Oct. 25,1915 
Temperature and Capillary Moisture in Soils 
153 
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 0 or 3 0 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: (1) 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 
