SOIL MOISTURE 13 



mum thickness, and, when this has been reached, there are 

 in average soils about 20 per cent of moisture. 



15. Hygroscopic coefficient. If a thoroughly dried 

 soil be exposed to air, which is always somewhat moist, 

 the attraction between the soil surface and the water 

 vapor in the air immediately becomes active. Some of 

 the water vapor condenses upon the surfaces of the soil 

 grains to form the beginnings of the film. This coating 

 of water is hygroscopic soil moisture. If the air sur- 

 rounding the soil is saturated with water vapor, the 

 largest possible quantity is condensed upon the soil. 

 The percentage of moisture representing the full con- 

 densation of water upon soil from such saturated air, 

 under given conditions of temperature, is known as the 

 hygroscopic coefficient. 



The water thus taken from the air is not wholly held 

 as surface film. In every soil are certain substances 

 (colloidal) that absorb water to form loose chemical com- 

 binations. Such materials are well represented by the 

 jellies which hold large quantities of water uniformly 

 distributed throughout their mass. Among the substances 

 with more or less strongly marked jelly-like properties 

 are clay, hydrate of iron, humus and decaying organic 

 matter generally, and a number of gums, among which 

 gum tragacanth is the most notable. 



The hygroscopic coefficient, therefore, increases as 

 the fineness of the soil increases, and as the quantity of 

 the water-absorbing substances increases. For example, 

 Lyon and Fippin found that very fine sand absorbed 1.8 

 per cent of hygroscopic moisture; silt, 7.3 per cent; clay, 

 16.5 per cent, and a muck soil absorbed 48 per cent of 

 water from saturated air. Hilgard examined three soils 

 very much alike, except that one contained 4 per cent, 



