176 Physics of the Soil. 



evaporates readily nor becomes easily available to plants. 

 and this may be regarded as the hygroscopic moisture. 



205. Relation of the Diameter of Soil Grains to the 

 Hygroscopic Moisture. It was shown in (163) that with 

 the same thickness of water surrounding the soil grains 

 the per cent, of water was necessarily much higher 

 in the soils having the smallest soil grains. In (192) is 

 given Quincke's observation of the distance across which 

 the force of cohesion is sensible, or TnroW inch. Since 

 this attraction of the soil for water is stronger than that 

 of the water for the water it appears likely that a layer 

 of water surrounding the % soil grains, at least as thick as 

 this, would not be as free to evaporate or to otherwise move 

 about as that much farther removed from this cohesive 

 attraction, and if so it is important to know what per 

 cents of soil moisture a water-film of such a thickness would 

 represent. This may be computed for spherical soil 

 grains with the formula 



TT (d 4- 2t) 3 it d a 



Per cent, of water 



_ 

 it d 3 sp. gr. 



where d = diameter of soil grain in c. m. 



t = thickness of water film. 

 sp. gr. = the specific gravity of the soil. 



Taking a very fine soil having grains with a diameter 

 of .00508 m. m. and a coarse one with a diameter of .1 

 m. m., a film of moisture on each, having the thickness of 

 the range of sensible cohesive attraction, as given by 

 Quincke, would make the per cent, for the finest soil 2.31 

 but for the coarse soil only .1153. ~No crop can survive in 

 soils as dry as these; and air-dry soils whose grains range 

 between those given will generally contain more than these 

 amounts of moisture. It follows from these considera- 

 tions, therefore, that what has been regarded as the hygro- 

 scopic moisture is more than that held within the range 



