1 68 



Journal of Agricultural Research 



Table IV. — Movement of moisture from a moist aiid 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 ; 



Percentage of moisture in soil. 



Clay: 



At beginning of experiment 



Movement from moist column at 20° to dry column 



ato°C 



Movement from moist column at o"^ to drj^ column 



at 20° C 



Movement from moist column at 40° to dry column 



at o'' C 



Movement from moist column at 0° to drj' column 



at 40° C 



23.29 



• 923 

 .796 



1-552 



• 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 w^ater 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° C. higher than that of the air above. 

 From the surface downward the soil temperature decreases, but in the 

 upper I or 2 inches the diminution is far more rapid than at the lower 

 depths, amounting sometimes and in certain soils to more than 1 1 ° 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° C. at this Station. In arid regions this diflfer- 

 ence must be far greater. 



