26 SOILS AND FERTILIZERS 



21. Hydroscopic Water, By hydroscopic water is 

 meant the water content of the soil absorbed from the 

 atmosphere. The air which occupies the non-capillary 

 spaces of the soil is charged with moisture in propor- 

 tion to the water in the soil. Under normal condi- 

 tions the soil atmosphere is nearly saturated. When 

 soils have exhausted their capillary water, the water 

 in the soil atmosphere is correspondingly reduced. 

 The available supply in other forms being exhausted, 

 the hydroscopic water cannot contribute to plant 

 growth unless the soil is supplied with hydroscopic 

 water from heavy fogs. 



22. Loss of Water by Percolation. Whenever a 

 soil becomes saturated, percolation or a downward 

 movement of the water begins. The extent to which 

 losses by percolation may occur depends upon the 

 character of the soil and the amount of rainfall. 

 When soils are covered with vegetation, the losses by 

 percolation are less than from barren fields. In all 

 soils which have only a limited number of capillary 

 spaces and a large number of non-capillary spaces, the 

 amount of water which can be held above the bottom 

 water is small. From such soils the losses by perco- 

 lation are greater than from soils which have a larger 

 number of capillary spaces, and a smaller number of 

 non-capillary spaces. In coarse sandy soils many of 

 the spaces are too large to be capillary. 



If all of the water which falls on some soils could be 

 retained and not carried beyond the reach of crops by 

 percolation, there would be an ample supply for agri- 

 cultural purposes. To prevent losses by percolation, 



