SOME PHYSICAL PBOPERTIMS OF THE SOIL 123 



surface exposed, are far from correct, as we can arrive 

 at no definite figure as to the average size of grain. Neither 

 do we know the actual structural conditions. In con- 

 sidering these maccuracies Iving^ decided that we were 

 in need of a single term which not only would give an 

 indication regarding the size of grain, but also would 

 carry with it definite ideas as to the arrangement of the 

 particles, particularly as to the rate at which they would 

 allow air and water to pass through. This would bring 

 the considerations nearer to the plant, as permeability 

 very largely determines the conditions for plant develop- 

 ment. King, while he could obtain neither the mean 

 diameter of particle nor the actual internal surface, found 

 that he could determine with considerable accuracy, 

 particularly in sands, the diameter of grain which if sub- 

 stituted for the actual one would permit under like con- 

 ditions the same rate of air and water movement. This 

 size of grain he designated as the effective mean diameter 

 of particle for that particular soil. 



The theory of the method is presented by Schlicter,^ 

 and is based on the flow of fluids through capillary tubes. 

 From the observed rate of the flow of air through a soil 

 column under controlled conditions, it is possible to 

 calculate the effective diameter of the interstitial spaces. 

 From these data the size of the spherical grains which 

 would be necessary to form such pore spaces, or capillary 

 tubes, is computed by appropriate formulse. Such a 

 figure represents the effective mean diameter of the soil. 



1 King, F. H. Physics of Agriculture, pp. 119-120. Pub- 

 lished by the author, Madison, Wisconsin 1910. 



2 Sehlicter, C. S. Theoretical Investigation of the Move- 

 ment of Ground Waters, U. S. Geol. Survey, 19th Ann. Rept., 

 Part II, pp. 301-384. 1899. 



