DUST MULCHES '20'.] 



climate. Mulches formed from the natural soil are 

 commonly termed "dust mulches," or more expressively 

 "dust blankets." A dust mulch is simply an air-dry 

 layer of the natural soil covering the moist soil below. 

 It may be in a compact condition, but ordinarily it is 

 loose and friable. Its creation is dependent on the prin- 

 ciples explained on pages 172 and 189 concerning capil- 

 lary movement and diffusion of water-vapor. Under arid 

 conditions where the atmosphere is dry and hot, and in 

 free circulation, the surface soil is quickly dried out 

 after an application of water. This drying takes place 

 so rapidly that the capillary films quickly become so 

 thin that movement is stopped, and no more water is 

 brought to the surface. The soil may be ever so hard 

 and compact, but so long as it is kept dry it very effec- 

 tively preserves the moisture below. The more rapid 

 the loss, the more quickly will the mulch condition be 

 created, and therefore the less the total loss of water 

 is likely to be. This has been demonstrated by Bucking- 

 ham in some experiments in which arid climate conditions 

 were created at the surface of a capillary column 

 forty-six inches in height. The soil was a fine sandy 

 loam, the equilibrium distribution of water in which 

 is shown in the curve on page 148. At first, the loss under 

 the arid conditions was very rapid and exceeded the 

 humid conditions, but the rate of loss soon dropped 

 considerably below the humid column, and continued 

 to fall behind during the twenty days of the experiment. 

 This experiment was conducted under the most difficult 

 conditions for creating a mulch, since the soil used was 

 of intermediate fineness and had a large effective capil- 



