WATER OF SOIL IN ITS RELATION TO PLANTS 199 



and a wilting coefficient of 2.6 per cent, has 1.1 per cent, of 

 water of a capillary nature unavailable. A clay loam having 

 a hygroscopic coefficient of 11.4 per cent, and a wilting coeffi- 

 cient of 16.3 per cent, would contain 4.9 per cent, of capillary 

 water unavailable to crops. It must be remembered, however, 

 that under certain conditions plants may reduce the capillary 

 moisture almost to the hygroscopic coefficient. 1 The moisture 

 so obtained is probably not utilized for growth activities. 



Advancing from the wilting, or critical, moisture content of 

 a soil, all the remaining capillary water is found to be avail- 



HYGRO. WILTING . LENTO- MAX. FIELD 



COEFF. X (JPOINT (CAPILLARITY CAPACITY 



*\ ^k. V OPTIMUM WATER ZONE fl/ 



OPTIMUM WATER ZON E 



HYGROS — .]« CAPILLARY WATER +GRAVITAT10NAL 



COPIC WATER 1 i ! WATER 



y >r*j y ^ y 



UNAVA1LABLE\ AVAILABLE SUPERFLUOUS 



AVAILABJ-E UNDER 

 CERTAIN CONDITIONS 



Fig. 35. — Diagram showing the various forms of water that may be 

 present in the soil and their relations to higher plants. 



able for normal plant use. However, when free water begins 

 to appear, a condition detrimental to growth is established, 

 conditions becoming more adverse as the saturation point is 

 approached. This free water is designated as the superfluous 

 water and its presence generates conditions unfavorable to 

 plants. The bad effects of free water on the plant arise 

 largely from the poor aeration that results from its presence. 2 

 Not only are the roots deprived of their oxygen, but toxic 

 materials tend to accumulate. Favorable bacterial activities, 

 such as the production of ammonia and nitrates, are much re- 



1 Alway, F. J., Studies on the Relation of the Non-available Water 

 of the Soil to the Hygroscopic Coefficient; Nebr. Agr. Exp. Sta., Res. 

 Bui. 3, 1913. 



2 It must be kept in mind that in a clayey soil the superfluous water 

 may include some of the upper capillary moisture. 



