112 



Water 



water between the full field capacity, as a maximum, and the wilting 

 coefficient, as a minimum, represents the available water. As indi- 

 cated in Fig. 4.9, a considerable amount of water may still remain in 

 the soil after the wilting coefficient has been reached. A fraction of 

 the capillary water, the hygroscopic water, and the combined water, 

 as well as the water vapor, cannot be obtained by the plant, and these 

 constitute the non-available water. 



SOIL WATER 



Pore space- 



Field capacity < 



Gravitational 



Capillary 



> Available 



Wilting coefficient 



Hygroscopic 



Combined 



Non-available 



Fig. 4.9. Generalized diagram of the forms of soil water. The relative propor- 

 tions and the value of the wilting coefficient differ according to the nature of 



the soil. 



Soils vary considerably in the relative proportions of the different 

 categories of soil water. In some instances the amount of non-avail- 

 able water may actually be larger than the available water. The 

 fact that a portion of the water in the soil is unavailable to land or- 

 ganisms and that water in the sea is unavailable to aquatic organisms 

 not adapted to its osmotic pressure contributes further to the general 

 concept of the universality of the water problem. From the ecologi- 

 cal point of view the actual amount of water present in any habitat is 

 not as important as its availability. 



Moisture in the Air. For many terrestrial organisms the moisture 

 in the soil is chiefly important as constituting the principal source of 

 water, whereas the moisture in the air is mainly significant as con- 

 trolling the loss of water. For this reason the absolute humidity, or 



