256 SOILS: PBOPJERTIES AND MANAGEMENT 



Crop Yicld in Pounds to the Acrd as Influenced by 

 DiprERENT Amounts op Watek. Widtsoe 



Inches 

 OP Water 



18.74 

 21.24 

 23.74 



28.74 

 38.74 

 48.74 

 63.74 



Drt 



Inches 



Dry 



Inches 



JMattkr 



or 



Matter 



or 



Wheat 



W\TEE 



Corn 



Water 



4,969 



13.04 



10,757 



11.17 



5,545 



15.54 



12,762 



13.67 



5,684 



20.54 



13,092 



16.17 



6,279 



25.54 



13,856 



21.17 



6,672 



30.54 



14,606 



26.17 



7,229 



35.54 



15,294 



36.17 



7,999 



60.54 



12,637 



51.17 



Dry 



Matter 

 Potato 



2310 

 2730 

 2925 

 3405 

 4005 

 3660 

 3797 



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 Pig. 37.) As the 

 zone of hygroscopic influence of the particle is approached 



