182 NATURE AND PROPERTIES OP SOILS 



order to prevent solution. The soil must of course be placed 

 in the tanks, this causing a disturbance of its structural con- 

 dition. As a consequence, data as to rate of flow and com- 

 position of the drainage water are rather unreliable for the 

 first few years. Such an experiment must necessarily be of 

 considerable duration. 



96. Thermal movement of water. — Little has been said 

 as yet regarding this mode of water movement, the vapor 

 flow, which is not peculiar to one form of soil-water but affects 

 them all. It is at once apparent that the movement of water- 

 vapor can be of little importance within the soil itself, since 

 it depends so largely on the diffusion and convection of the 

 soil-air. While the soil-air is no doubt practically always 

 saturated with water-vapor, the loss of moisture by this means 

 is slight. Buckingham x has shown that, while sand allows 

 such a movement to the greatest degree, the loss through any 

 appreciable depth of layer is almost negligible. The question 

 of the thermal movement of water at the soil surface, however, 

 is vital in farming operations. At this point the moisture is 

 exposed to sun and wind, and drying goes on rapidly, the free, 

 capillary, and a part of the hygroscopic water vaporizing in 

 the order named. If the loss of the moisture in the surface 

 layer of soil was the only consideration, the problem would 

 not be serious; but the movable water of the whole soil sec- 

 tion must be considered also. As the films at the surface be- 

 come thin, a capillary movement begins, and if the evapora- 

 tion is not too rapid a considerable loss of water may occur in 

 a short time. The moisture thus lost is that of most value 

 to plants. The evaporation from the bare soil in the Rotham- 

 sted lysimeters 2 averaged about seventeen inches a year, with 



1 Buckingham, E v Studies on the Movement of Soil Moisture; U. S. 

 Dept. Agr. Bur. Soils, Bui. 38, pp. 9-18, 1907. 



See also Bouyoucos, G. J., Effect of Temperature on Movement of 

 Water Vapor and Capillary Moisture in Soils; Jour. Agr. Ees., Vol. V, 

 No. 4, pp. 141-172, Oct., 1915. 



2 Warington, E., Physical Properties of the Soil, p. 109; Clarendon 

 Press, Oxford, 1900. 



