28 SOILS AND FERTILIZERS 



capillarity depends upon the size and arrangement of 

 the soil particles. Water may be raised by capillarity 

 to a height of several feet. Ordinarily, however, the 

 capillary action of water is confined to a few feet. The 

 arrangement of the soil particles influences greatly the 

 capillary power of the soil. Usually from 30 to 60 per 

 cent, of the bulk of a soil is air space : by compacting, 

 the air spaces may be decreased ; by stirring, the air 

 spaces are increased. In some soils of a close texture an 

 increase in air spaces results in an increase of capillary 

 spaces and of water-holding capacity, while in other 

 soils, as coarse sandy soils, increasing the air spaces 

 decreases the capillary spaces and the water-holding 

 capacity. The best conditions for crop production 

 exist when the soil contains water to the extent of 

 about 40 per cent, of its total capacity of saturation. 



21. Hydroscopic Water. — By hydroscopic water is 

 meant the water content of the soil atmosphere. 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 atmos- 

 phere 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. 



22. Loss of Water by Percolation. — Whenever a 

 soil becomes saturated, percolation or a downward 



