72 Physics of the Soil. 



true when single cases are compared, and this is shown in 

 a striking manner in the single case of clay soil given below 

 the line of averages in the table of sandy and clayey soils. 

 This is described by Hilgard as a fair upland soil yielding 

 TOO to 800 pounds of cotton per acre, gray in color, not 

 heavy, 6 to 8 inches deep, and underlaid by a subsoil quite 

 heavy in tillage and dark orange in color ; and yet its in- 

 soluble residue is about 91 per cent, and there are two of 

 the sandy soils where the per cents, are 90 and 92 respec- 

 tively, showing that the two are more nearly alike chemi- 

 cally than they are physically. 



83. Observed Chemical Differences, Partly Due to Differ- 

 ences in Amount of Soil Surface. It is a common experience 

 that the more finely a substance is subdivided the more 

 rapidly will it dissolve. Fine salt and powdered sugar, 

 for example, dissolve much more rapidly in water than the 

 coarser grained varieties do. In the clay soils the particles 

 have a much smaller diameter than they do in the sandy 

 soils and hence the number of grains in a given weight of 

 soil will be much larger, but the number of grains cannot 

 be increased without also increasing the surface upon 

 which the solvent may act, and hence with the same 

 strength and amount of acid, for equal weights of the coarse 

 and fine grained soil, having exactly the same chemical 

 composition, there should be dissolved in equal times a 

 larger per cent, of the soil having the largest amount of sur- 

 face. The sandy soils therefore are not likely to be as dif- 

 ferent from the clayey ones as the table of analyses indi- 

 cate. 



84. The Chemical Differences Between Soils and Their 

 Subsoils. In humid climates there is usually a marked dif- 

 ference in the producing capacity of the soils and their sub- 

 soils as was pointed out in (65), and a study of the table 

 of subsoils, pp. 74, 75, will show that 'there is a chemical 

 difference also. It will be seen that the surface soils con- 

 tain more lime, phosphoric acid and organic matter, less 

 soluble silica, alumina and iron and about the same 

 amounts of potash, magnesia and sulphuric acid. 



