106 Physics of the Soil. 



It can be seen that the nitric nitrogen has increased in 

 both series to a depth of 3-inch cultivation and it has in- 

 creased with the frequency of the cultivation. 



123. Soluble Salts Affect the Movement of Soil Moisture 



The varying strength of salt solutions in soil moisture mod- 

 ify both the movement of moisture in the soil and its rate 

 of loss from the surface. These movements are influenced 

 (1) by changes in the intensity of surface tension; (2) 

 by changes in the internal friction of the soil moisture or 

 its viscosity; and (3) by modifications of the surface of 

 the soil due to deposits of salts upon and within it, where 

 evaporation is taking place. 



124. Modification of Surface Tension by Soluble Salts 

 As a general rule the surface tension of a strong soil solu- 

 tion is greater than that of a weaker one, or of pure water, 

 and in so far as this influence is operative it tends to in- 

 crease the rate of capillary movement toward the surface 

 or toward the roots of plants. 



125. Salts in Solution Lessen Rate of Evaporation. When 

 water has been brought to the surface of the soil by capil- 

 larity it has yet to evaporate and unless this takes place the 

 surface soil would become capillarily saturated with water 

 and remain so. Since salts in solution increase the sur- 

 face tension it will require a greater energy a higher 

 temperature to throw the water molecules off into the air 

 than would be required to do so from the surface of pure 

 water and hence the evaporation from soil solutions rich 

 in salts is slower than it is from weaker ones under other- 

 wise like conditions. As the salts become concentrated at 

 the surface by evaporation the moisture becomes a stronger 

 and stronger solution and hence the rate of evaporation be- 

 comes less and less so far as it can be influenced by this 

 factor, in this way. 



126. Viscosity of Soil Water Modified by Soluble Salts. 

 The internal friction of soil moisture is made greater by 



