THE MOISTURE IN THE SOIL 49 



67. Hygroscopic water is that which is held 

 firmly as a film surrounding each particle of 

 soil. It does not move under the influence of 

 gravity or capillarity, and it is held so firmly 

 that it is driven off only when the soil is 

 exposed to a temperature of 212 Fahr. The 

 dryest road -dust firmly holds its hygroscopic 

 water, and it may constitute from 2 to 3 per 

 cent or more of the weight of the soil. If of 

 service to plants in any way, it is only dur- 

 ing the most excessive droughts, in which case 

 it may sustain the plants for a time, until 

 capillary water is supplied. 



68. Both capillary and hygroscopic water are 

 frequently referred to as "film moisture," from 

 the fact that they are held as a film of greater 

 or less thickness around the soil particles. 

 That part which has the most intimate and 

 permanent contact with the particle is the hygro- 

 scopic water, and the outer part of the film, 

 which may move away from the soil particle, is 

 the capillary water. Very wet land is that 

 which contains too much free water ; whereas, 

 soils which are dryish and crumbly usually 

 contain sufficient water for the growing of 

 plants. That is, lands in good condition for 

 the growing of crops are moist, not wet ; and 

 we may, therefore, speak of the moisture of the 

 soil rather than the water of the soil. 



