256 SOILS: PROPERTIES AND MANAGEMENT 



Crop Yield in Pounds to the Acre as Influenced by 

 Different Amounts of Water. Widtsoe 



177. Availability of the water in the soil. — From the 

 discussion already presented regarding the forms of water 

 in the soil, the ways in which they are held, and their 

 movements, it is evident that all the moisture present 

 in a soil is not available for plant growth. Three divisions 

 of the soil water may be made on this basis : unavailable, 

 available, and super-available. 



178. Unavailable soil water. — As has been shown in 

 a previous paragraph, free or capillary water may become 

 of little use to a plant through distance, since capillarity 

 is unable to pump the water fast enough to supply ordinary 

 crop needs. Water near at hand or in the immediate 

 zone of the rootlet may also become unavailable through 

 the obstruction of capillarity, friction instead of distance 

 being the cause in this case. As the rootlet thins the 

 interstitial film at any point, capillarity occurs and 

 water moves toward the absorbing surface. This move- 

 ment is rapid enough for plant needs until the film chan- 

 nels on the particles become thin. (See Fig. 37.) As the 

 zone of hygroscopic influence of the particle is approached 



