INTRODXTCTION 



Affinity for water is one of several soil properties that determine infiltration 

 rates. In some soils, particularly sandy soils, there are zones that have little 

 affinity for water and that, in fact) repel water because surfaces of the minerals in 

 such zones are coated with hydrophobic organic substances. The extent to which water 

 repel lency affects infiltration depends not only on its severity, but also on its dis- 

 tribution within the soil mantle. If repellency is mild or is limited to a few scat- 

 tered patches, its effect on infiltration is minor. At the other extreme, no infiltra- 

 tion is possible if the entire soil surface is severely repellent. 



The mechanisms involved in the formation of water-repellent soils are not well 

 understood, partly because the hydrophobic compounds have not been identified. These 

 compounds are formed and released from plants and plant residues by decomposition and 

 pyrolysis. They may be formed in the soil by fungi and other microflora (Bond and 

 Harris 1964) or they may move into the soil after formation elsewhere. 



It is known that hydrophobic compounds are released by combustion of litter and 

 move as vapors into the soil (DeBano 1966). Similarly, the high temperatures induced by 

 fire pyrolyze soil organic matter and vaporize hydrophobic compounds already present 

 in the surface soil. A portion of these vapors escape to the atmosphere, but some dif- 

 fuse downward in the soil to condense on cooler mineral surfaces (DeBano and Krammes 

 1966). This action results in a soil profile that has a wettable surface but a layer 

 of water-repellent soil exists an inch or so below the soil surface. Infiltration into 

 such a soil may cease entirely when the wettable surface layer becomes saturated. 



The following hypothesis is offered to explain water repellency that has not formed 

 in place and has not been caused by fire. Decomposition of plant litter may release 

 compounds that are carried by water into the soil where they are absorbed on mineral 

 surfaces and become hydrophobic as the soil dries. This process is probably slow; 

 several years may be required for formation of severe repellency. Its existence may be 

 difficult to prove. 



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