Oct. 25. 191S Temperature and Capillary Moisture in Soils 169 



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 the 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 ten- 

 sion of the soil water are increased by the low temperature, the tendency 

 of the soil moisture is to move upward very energetically. To what 

 extent this movement occurs can not 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 a farther 

 depth below, satisfies the absorptive power, and thickens the surface and 

 capillary films. 



Furthermore, the amount of water moved will depend upon the 

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

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

 gradient at night is most marked during the summer and fall and is 

 smallest 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 be, then, that an appreciable amount of water 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 

 conductor 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 experi- 

 mental 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 importance as well as of high interest to know whether these 

 deductions can be venfied experimentally. The type of experiment 

 which the writer probably would have performed to test out whether or 

 not the temperature does tend to conserve moisture in the soil has 



