SOILS OF THE EASTERN UNITED STATES AND THEIR USE XXVII. 



THE HOUSTON BLACK CLAY. 



GEOGRAPHICAL DISTRIBUTION. 



The Houston black clay is the most widespread and important of 

 the soils found in the Black and Grand Prairie sections of southern 

 Oklahoma and of central Texas. Its occurrence, so far as it has been 

 encountered in the soil surveys, is almost entirely confined to this 

 region, although small areas undoubtedly exist in the Cretaceous 

 praiiie region of central Alabama and northeastern Mississippi. It 

 is within the great prairie region of Texas that this type is principally 

 to be found. The Houston black clay has been encountered in 15 

 different soil survey areas located in Alabama, Oklahoma, and Texas 

 and mapped to a total extent of 1,402,392 acres. The region formed 

 by the Black and Fort Worth Prairies includes far greater areas of the 

 Houston black clay than those already mapped. It is safe to esti- 

 mate that in southern Oklahoma and in central Texas, together with 

 several areas in other States, there will ultimately be found over 

 15,000,000 acres of this important soil type. 



CHARACTERISTICS OF SOIL AND SUBSOIL. 



The surface soil of the Houston black clay is a drab, dark-brown or 

 jet-black clay, having an average depth of about 10 inches. The 

 soil is friable when well cultivated and in a condition of moderate 

 moisture, but it is exceedingly waxy and sticky when wet, and in this 

 condition its structure may be seriously injured if it is plowed or 

 otherwise tilled. The waxy nature of both the surface soil and sub- 

 soil of this type has given rise to the popular name of ' ' Black Waxy 

 Prairie" for the section where it is prevalent. The subsoil is a drab, 

 light-brown, or sometimes blue or gray, tenacious clay which is very- 

 waxy and stiff. It extends to a depth usually exceeding 3 feet, but 

 may be underlain at about this depth by the gray or chalky marl of 

 the Austin chalk and Taylor marl formations. In Alabama and hi 

 Mississippi the soil is derived chiefly from the Selma chalk. 



Although the surface soil and subsoil of the Houston black clay are 

 exceedingly waxy and tenacious when wet, the surface soil has the 

 property of cross-checking into innumerable fine granular aggrega- 

 tions of clay when it is partly dried. If plowed or otherwise tilled 



15974 Cir. 50 12 3 



