CHAPTER V. 



PHYSICAL COMPOSITION AND CLASSES OF SOILS. 



Soils are composed of particles of different sizes, ranging from 

 over 2 mm. to o.oooi mm. or less, in diameter. Many of the 

 physical properties of soils are closely related to the relative 

 abundance of particles of different sizes. The surface area of a 

 cubic foot of the particles increases with their fineness of division. 

 The retentive power for moisture, the area exposed for chemical 

 action and for the feeding of roots, the capillary action of the 

 soil, etc., are closely related to the size of the soil particles. 



Soil particles may be found independent of one another, but 

 they are usually more or less united into crumbs, compound 

 particles, or lumps. 



Mechanical Analysis. By the mechanical analysis of a soil, we 

 mean the estimation of the relative quantities of soil particles of 

 different sizes. As the particles which make up the soil have 

 almost an infinite variety of size, all that can be done is to group 

 them, by placing all that are between certain dimensions in a certain 

 group. The sizes selected for the groups, and the name given to 

 each, are purely arbitrary. A number of systems of soil analysis 

 is possible. The principal groupings of soil particles in mechan- 

 ical analysis used in the United States are those of Hilgard, and, 

 those of the Bureau of Soils. Other systems are used abroad. 

 The Bureau of Soils makes seven separations. Dr. E. W. Hilgard 

 has made a number of analyses based on the velocity of a current 

 of water holding the particles in suspension, stated in millimeters 

 per second (hydraulic value). For example, sand of 0.5 to 0.30 

 mm. in diameter is held in suspension by a current of water 

 moving at the rate of 64 millimeters per second. 



There has been little or no work to determine the classification 

 of soil particles which would best correlate the properties of the 

 soil with the physical analysis. So far as the writer has been able 

 to find, the division into groups is arbitrary. 



The following table compares the systems of Hilgard and of the 

 Bureau of Soils : 



