WATER IN SOILS 17 



2. Physical Water in Soils. 



The term physical water is used in this discussion to des- 

 ignate the water a soil may retain at any time. This is limited 

 only by the water capacity. The maximum amount of 

 physical water occurs in streams, lakes, etc. The minimum 

 amount is found in epiphytic or xerophytic situations. The 

 per cent of physical water in the soil is not a factor of abso- 

 lute value, since only the water available for the use of the 

 plant can be a stimulus to its growth. 



A plant growing in saline soil requires a higher per cent 

 of physical water in order to live. This is illustrated by the 

 data gathered from plants grovcing on the salt-bottoms near 

 Lincoln. Hfliantfius annuus and Panicum crus-galli were 

 found dying in a saline soil with 14.9 per cent of physical 

 water present, while in loam they begin to die only when 

 the physical water drops to less than 10 per cent. Plants 

 can grow well in sandy soil with even less than 5 per cent 

 and in pure sand do not begin to die until the water content 

 falls below 2 per cent. The whole matter seems to hinge 

 upon the ability of the plant to take the capillary and hy- 

 groscopic water from the soil. This is dependent upon the 

 vigor of the plant as well as upon the density of the soil so- 

 lution. The imbibition of water by the roots of the plant 

 is greatly retarded in soils which contain chemicals detri- 

 mental to the assimilatory processes of the plant. The salt 

 marsh plants such as Atriplex argentea seem able to take in 

 a soil solution of greater salinity than is capable of absorp- 

 tion by ordinary plants. The leaves of Atriplex often con- 

 tain crystals of sodium chloride. May it not be that this is 

 formed as a bye-product and serves as a means of elimination 

 of the injurious substances in the soil? Atriplex grows 

 and thrives where a common ruderal plant would die from 

 the effect of the saline soil. 



The welfare of the plant is dependent directly upon the 

 amount of nutriment it may obtain from the soil solution 

 and use in its economy. The soil solution may be so dilute 

 in hydrophytic situations that it is necessary for the plant 



