16 STUDIES IN THE VEGETATION OF THE STATE 



Soils in nature always overlie strata of other kinds, and 

 the water contained in them tends to settle by gravity to the 

 stratum below. The rate of percolation depends upon the 

 capillarity of the soil, and upon the impeding action of the 

 hygroscopic water; probably upon even more obscure fact- 

 ors such as are involved in the varying chemical composi- 

 tion of the soil solutions. 



According to Briggs* "The water contained in the soil in 

 regard to the manner in which it is retained may be consid- 

 ered to be of three kinds, gravitation water, capillary water 

 and hygroscopic water. Gravitation water is thab portion 

 which is in excess of the amount which the soil is able to 

 retain under existing conditions, and is consequently free to 

 drain away. The capillary water is that part which would 

 be retained in the capillary spaces under these conditions, 

 and which is capable of movement through capillary action. 

 The hygroscopic water is that found on the surface of the 

 grains, which is not capable of movement through the action 

 of gravity or capillarity. 



"The maximum amount of water which a given soil may 

 contain depends upon the resultant effect of two forces — 

 gravitation and surface tension. The force due to gravity 

 is proportional to the mass of the liquid considered and is 

 always directed vertically downward. In other words, it is 

 the weight of the liquid. This mass of liquid would there- 

 fore leave the soil if unopposed by the action of some other 

 force, the vertical component of which is acting along the 

 same line as the force of gravity but must be equal to it and 

 in opposite direction." 



Prom this it may be readily deduced that in sandy soils 

 the amount of gravitation water is at a maximum, and capil- 

 lary water as well as hygroscopic water would be at a min- 

 imum. The clays, being the finest grained, hold the maxi- 

 mum amount of capillary and hygroscopic water, but here 

 the gravitation water would be at a minimum. The other 

 soils in quality vary between clay and sand. 



♦Briggs, Lyman J., Bull. 10, U. S. Uept. of Agr. Division of Soils, p. 

 6, 1897. 



