POROSITY OF SOIL 93 



Another and more simple formula which may be 

 used in the calculation of the pore space is as follows: 



P = io Q - fp- s Pg r xioo 



Ab. sp. gr. 

 Where P = Per cent of pore space. 

 Ap. Sp. = Apparent specific gravity or volume weight. 

 Ab. Sp. = Absolute specific gravity of soil material. 

 100% = Total space occupied by soil mass. 



This relation between texture and pore space is 

 exhibited by the following table of figures for soils in 

 field condition. 



Per cent by volume 



1. Clean sand 33.50 



2. Coarse sand 40.00 



3. Medium sand 41.80 



4. Fine sand 44.10 



5. Sandy loam. 51.00 



6. Fine sandy loam 50.00 



7. Silt loam 53.00 



8. Clay loam 54.00 



9. Clay 56.00 



10. "Gumbo" clay (Wedgefield) 58.46 



11. Heavy clay (Potomac puddled) 47.19 



12. Very heavy clay (pipe clay) 65.12 



The reason for the greater porosity of the finer soils 

 appears to be, that the smallest particles are so light 

 that they do not settle so closely together in proportion 

 to their size as do the sand particles, because of the 

 greater friction between their surfaces. When this is 

 overcome by mixing in water, such material becomes 

 dense. Treatment greatly affects the structure and 

 therefore the porosity of the soil. This is well shown 



