Oct. 25. 191S Temperature and Capillary Moisture in Soils 1 53 



experiments the temperature amplitudes of 20° and 40° C. were employed. 

 In nature, however, so large and sharp variations in temperature between 

 adjacent depths never occur during the night; they do occur, however, 

 at the upper depths between day and night. Soil-temperature investiga- 

 tions which are being conducted at this Station show that in the early 

 morning, when the temperature gradient is most marked, the tempera- 

 ture of the bare mineral soils increases sometimes in the summer and fall 

 at the average rate of about 2° or 3° for each inch of depth down to 

 about 4 inches, and then this rate becomes less. In cropped soils, .where 

 the temperature remains more constant, this rate of increase of tempera- 

 ture with depth is still less. Hence, the amount of thermal trans- 

 location of water that would occur during a single night would be very 

 small. On the other hand, the maximum thermal translocation of 

 water obtained in the preceding series of experiments was procured 

 from a column of soil with uniform moisture content. As will be shown 

 subsequently, there is no doubt whatever that this maximum thermal 

 translocation of water in the various soils would have been far greater 

 if the moisture content of the cold column was less than that of the 

 warm column of soil. In nature, as already mentioned, the moisture 

 exists in a gradient form ; consequently the movement of water is upward 

 and the forces of the factors which cause this upward movement are 

 increased during the night. Therefore, while the amount of thermal 

 translocation of water during a single night in soils under field conditions 

 may not be as great as that obtained in the foregoing series of experi- 

 ments, yet it will be quite appreciable; and since the process is repeated, 

 the sum of the translocation for all the nights during the vegetative 

 season will probably be considerable. 



The moisture content at which the maximum thermal translocation 

 of water occurs, or what has been designated as the thermal critical 

 moisture content, is very significant and needs further consideration. It 

 would be of very great interest to know, for instance, the thickness of 

 the water film around the particles at this degree of moisture. This 

 thickness could be calculated if all the soil grains were solid and spherical. 

 The particles of the soils used, however — and these are the commoner 

 types of agricultural soils — are neither spherical nor solid. Nearly all 

 the particles in agricultural soils can be said to be irregular in shape. 

 Some of them are solid and enveloped with a colloidal coating; others 

 are compound aggregates, or "crumbs," and are porous; and still others, 

 mainly of the peat nature, are of a sponge structure and are necessarily 

 porous. The particles of a soil or soils may be classified under two 

 categories: (i) Particles which are solid and have only an external 

 surface and (2) particles which are partly or wholly porous and possess 

 both an external and internal surface. In the solid and cleaned surface 

 particles the water film is soread over the surface, but the film of water 



