84 THE PRINCIPLES OF SOIL MANAGEMENT 



From this table it appears that one pound of the 

 average agricultural soil may have from about 400 

 square feet, in the case of coarse sand, to 2,000 square 

 feet internal surface area, in the case of the average 

 clay. A more reasonable basis of comparison, because 

 of differences in volume weight, is that of one cubic 

 foot of the material, as shown by the fourth column, 

 from which it appears that these soils have from one 

 to three acres of surface area. These are striking dif- 

 ferences, particularly those between soils 8 and 9, 

 which represent extremes in light and heavy soils, 

 respectively. Number eight is the sand-hill soil of the 

 Carolinas, and is of exceedingly low agricultural value. 

 Number nine, Hobart clay, occurs in eastern North 

 Dakota, and is derived from shale rock. The range in 

 surface area per cubic foot of these soils is from one- 

 twelfth of an acre, for the sand, to almost five acres for 

 the clay. The latter contains 76 per cent of clay in the 

 subsoil, the former 2 per cent. 



31. Chemical composition of the soil separates. 

 There is some relation between the soil classes or 

 separates and their chemical composition. Quartz, for 

 example, in the original rock resists decay and comes 

 through largely as sand particles, while the silicate min- 

 erals undergo much more decay which results in a larger 

 proportion of clay particles, and this partial difference 

 in derivation is reflected in the composition of the sepa- 

 rates. The distribution of plant-food constituents and 

 the general chemical composition of the classes of a soil 

 is shown by the following table of results of acid anal- 

 ysis, obtained by Loughridge as reported by Merrill. 



