SEPARATING SOII, PARTICLES 15 



and clay is purely an arbitrary one. Various authors 

 use these terms in different ways, and when compar- 

 ing soils reported in different works, one may avoid 

 confusion by omitting the names and noting only the 

 sizes of the particles. A division has recently been 

 suggested by Hopkins 7 , in which the square root of ten 

 is taken as the constant ratio between the grades of 

 soil particles. 



9. Number of Particles per Gram of Soil. It has 

 been estimated that a gram of soil contains from 

 2,000,000,000 to 20,000,000,000 soil particles ; soils 

 which contain less than 1,700,000,000 are unproduc- 

 tive. The number of particles in a given volume of 

 soil varies with their size and form. According to 

 Whitney 8 the number of particles per gram of differ- 

 ent soil types is as follows : 



Early truck 1,955,000,000* 



Truck and small fruit 3,955,000,000 



Tobacco 6,786,000,000 



Wheat 10,228,000,000 



Grass and wheat 14,735,000,000 



Limestone 19,638,000,000 



Assuming that the particles are all spheres, it is es- 

 timated that in a cubic foot of soil a surface area of 

 from two to three and one-half acres is presented to 

 the action of the roots. 



i o . Methods Employed in Separating Soil Particles . 

 Sieves with circular holes 0.5, 0.25 and o.i mm. 

 are employed for the purpose of separating the three 

 coarser grades of sand. The sieve a, 0.5 mm. size, is con- 



* Figures below sixth place omitted and cyphers substituted. 



