PHYSICAL PROPERTIES OF SOILS 1 7 



often 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 unproduct- 

 ive. 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. 



10. Methods Employed in Separating Soil Particles. 



— Sieves with circular holes 0.5, 0.25, and o. 1 mm. 

 are employed for the purpose of separating the three 

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

 nected with the filtering flask c by means of the tube 

 b, and the flask is connected at point d with a suc- 

 tion-pump. Ten grams of soil, after treatment with 



* Figures below sixth place omitted and ciphers substituted. 



