Journal of Agricultural Research 
Vol. V, No. 4 
168 
Table IV .—Movement of moisture from a moist and warm column of soil to a dry and 
cold column of soil and from a moist and cold column of soil to a dry and warm column 
of soil —Continued 
Kind and temperature of soil. 
Clay: 
At beginning of experiment. 
Movement from moist column at 20° to dry column 
at o° C. 
Movement from moist column at o° to dry column 
at 20 0 C. 
Movement from moist column at 40 0 to dry column 
at o° C. 
Movement from moist column at o° to dry column 
at 40° C... 
Percentage of moisture in soil. 
17 - 05 
21. 88 
23.29 
• 5*4 
•653 
• 923 
.436 
.502 
.796 
1.180 
1.482 
*• 55 * 
.380 
00 
cn 
O 
.873 
Obviously, then, the temperature has a tremendous influence upon the 
absorptive power of soils for water. This is what might be expected from 
the laws of kinetic energy. According to this law, the energy or motion 
of the molecules increases with temperature, and consequently the ad¬ 
hesive and absorptive forces of the solid matter for liquids or gases 
decreases. These results, then, tend to confirm postulate 2 (p. 148), 
that the attractive forces of the soil for water decrease with a rise in 
^temperature. 
The foregoing experimental results and theoretical considerations sug¬ 
gest very strongly that the efficiency of the soil mulches in conserving 
moisture in the soil is not dependent solely upon their thickness and 
degree of capillary discontinuity between themselves and the moist soil 
below, but also upon their temperature. It is well known that the 
temperature of the surface soils during sun insolation is many degrees 
higher than that of the air immediately above. In some parts of the 
world where the sky is clear and the sun insolation very intense, the 
surface soil may attain a temperature about 40° C. higher than that of 
the air about 4 feet from the ground. Even at this Station the surface 
soil temperature of the mineral soils, and especially of the light sandy 
soils, is very often approximately 15 0 C. higher than that of the air above. 
From the surface downward the soil temperature decreases, but in the 
upper 1 or 2 inches the diminution is far more rapid than at the lower 
depths, amounting sometimes and in certain soils to more than n° C. for 
each inch in depth. When the surface soil is disturbed and a mulch is 
formed, its heat conductivity is decreased, and the high temperature 
attained at the surface is not all conducted downward but is compelled to 
accumulate on the dry mulch and then is radiated back into space. The 
difference in temperature between the mulch and the moist soil below is 
sometimes as high as 15 0 C. at this Station. In arid regions this differ¬ 
ence must be far greater. 
