524 STATE BOARD OP AGRICULTURE. 



inch 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 accumu- 

 late on the dry mulch and then is radiated back into space. The dif- 

 ference in temperature between the mulch and the moist soil below is 

 sometimes as high as 10° C. at this Station. At the arid regions this 

 difference must be far greater. This excessively greater temperature of 

 the dry mulch diminishes the adhesive and absorptive forces of the dry 

 soil so that its capacity and intensity to withdraw water from the moist 

 soil below are either entirely prohibited or greatly reduced. The result 

 is that the water is saved from direct evaporation. On the other hand, 

 during the night the soil temperature reverses itself and becomes lowest 

 at the surface and increases with the depth, but its difference between 

 the mulch and moist soil is generally not as great during night as during 

 the sun insolation. Since the attractive and adhesive force of the dry 

 soil and the surface tension of the soil water are increased by the low 

 temperature, the tendency of the soil moisture is to move upwards very 

 energetically. To what extent this movement occurs cannot be stated 

 with certainty because the moisture, not very far below the mulch, is 

 held with a great force and is given up with great reluctancy, unless 

 moisture moves from further depth below and satisfies the absorptive 

 power, and thickens the surface and capillary films. 



Furthermore, the amount of water moved will depend upon the tem- 

 perature gradient, that is, upon the range of temperature between the 

 surface and lower depths. As already stated, this temperature gradient 

 at night between adjacent depths is always small but is most marked 

 during the summer and fall and least during the spring. Any water, 

 however, that the mulch pulls up during the night is certain of being 

 evaporated during the day. May it not le then that an apprecialjle 

 amount of icuter is lost from the soil in this manner? 



Temperature not only tends to conserve moisture in the soil after the 

 mulch is formed but also aids and hastens the formation of this mulch. 

 It has been seen that as the temperature of the moist soil at the upper 

 depth increases the surface tension of the soil water and the adhesive 

 and absorptive forces of the soil, decrease. The upward pulling force, 

 therefore, is diminished, and the water is not brought up with sufficient 

 rapidity to keep the upper layers moist, so that a mulch is formed at the 

 top. The diminution of the surface tension of the soil water at or near 

 the surface is very large during the sun insolation, and far greater than, 

 the increase during the night, because during the sun insolation the soil 

 absorbs heat from the sun very rapidly and since the soil is a poor con- 

 ductor of heat, the heat is allowed to accumulate at the surface and 

 raise its temperature far above that of the next layers. 



The foregoing considerations have been deduced from the experimental 

 data, and from the laws of kinetic energy of matter, surface tension of 

 liquids, etc., in their relation to temperature. It is now of great im- 

 portance as well as of high interest to know if these deductions can be 

 verified experimentally. The type of experiment Avhich the writer prob- 

 ably would have performed to test out whether or not the temperature 

 does tend to consen^e moisture in the soil, has fortunately been per- 

 formed by Buckingham^^ for another purpose. In his studies on the 



15U. S. Dept. of Agr., Bureau of Soils, BuUetin No. 38 (1907). 



