HYGROSCOPIC WATER IN THE SOIL 103 



sible to rid any natural substance wholly of water 

 without heating it to a high temperature. This 

 water which, apparently, belongs to all natural 

 objects is commonly called hygroscopic water. 

 Hilgard states that the soils of the arid regions con- 

 tain, under a temperature of 15° C. and an atmos- 

 phere saturated with water, approximately 5^ per 

 cent of hygroscopic water. In fact, however, the 

 air over the arid region is far from being saturated 

 with water and the temperature is even higher than 

 15° C, and the hygroscopic moisture actually found 

 in the soils of the dry-farm territory is considerably 

 smaller than the average above given. Under the 

 conditions prevailing in the Great Basin the hygro- 

 scopic water of soils varies from .75 per cent to 3|- per 

 cent ; the average amount is not far from 1^ per cent. 

 Whether or not the hygroscopic water of soils is 

 of value in plant growth [is a disputed question. 

 Hilgard believes that the hygroscopic moisture can 

 be of considerable help in carrying plants through 

 rainless summers, and further, that its presence pre- 

 vents the heating of the soil particles to a point 

 dangerous to plant roots. Other authorities main- 

 tain earnestly that the hygroscopic soil-water is 

 practically useless to plants. Considering the fact 

 that wilting occurs long before the hygroscopic water 

 contained in the soil is reached, it is very unlikely 

 that water so held is of any real benefit to plant 

 growth. 



