118 SOILS: pbop£jbti:es and management 



85. The number of soil particles. — The number of 

 particles in any given volume of soil is really determined 

 by texture, and, as this number determines very largely 

 the probable arrangement of the soil grains, structure 

 becomes in turn dependent on the size of grain. Since 

 soil particles run to very small diameters, the number 

 in any given volume is very large, especially when we 

 are dealing with fine-textured soils or with soils of com- 

 pact structural condition. Any calculation of the number 

 of particles present in a soil is open to considerable in- 

 accuracy, first, because it is impossible to get a correct 

 figure for the average diameter of the particles of any 

 soil or of the various groups of separates that go to make 

 it up, and, secondly, because it must be assumed in the 

 calculation that the particles are spherical. This assump- 

 tion is of course incorrect, as has already been demon- 

 strated; but it must be entertained in order to obtain 

 approximate ideas as to the number of grains in any soil. 



The number of particles in any soil sample may be 

 arrived at from a mechanical analysis and the diameters 

 that limit each group. Using the average diameter of 

 each group together with the percentage of the groups 

 in a given sample, the number of particles may be cal- 

 culated by the following formula : — 



Number of particles in a _ Weight of sample in grams 

 sample of soil "^ 1/6 ttD^X 2.70 



The formula 1/6 tt D^ is that used for determming the 

 volume of a sphere, the diameter in this case being ex- 

 pressed in centimeters. The volume of the sphere, then, 

 is obtained in cubic centimeters, which must be multiplied 

 by the absolute specific gravity of soil minerals, or 2.70, 



