198 MOVEMENTS OF SALTS IN THE SOIL 



water at the surface. After a short time the upper layer of 

 the soil, through which the column of water has passed, will 

 be found entirely free from soluble salts (saving of course 

 those contained in the rain, or those formed by a new chemical 

 action on the soil), while at the lower surface of the descend- 

 ing column there will be found a narrow layer of concentrated 

 salt solution, containing all the salts which have been re- 

 moved from the surface soil. If this narrow layer or band 

 of salt solution during its passage downwards reaches an 

 outfall, as for example a bed of gravel, the salts carried down 

 by the rain are permanently removed from the upper soil. 



For the action just described to take place in its most 

 striking manner, it is necessary that the rainfall should be 

 considerable and continuous ; for diffusion is always at work, 

 tending to re-establish the equal distribution of salts in the 

 wet soil. With a heavy rainfall, occurring in a short time, 

 the layer of salt solution carried downwards will be very 

 narrow and highly concentrated. If, on the other hand, the 

 rainfall is light and long continued, the layer of salt 

 solution will be wider and the solution weaker. In any case, 

 as soon as percolation ceases, the salts carried down will 

 begin to distribute themselves again in all directions through 

 the wet soil, unless in their downward course they have 

 reached an area of discharge, and have thus been removed 

 from the scene of action. 



If, however, the soil is moist when rain commences, 

 although we have, as before, the same descending column of 

 rain water, we have no longer a layer of highly concentrated 

 salt solution pushed before it ; the rain water now simply 

 displaces the solution already existing in the soil, and pushes 

 this before it in a practically undiluted condition. 



