Principles of Wetting Soil 333 



field by a sort of sub -irrigation, which "should be 

 utilized to the fullest extent possible, for then the 

 intervals between irrigations may be longest and the 

 duty of water will be highest. 



If the soil is allowed to become very dry before 

 watering, especially if the texture is close and the 

 grains fine, water will percolate downward less 

 rapidly, and it will move sideways and rise under the 

 influence of capillarity "more slowly, because the air of 

 the soil must be displaced ahead of the water. 



A fine soil, flooded under these conditions, will 

 take water very slowly, because the surface pores be- 

 come filled with water, which is retained with so 

 much force that air bubbles cannot readily rise through 

 it, and the conditions are similar to a jug filled with 

 air bottom upwards under water, the one cannot 

 escape nor the other enter. Such soils, therefore, 

 which must be flooded should not be allowed to reach 

 this dry condition. The case is not so bad when 

 furrow -irrigation is practiced, because the water pres- 

 sure in the furrow may displace the air laterally 

 where it can escape upward between the furrows 

 unhindered by the water. 



On the other hand, there are conditions when it is 

 desirable to take advantage of this hindrance of air 

 to percolation. Where a clover, alfalfa, grass or grain 

 field must be watered by flooding, and where the head 

 of water is small, the fall slight, and the distances 

 the water must be led long, the spreading will be 

 much more rapid and better when the surface soil has 

 become dry. Indeed we have repeatedly tried to 



