THE CONTROL OF SOIL MOISTURE 275 



very low. Moreover, any layer of air-dry soil resists 

 wetting, principally because of the resins and oils that 

 become deposited on the surface of the soil particles. 

 This material, called " agricere," has a low surface tension 

 and the capillary water film is not easily resumed under 

 such conditions. Again, if the soil is well granulated 

 it is able to assume a looser and more open structure. 

 The interstitial angles, which afford spaces for capillary 

 surfaces, are cut down, and the capillary pulling power 

 of the layer is much reduced even if it should assume a 

 film of water. It is evident that looseness and dryness 

 are the essentials in the efficiency of a soil mulch. As 

 long as a mulch is dry, texture is not a very important 

 factor in efficiency, a dry sand being about as effective 

 as a dry clay. Texture is important, however, in the 

 length of time that a mulch will remain effective. Due 

 to the fact that the capillary power of a clay is so great, 

 it will become moist from below after a few days ; while 

 a sand mulch, if there is no rain, will remain dry for an 

 indefinite period. On a heavy clay soil in fine tilth a 

 mulch may be destroyed by moist, foggy weather, or 

 by a number of days of very humid atmosphere ; such a 

 condition, by causing condensation of moisture on the 

 clay, hastens the reestablishment of capillarity with the 

 subsoil, thus allowing moisture to be pumped up and 

 lost. 



193. The soil mulch versus the dust mulch. — A few 

 words will not be amiss at this point regarding the term 

 " dust mulch," which is observed so commonly in soil 

 literature. This term would indicate that the mulch is 

 in a very fine condition, its granulation having been 

 broken down. Such a condition would not be conducive 

 to efficiency, as it would encourage capillarity, while at 



