PHYSICAL PROPERTIES OF SOILS 1 9 



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



been estimated that a gram of productive 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 : 



t 



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 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 plant roots. 



10. Method 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 coarser 

 grades of sand. The sieve a, 0.5 mm. size, is connected 

 with the filtering flask c by means of the tube b, and the 

 flask is connected at point d with a suction pump. Ten 

 grams of soil, after soft pestling with boiling water, are 

 placed in the sieve and water is passed through until the 

 washings are clear. All particles larger than 0.5 mm. 



* Figures below sixth place omitted and ciphers substituted. 



