SOIL OF THE YUMA MESA 



The soil of the Yuma Mesa has been classified by the U. S. 

 Department of Agriculture Bureau of Soils as Yuma Sand. It is 

 probably of marine origin and consequently is quite uniform over 

 a large area. In common with marine soils it does not contain a 

 large total amount of plant food elements. The abrupt edges of 

 the Mesa left by the erosion of the river valley show some stratifica- 

 tion. Bands of shale-like clay of varying thickness may be seen 

 where vertical sections are exposed. It is not definitely estab- 

 lished to what extent these clay bands reach out under the Mesa. 

 At several points clay strata reach the surface, but dip ofif rapidly 

 again beyond the reach of an ordinary spade. Drilled wells have 

 encountered clay in several locations. The clay strata are often 

 strongly alkiline. With the exception of these bands, the sands 

 are deep and well drained. 



The most striking character of the Yuma Sand is its highly 

 calcareous nature ; even the drifting sands and dune sands effer- 

 vesce strongly with acid. The lime does not exist as grains of 

 calcium carbonate, excepting to a small extent in the silt and clay 

 separates, but as a more or less uniform incrustation on all the soil 

 particles. This incrustation remains on the particles thru the 

 process of separation in mechanical analysis, and all separates 

 from the finest to the coarsest effervesce with acids. The incrusted 

 particles give the soil a characteristic appearance, which at once 

 suggests the name "tarnished sand." The lime content varies in 

 the vertical section, and usually a band of cemented soil a foot or 

 more in thickness is found at or near the surface. The surface 

 appearance of the Yuma Mesa is that of gravelly and sandy streaks 

 and patches alternating. By following the gravelly areas a light 

 automobile can be driven over the Mesa. Investigation shows that 

 the gravelly areas in a general way mark the places where the 

 lime cemented strata come near the surface. A vertical section 

 thru one of these lime cemented strata shows a mottled or marble- 

 like appearance, due to the cutting of limy concretions varying in 

 size from a wheat grain to a walnut. In places the lime becomes 

 so dominant as to form semi-chalky layers. These are hard when 

 dry, but soft as clay when wet. An explanation of the surprising 

 fertility of these seemingly barren sands may be found in the dis- 

 tribution of the calcareous incrustation over the surface of the soil 

 grains. This probably averages 8 or 10 percent of the total weight 



