152 
Journal of Agricultural Research 
Vol. 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 
curvature 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 water 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 (1) 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 
