152 Journal of Agricultural Research vo1.v,no.4 



maximum thermal translocation of water. After this point of inter- 

 section the willingness of the warm soil to give up water is large, but 

 since the. effective pull is being reduced to a minimum the water is not 

 moved. If a parabola is now drawn along the lines WP, with its maxi- 

 mum value at the point of intersection, then this theoretical curve 

 will agree almost perfectly with the real one in figure 3. 



The serious fault with the above illustration (fig. 4) is that the total 

 effective pull tends to become zero, and theoretically this should not 

 be the case, because while the pull due to the attractive power of the 

 soil for water and to the curvature of the capillary films will ultimately 

 become zero, the pull due to the increased surface tension of the soil 

 water should not become zero, but should remain the same for all mois- 

 ture contents. Hence, figure 4 illustrates more correctly only the ther- 

 mal translocation of the water as due to all the other forces except 

 the surface tension of water. 



The next important question to consider is the mode and amount of 

 thermal translocation of water in field soils as suggested by the forego- 

 ing laboratory experimental data. Under field conditions the soil mois- 

 ture exists practically always in a gradient form. As the water content 

 tends to decrease upward from the water level, the forces due to the 

 cur\'ature of the capillary film and to the attractive power of the soil 

 for water increase correspondingly; consequently the pull is upward. 

 The soil temperature also exists in a gradient form, but this reverses 

 itself diumally and therefore modifies these pulling forces. During the 

 day the temperature at the upper depths is higher than that below; the 

 attractive and adhesive forces of the soil for water and the surface ten- 

 sion of water are decreased, so that the total upward effective pull is 

 diminished correspondingly. Inasmuch as the temperature below is 

 less than that above, the effective pull due only to the increased attrac- 

 tive and adhesive forces of the soil for vv^ater and to the surface tension 

 of the soil water should occasion a downward movement of moisture. 

 Since, however, the water-attractive forces of the soil below are more 

 satisfied than those of the soil above, the downward pull due only to 

 the attractive adhesion and surface tension as increased by a lower 

 temperature is very small in comparison with the upward pull. Hence, 

 during the day the moisture movement is upward. During the night 

 nearly all of the above forces act in a parallel direction and favor an 

 upward movement. Therefore, the thermal movement of moisture in 

 soils is always upward and never downward. 



The extent to which moisture will move during the night from the 

 warmer soil below to the colder soil above will depend (i) upon the 

 soil temperature gradient — ^that is, upon the difference in temperature 

 of the various adjacent depths — and (2) upon the gradient or amount of 

 moisture content at the various depths. In the preceding series of 



