Physical Effects of Soluble Salts. 107 



the presence of salts in solution and the more concentrated 

 the soil solution is the greater is the internal friction, and 

 hence the slower must be the rate of flow, and it may be that 

 the much slower rate of capillary movement in a compara- 

 tively dry soil is to a considerable extent due to this in- 

 creased viscosity or internal friction. But as one effect of 

 the salt in solution is to increase the surface tension, while 

 the other decreases the flow by increasing the friction, the 

 two influences work against each other, making the com- 

 bined result less than it would be could either act alone. 



127. Deposits of Salts after Evaporation May Lessen Loss 

 of Soil Moisture. Where water rich in salts is being evap 

 orated from a soil these salts may accumulate upon the sur- 

 face and form a sort of mulch more or less effective accord- 

 ing to its texture ; or they may be deposited as a crust upon, 

 over and between the soil grains, which may nearly close 

 the capillary pores and in this way lessen the loss of water 

 by evaporation. Such a closing of the pores is likely to be 

 more harmful in shutting out the air and in lessening the 

 freedom of entrance of water after rains than it can render 

 assistance in conserving soil moisture. 



