EVIDENCE OF HISTORY 245 



year or a generation, but over thousands or 

 hundreds of thousands of years they accom- 

 plish enormous results. 



These same agencies are operating at all 

 times in the soil. The most important to the 

 the farmer of to-day is the movement of 

 ground waters. 



Water exists in the soil in four different 

 forms: the water of hydration, hygroscopic 

 water, capillary water, and gravitational water. 

 The water of hydration is that which combines 

 chemically with the minerals as hydrates and 

 is not, as a rule, available for plants. 



Hygroscopic water is that which adheres to 

 the soil grains as films and contains the min- 

 eral nutrients of plants under high pressure. 



Capillary water is that which moves freely 

 between the soil grains by capillary attrac- 

 tion. This is the principal source of food for 

 plants, the "carrier" as it were of the food. 



Gravitational water is that which escapes to 

 the depths, as excess beyond the ability of the 

 soil to retain it, and in the best farming prac- 

 tice is carried off by means of drains. 



The point to remember in this connection 

 is that the water which goes down the gravi- 

 tational water following a rain is not of so 



