32 SOILS AND FERTILIZERS 



21. Hydroscopic Water. By hydroscopic water is 

 meant the water that is held mechanically in the soil 

 and is not removed by air drying. The air which 

 occupies the non-capillary spaces of the soil is charged 

 with moisture in proportion to the water in the soil. 

 Under normal conditions the soil atmosphere is nearly 

 saturated. When soils have exhausted their capillary 

 water, the water in the soil atmosphere is correspond- 

 ingly reduced. The available supply in other forms 

 being exhausted, the hydroscopic water cannot con- 

 tribute to plant growth unless supplemented by 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 

 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 percolation are 

 greater than from soils which have a larger number 

 of capillary spaces, and a smaller number of non-capil- 

 lary 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 



