44 LAND DRAINAGE 



tively small, and the capillary supply in the soil has been 

 reduced, it distributes itself, as it moves downward over 

 the walls of the soil particles, as capillary water. If the 

 quantity is large, it distributes itself in the same manner 

 until all the soil particles are fully invested with capillary 

 water, after which any residue moves downward as gravi- 

 tational water. The capillary supply is thus brought 

 to maximum. 



65. Assists in soil ventilation. One of nature's agents 

 in soil ventilation is gravitational water. This water, 

 entering the soil, fills all the open space, thus driving out 

 the air which previously occupied the open space. As 

 the gravitational water passes downward, fresh air enters 

 to take its place. The operation results in the removal 

 of the air in the soil and replacing it with a supply of 

 new air a complete change of air. 



66. A cleanser of soils. It is said that in the dis- 

 integration of soils, and in the activities of plant life in the 

 soil, there develop certain salts, conventionally spoken of 

 as alkalies, and possibly poisonous by-products of plants, 

 sometimes spoken of as toxines, 1 which, if permitted to 

 accumulate, would work injury to growing crops. In 

 regions of fair rainfall, the frequent passage of gravita- 

 tional water prevents the accumulation of any injurious 

 salts and, to some extent at least, the poisonous by- 

 products of plants. 



67. Standing water or gravitational water in fields 

 destroys soil structure. It has been previously stated 

 that the crumby condition, so desirable and so charac- 

 teristic of a mellow soil, is due, in large measure, to the 

 capillary water investing the soil particles, which in turn 

 make up soil crumbs. The outer film of the capillary 



1 Schreiner and Skinner, Bui. 70, Bureau of Soils. 



