152 NATURE AND PROPERTIES OF SOILS 



If an absolutely dry soil is exposed to a moist atmosphere, 

 it will absorb moisture rather rapidly until the colloidal sur- 

 faces are in equilibrium with the air as far as water vapor is 

 concerned. Other conditions being equal, maximum water 

 will be taken up from an atmosphere which is saturated with 

 moisture. The moisture thus taken up is called hygroscopic 

 water, its amount being determined quite largely by the mag- 

 nitude of the colloidal material present in the soil. 



On adding more water, it will be found that the absorptive 

 power of the soil has been by no means satisfied by the hygro- 

 scopic water. Moisture will still be taken up by the colloidal 

 complexes and it will also collect in the interstices between 

 the soil particles. This water which is above and beyond the 

 hygroscopic is generally called the capillary. That part held 

 by the colloidal complexes is very similar in characteristics to 

 the hygroscopic water in that it is tightly held and is more 

 or less immovable. That portion in the interstices, especially 

 the larger spaces, is in the form of a film, is loosely held, and 

 responds to capillary action. While typical capillary water 

 is much different from hygroscopic moisture, it grades into 

 the latter with no sharp line of demarcation. 



Once the capillary capacity of the soil is satisfied, a third 

 form of water may appear. This water is but slightly in- 

 fluenced either by the colloidal complexes or the larger soil 

 particles and consequently is free to respond to the pull of 

 gravity. It is called the free or gravitational moisture and 

 is the water which passes through the soil and appears in 

 streams and rivers bearing in solution the tremendous amounts 

 of soluble salts which are every year lost from the land. 



86. Hygroscopic water. — The hygroscopic water in a 

 soil has been spoken of as the water of condensation, or ab- 

 sorption. It is, however, quite distinct from water condensed 

 on a surface colder than the moist atmosphere in which it is 

 placed. All bodies possess the power, to a greater or less de- 

 gree, of absorbing water even when at the same temperature 



