418 Mineral Nutrition of Plants 



the vertical distance from the cup down to the free surface of the 

 water in the manometer. This tension can be read directly from a scale 

 on the mercury manometer {102). Tension readings measure the tenac- 

 ity with which water is held by soil, but are not simply related to mois- 



n 



r 



L ab 



L bc 



-cd 



fR\ 



1 



I 



I 



V\ \\W\\\V 



Hab 



T 



H 



cd 



Figure 2. Porous cup-manometer system for measuring the 

 hydraulic head and hydraulic gradient of water in unsaturated 

 soil. See page 41J. 



ture movement because gravity force is not taken into account. Hy- 

 draulic head and hydraulic gradient on the other hand do take gravity 

 into account. 



As defined above, the hydraulic head of the water in the soil adjacent 

 to a porous cup is equal to the elevation of the free water surface in the 

 corresponding water manometer. In a connected soil moisture system, 



