Thermal Movements of Soil Moisture. 175 



THERMAL MOVEMENTS OF SOIL MOISTURE. 



Besides the gravitational and capillary movements of 

 soil moisture there are others due to the molecular vibra- 

 tions set up in the soil-air and water by the absorbed solar 

 energy. 



203. Hygroscopic Soil Moisture. It is seldom if ever true 

 that any solid surface, even when in the dryest air, can 

 be found which is not invested with a film of moisture 

 of greater or less thickness. It is also true that even when 

 all moisture has been driven from the surface of a solid 

 by drying at the high heat of 200 C., the same body will 

 again become coated with moisture when exposed to a 

 moisture-bearing atmosphere. Water thus collected on 

 the surface of solids is called hygroscopic moisture. 



204. The Movements of Hygroscopic Moisture, It will be 

 seen that the movements of hygroscopic moisture are the 

 same as those of evaporation. The same molecular at- 

 traction which causes the capillary rise of water in a glass 

 tube tends to collect the water molecules, which may be 

 moving about in the air, upon solid surfaces. So when 

 a dry soil is exposed to a damp atmosphere some of the 

 moving water molecules are brought in contact with, and 

 retained by, the surfaces of the soil grains. The moisture 

 will go on accumulating upon the soil grains until the rate 

 of evaporation from them equals the rate of condensation. 

 Since the water molecules are attracted to the soil grains 

 more strongly than they are attracted to one another the 

 water in immediate contact with the soil grains cannot 

 evaporate as readily as that which is further removed when 

 the water films are thick, as they are in a well saturated soil. 



Neither can the innermost layers of molecules adhering 

 to the spil grains escape to enter the root hairs of plants by 

 osmotic pressure as readily as those from the layers farther 

 removed, and hence there must always be a certain quan- 

 tity of water upon the surfaces of soil grains which neither 



