178 



SOIL PHYSICS AND MANAGEMENT 



6. Number of Particles. From the work preceding, especially 

 the tables giving the diil'erent grades of soil material, it will be 

 seen that many soil particles are extremely small, and the number 

 of these in a certain volume or weight of soil is very great. If the 

 largest particles of clay, 0.001 millimeter in diameter, were spherical 

 and could be arranged in columnar form in a cubical box one inch 

 each way, it would contain 15,625,000,000,000 particles. The de- 

 termination of the number of particles in a definite weight of soil 

 can be made by dividing the weight of soil by the weight of a 

 single average-sized particle, as given in the following formula : 

 ,, _ Weight of soil (grams) 

 Weight of a single particle 



Weight of one particle =%7rD 3 X Sp. gr. 



N = the number of particles, D = the mean diameter of the soil particle 

 in centimeters and ^D 3 = the volume of a sphere. The specific gravity 

 taken is 2.65. 



This, of course, assumes that the soil particles are spheres and 

 are all reduced to the average diameter. The following table gives 

 the number of soil particles per gram of soil : 



Number of Particles and Internal Surface of Soil Separates 

 Bureau of soils groups 



Illinois experiment station groups 



8. Shape of Particles. Particles of many shapes and sizes 

 exist an all soils. The shape varies with the origin. Soils formed 

 from volcanic ash or dust are most irregular in shape and those of 

 wind origin are more nearly uniform. The former have many 



