Water Capacity of Soils. 137 



so is because the thickness of the water film surrounding 

 the grains is nearly the same in all the cases. 



The truth of this statement will be evident if we com- 

 pute the per cent, of moisture in a soil which a given thick- 

 ness of film surrounding the grains will produce. 



163. Relation of Thickness of Moisture Films to Per Cent. 

 of Soil Moisture. If the data in the table of (145) is used 

 the per cent, of soil moisture a given thickness of film will 

 produce may be computed approximately from the formula 



where P = the per cent, of moisture in the soil. 

 K= a factor, Log. 2.497532 = .0314355 

 S = surface of soil per cu. ft. taken from (145) 

 T = thickness of film of moisture 



Q = per cent, of dry soil obtained by subtracting the pore 

 space in (145) from 100. 



Using this formula and the data in (145) it will be 

 found that the per cents, of moisture stand as given below : 

 "With thickness of film Trru&W inch the per cent, of water 

 will be, in the 



Heavy red clay .......................... 14 .24 per cent. 



Loamy clay .............................. 11.28 per cent. 



Loamy clay .............................. 8. 74 per cent. 



Loam .................................... 7.20 per cent. 



Sandy loam .............................. 5.21 per cent. 



Sandy soil ............................... 2.09 per cent. 



Sandy soil ............................... 1.41 per cent. 



Coarse sandy soil ........................ 1.11 per cent. 



From this table it will be seen that the coarse sandy 'soil 

 contains only 1.1 per cent, of its dry weight of moisture 

 when the heavy red clay contains 14 per cent, with the same 

 thickness of film surrounding the soil grains. 



Comparing these per cents, of moisture with those con- 

 tained in the soil in which the corn ."wilted, it will be seen 

 that the sand of that soil was really the wettest soil there, so 

 far as the available moisture is concerned, there being at 



