122 SOILS: PROPERTIES AND MANAGEMENT 



APPROXIMATE INTERNAL AREA EXPOSED BY AVERAGE CLASSES 

 OF UNITED STATES SOILS 



It is at once apparent that the amount of surface 

 exposed by the soil grains of even a sand is tremendous. 

 It is not to be wondered at that the slowly soluble minerals 

 are able to supply sufficient food to the crop growing on 

 the soil, when such a large amount of surface is continually 

 available for chemical action. The figures presented for 

 an acre-foot of soil are almost too large for adequate com- 

 prehension. It is quite evident that the finer the soil, 

 the greater is the amount of internal surface. For ex- 

 ample, a sandy loam weighing 90 pounds to the cubic 

 foot would present 60,390 square feet of surface, while 

 a clay weighing 75 pounds to the cubic foot would expose 

 about 154,275 square feet. This is equivalent to 1.39 

 and 3.54 acres, respectively. 



87. The effective mean diameter of soil particles. - 

 It is very evident that the calculations presented above, 

 both as to the number of particles and as to the internal 



