and separation of the clay effected by sedimentation, using Stoke's law to determine settling rates. The material remaining in suspension to a predetermined depth was siphoned off. The clay was flocculated with magnesium G. W. KuNzE, E. H . TEMPLIN, AND J. B. PACE 375 acetate, transferred to a 250 ml. centrifuge tube and excess salts removed with several washings of distilled water. Free iron oxides were removed from the Amarillo and Ruston soil samples according to the procedure by Aguilera and Jackson (1953, p. 359). Samples for X-ray diffraction analyses were prepared by placing 100-150 rag. of clay in a 100 ml. centrifuge tube, 50 ml. of a 1.0 N solution of magnesium acetate added andj the samples boiled gently for 10 minutes, after which they were centrifuged and the supernatant liquid decanted. Excess salt was removed by two washings with 25 ml. of distilled water and the samples mounted on glass slides while still in a moist state. The samples were allowed to air dry, after which they were placed in a small tin can, 4 inches in diameter, which contained a small amount of ethylene glycol and placed in an oven for an hour at a temperature of 65 ~ C. Condensation of ethylene glycol vapors on the samples served to saturate them with ethylene glycol. After an hour they were removed from the can, placed in the oven at 65 ~ C and left until the droplets of ethylene glycol adhering to the samples and glass slides evaporated. The samples had to be checked every 5 to7-nlinutes to prevent excessive drying. Following drying, they were placed in a desiccator containing ethylene glycol for a period of 12-24 hours to come to equilibrium, after which they were irradiated. A duplicate set of samples was saturated with potassium, which was accomplished by the same procedure, except that a 1.0 N solution of potassium acetate was used and these samples were not saturated with ethylene glycol. The diffraction patterns for samples from a profile were all recorded at the same intensity settings. The X-ray diffraction unit used was a North American Philips high angle goniometer model equipped with a copper target tube. RESULTS AND DISCUSSION Figure 1, a map of Texas, shows the rainfall belts, locations for the soil profiles investigated and the geological regions considered for this study. Amarillo fine sandy loam, the representative of the High Plains region, dominates the soil pattern in the southern half of that area in Texas and comprises several million acres. The parent sediments throughout this area are of terrestrial origin and were deposited as a vast alluvial apron along the eastern foot of the Rocky Mountains during the Pliocene and more or tess reworked by wind action during the Pleistocene. They represent sediments from relatively dry regions which have not passed through a cycle of strong weathering under a humid environment. Figure 2 shows the X-ray diffraction patterns for the < 2 ~ fractions separated from a profile of Amarillo soil. Illite, kaolinite and quartz constitute the major crystalline portion of the < 2 ~ fraction. Of these illite appears as the dominating clay mineral. T h e relatively strong diffraction line commencing at approximately 6 ~ 2 0 is indicative of the presence of poorly crystallized material, which fails to show any definite peak. This type of diffraction pattern has generally been observed for most Texas soil 376 REPRESENTATIVE SOILS FROM FIVE GEOLOGICAL REGIONS OF TEXAS