214 



SOIL PHYSICS AND MANAGEMENT 



Available Moisture. The non-available moisture is that per 

 cent found in Soils when permanent wilting occurs. It is possible 

 that a small amount of this may be slowly available but insufficient 

 for rapid or even normal growth. It is supplied to the plant much 

 too slowly. When the capillary force equals the osmotic pressure 



Water offydration 

 Wafer free (at/oo'C.) 



Unavailable < 

 Moisture 



Available 

 Moisture 



(Jnavai/able 

 Moisture 



^ I 



-f- r Hygroscopic Coefficient 

 Wiltinq Coefficient 



> Moisture Equ/ro/ent 



Optimum Moisture Content 



for Ordinary Crops 



Moisture Holding Capacity 



Completely Saturated 



Fia. 94. Diagram showing the relation of different forms of moisture to the available and 

 unavailable moisture of soils. 



or force of the plant the water may be said to be non-available. 

 The difference between the wilting coefficient and the maximum 

 capillary capacity gives the maximum amount of available water. 

 Somewhere between these lies the critical or optimum water con- 

 tent. Under field conditions the difference between the amount of 



