140 MANUFACTURE OF FERTILIZING MATERIALS 



resistance to drought, etc., will depend essentially 

 upon the nature of the substratum. But in or- 

 dinary eases 10 or 12 in. of subsoil will be suf- 

 ficient for the purposes of examination in the 

 laboratory. The specimen should be obtained in 

 other respects precisely like that of the surface 

 soil, while that of i?he material underlying this 

 subsoil may be sampled with less exactness, 

 perhaps at some ditch or other easily accessible 

 point, and should not be broken up, but left, as 

 nearly as possible, in its original state. Mix each 

 of these soils intimately, remove any stones, shake 

 out all roots and foreign matter, expose in thin 

 layers in a warm room till thoroughly air-dry, 

 or dry in an air-bath at a temperature of 40 C. 



The soil is rapidly dried to arrest nitrifi- 

 cation. It is not heated above 40 lest there be a 

 dissipation of ammonium compounds, or a change 

 in the solubility of the soil. The normal limit 

 to which the soil may be heated in place by the 

 sun's rays should not be exceeded in preparing 

 a soil for an agricultural chemical analysis. 



Five hundred gms. or more of the air-dried 

 soil, which may be either the original soil or 

 that which has been passed through a sieve of 

 coarser mesh, are sifted through a sieve with 

 circular openings one-half millimeter in diameter, 

 rubbing, if necessary, with a rubber pestle in 

 a mortar until the fine earth has been separated 

 as completely as possible from the particles 

 that are too coarse to pass the sieve. A three- 



