CHAPTER X 

 MINERAL CONSTITUENTS 



I. SOIL PARTICLES AND THEIR SEPARATION' 



THE forces at work on rocks break them down into soil material, 

 the particles of which are of various sizes and shapes. The relative 

 proportion of the different sizes is a very important factor in the 

 character of a soil. As a general rule where soils contain large per- 

 centages of a certain grade of particles, one or two per cent makes 

 very little difference in the physical phenomena that take place. It 

 is, however, of considerable importance to know the approximate 

 physical composition or texture, as it usually gives some idea of the 

 capillary power, aeration, percolation and other properties of the 

 soil. 



There are frequent exceptions to this, however. The physical 

 composition gives no idea of the arrangement of the particles or 

 structure of the soil. The aggregation of the particles into granules 

 or crumbs plays a most important part in the physical phenomena 

 that take place. Some expression for showing this is very desirable. 



Mechanical or physical analysis, which is the process of sepa- 

 rating a soil into the different grades of particles according to size, 

 is an attempt to accomplish this. 



As yet, however, no very scientific grouping of the soil particles 

 has been devised. That of Dr. Hopkins is without doubt one of the 

 best, as it recognizes a constant factor or ratio, the square root of 

 ten- Between groups. In other methods or schemes the ratio be- 

 tween grades varies quite widely. The result is that when an 

 analysis is made of soils of a regularly decreasing or increasing size 

 of particles no uniformity is shown. 



Several systems have been devised, of which the principal ones 

 in this country are given in the accompanying table. 



It will be noted that in the Osborne system the factors vary from 

 2 to /> ; in the Bureau of Soils from 2 to 10; in Hilgnrd's from 1.:> 

 to 3 ; in the Hopkins system the constant factor is 3.1G or the square 

 root of 10. 



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