12 IRRIGATION PRACTICE 



constituting the soil. This must of necessity be so, for, 

 as has been shown, the smaller the soil particles the 

 larger is the surface exposed; and the larger the surface, 

 the thinner will be the film produced by a given quantity 

 of water. The thickness of the soil-moisture film is of 

 considerable importance, for from it plants secure the 

 water needed in their growth. If the film be too thin, 

 that is, if it is held very firmly, plants are not able to move 

 it from the surface of the soil particle. 



14. Thickness of film and diameter of particle. A 

 definite mathematical relationship exists for any per 

 cent of moisture between the thickness of the soil-mois- 

 ture film and the diameter of the soil grains. If a soil of a 

 specific gravity of 2.75 contains 5 per cent of water, the 

 thickness of the soil-moisture film is slightly more than 

 two hundredths of the average diameter of the soil grains; 

 if 10 per cent, a little more than four hundredths; if 20 

 per cent, not quite eight hundredths; if 30 per cent, a 

 little more than eleven hundredths; and if the soil con- 

 tains 40 per cent of water, the thickness of the soil-mois- 

 ture film is about fourteen hundredths of the average 

 diameter of the soil grains. 



That is, the thickness of the soil-moisture film in soils 

 that contain from 5 to 40 per cent of moisture, varies 

 from two hundredths to fourteen hundredths of the 

 diameter of the average soil grains. When the very small 

 sizes of the particles themselves are considered, this 

 shows the extreme thinness of the soil-moisture films, 

 with which agriculture has to deal. Meanwhile, it must 

 be remembered that only very fine soils can hold as much 

 as 40 per cent of water. When the thickness of the soil- 

 moisture film is somewhere in the neighborhood of one 

 fifty-thousandth of an inch, it is probably near its maxi- 



