142 THE PRINCIPLES OF SOIL MANAGEMENT 



ditions, as for example the swamp type of vegetation. 

 About the only cultivated crops of this sort are rice 

 and cranberries. Practically all of the common culti- 

 vated crops, from vegetables to fruit trees, are adapted 

 to growing in soil from which the gravitational moisture 

 has been removed. The gravitational water is directly 

 injurious to the growth of these plants, and its practical 

 removal from the soil constitutes the practice of agri- 

 cultural drainage, later to be considered as a phase 

 of soil management, It may therefore be stated that 

 gravitational water in the root zone is injurious to most 

 farm crops, and consequently it is in a sense unavailable. 

 It is the film or capillary moisture which supports plants. 

 The roots of ordinary crops are adapted to take the 

 moisture needed by threading their way between the 

 soil particles, where they may come in intimate contact 

 with these moisture films and absorb the needed supply 

 of water, without being excluded from the air supply 

 which promotes their growth. For, in the capillarily 

 moist soil, the water is retained chiefly in the very 

 small spaces, and the large spaces are occupied by air. 

 While capillary moisture is practically the only form 

 upon which plants depend, it is not possible for them 

 to use all of this form of moisture in the soil. They take 

 their supply most readily when the films are relatively 

 thick, and when the globules between the particles 

 are large. But, as the thickness of the films is reduced 

 by the use of the plant and by evaporation, it becomes 

 increasingly difficult for the plant roots to take their 

 needed supply. Before all of the capillary moisture has 

 been removed, this difficulty becomes so great that it 



