88 SOILS. 



substance remains adherent to the surface of the larger grains, 

 and plastic clay is formed to a much less extent. Much avail- 

 able plant-food may therefore, in arid lands, be present even in 

 rather coarsely sandy soils almost devoid of clay; such as in 

 humid climates would he likely to be found wholly barren. 

 (See chapt. 19). 



PHYSICAL ANALYSIS OF SOILS. 



Use of Sieves. Down to a certain point the separation of 

 the soil into its several grain-sizes may be accomplished by 

 means of sieves. We may thus separate coarse gravel from 

 fine gravel and from sand ; and the latter may itself be sepa- 

 rated into several sizes by the same means. This presupposes, 

 of course, that the soil has been previously prepared for the 

 purpose by crushing the lumps consisting of aggregates of 

 finer particles, that in the operation of tillage would again be 

 resolved into their fine constituents, or be penetrated by roots. 

 But this preparation of the soil for sifting must not be carried 

 beyond the point mentioned, for a grain consisting of particles 

 somewhat firmly cemented together will under ordinary con- 

 ditions play in the soil precisely the same part as a solid sand- 

 grain, and must not therefore be broken up, if the soil is to be 

 examined in its natural condition. The pressure of the fingers 

 or of a rubber pestle is as far as trituration should go. The 

 disintegration of these compound particles by means of acids, 

 as prescribed and practiced by the French soil chemists, may 

 wholly change the physical nature of the soil by the breaking- 

 up of mechanical aggregations which in the usual course of 

 tillage would remain intact. This is especially true of 

 strongly calcareous soils, and particularly those containing 

 calcareous sand. 



The sieves used for this purpose should not be ordinary wire sieves, 

 but should have bottoms of sheet brass perforated by round holes of 

 the various diameters desired, of fractions of inches, or preferably of 

 millimeters. For the finer grain sizes, silk bolting cloth is used by the 

 U. S. Bureau of Soils. 



In the sifting process it will be found that so soon as the finer grain- 

 sizes of the sand are approached, the sieve fails to act satisfactorily ; 

 the more so, the more clay was originally contained in the material. 



