76 



SOIL PHYSICS AND MANAGEMENT 



network of roots whose partial decay has provided the soil with an 

 abundance of organic matter. This extends to a depth of 13 to 24 

 inches in amounts sufficient to impart the predominating dark color. 

 The prairie soils contain a larger amount of lime than the timber 

 soils, and this may be one important factor in their origin. They 

 usually give an alkaline or neutral reaction. An exception to this 

 is found in Southern Illinois, some parts of Missouri, and Arkansas. 

 These prairies are acid and have a tight clay or so-called " hardpan " 

 subsoil. 



Prairie soils have had sufficient rainfall to leach out the larger 



ana Tropical fore a 



FIG. 63. Map of United States, showing timber and prairie areas. Unshaded area, 

 Prairie. Dark shade in Northeast and North, Central forests. Lighter shaded area in South- 

 east, Southern forests. Very lightly shaded area, Rooky Mountain forests. Deep shade, 

 Pacific Coast forests. (Graves, U. S. D. A. Forest Service.) 



part of the soluble salts, so that alkali is found only in small areas, 

 and then consists principally of the more insoluble magnesium car- 

 bonate. The second table on page 74 shows 0.048 per cent of solu- 

 ble salts in prairie soils, as compared with 0.013 for timber soils. 



The prairie soils extend from Southern Texas northward into 

 Canada, widening to the east into west central Indiana. A belt which 

 is not shown in figure 63 extends across Mississippi and Alabama 

 and over into Texas. 



(6) Timber Soils. The timber soils are characterized by a 

 lighter color, due to a small amount of organic matter. This has 



