HYGROSCOPIC MOISTURE IN SOIL 143 



practically amounts to the prohibition of further extrac- 

 tion by the plant. At this stage, if evaporation from 

 the leaves continues, the plants wilt, because they are 

 not supplied with moisture by the roots as rapidly as 

 it is being lost. 



Since plants cannot utilize all of the capillary moisture 

 it is manifestly impossible for them to derive any benefit 

 from the hygroscopic moisture, which is held much 

 more intimately by the soil particles than is the capil- 

 lary moisture. In other words, the hygroscopic moisture 

 capacity of a soil represents that much water unavail- 

 able to plants, to which must be added the proportion 

 of the capillary moisture which is also unavailable. 



69. Amounts of each form. The relative amount 

 of each form of water varies with the soil, and is deter- 

 mined by its physical properties. The forms of water 

 merge one into the other. 



70. Hygroscopic water. The amount of each of 

 the three forms of soil water depends on the physical 

 properties of the soil. These are best explained by first 

 considering the hygroscopic capacity. This depends 

 on the texture of the particles and the content of organic 

 matter. Since hygroscopic moisture is a function of 

 the surface exposed, it results that the larger the surface 

 area exposed by the soil particles, the greater the hygro- 

 scopic capacity of the soil. Reference to the table 

 on page 83 shows fine-textured or clay soils to have the 

 greatest surface area, and these hold the most hygro- 

 scopic moisture. Sand soils, with a relatively small 

 surface area, hold a small amount of this form of water. 

 This fact is illustrated by the following table. 



