92 THE PRINCIPLES OF SOIL MANAGEMENT 



It is well known that a box of baseballs, or a pile 

 of boulders, or even a box of sand, does not adhere 

 together to any appreciable extent. That is, in all the 

 coarser -textured classes, certainly down to the size 

 of very fine sand, there is very little tendency to granu- 

 late. But in the silt, to a small extent, and in the clay, 

 to a very great extent, granulation is strong. 



36. Porosity. In a mass of particles there is some 

 unoccupied or pore space. If the particles are fine, then 

 the intervening spaces are correspondingly small; if 

 large, the spaces are large. In the discussion of ideal 

 particles above, it was shown that the pore space is 

 theoretically independent of the size of the particles, 

 with any given arrangement. There would be as much 

 pore space in a cubic foot of buckshot as in one of 

 marbles. But in the soil this is not true. For, the finer 

 the particles, the larger the proportion of pore space 

 is found to be. 



A clay has much more total pore space than a sand, 

 although the individual spaces or openings between 

 the particles are much smaller in the clay. The approxi- 

 mate per cent of pore space in a soil may be calculated 

 by use of the following formula. 



v Vw 



2.65 Vd 



*>- -xioo= ^Exioo 



Vs Vs 



Where P = Per cent of pore space. 



Vs = Volume in c.c. occupied by the soil. 

 Vw= Weight of water equal to weight of soil in grams. 

 Vp = Volume in c.c. of pore space in soil. 

 2.65 = Specific gravity of soil particles. 



