ABSORPTION BY SOILS. 269 



mineral ingredients from solution, and retaining them in such manner 

 as to be readily available to plant growth. (See chap. 8, p. 1 24.) 



In view of the almost invariable conjunction of physical and 

 chemical effects, it may be fairly said that no solution, at least 

 of mineral salts, can pass through the soil without being 

 changed in its concentration and chemical composition. It is 

 sometimes difficult to decide to which of the two classes of 

 effects the several changes may be due. 



Purifying Action of Soils. The disinfecting action of dry 

 soil, absorbing offensive gases from manure piles and from 

 earth closets, has already been alluded to. Similarly it is a 

 matter of common experience that the colored and otherwise 

 offensive drainage from manure piles, tanneries, dyeworks, 

 etc., is not only deodorized but also decolorized when passed 

 through a sufficiently thick layer of clay soil. The filtration 

 through fine sand by which the drinking waters of cities are so 

 commonly purified before delivery to the consumer are famil- 

 iar examples of the same effects. 



Equally familiar, however, is the fact that this power of 

 decolorization and retention of offensive compounds is limited ; 

 that after a while the filtering earth or sand becomes saturated, 

 and afterwards the water or drainage will pass through with- 

 out any sensible purification. 



It is therefore clear that this purifying effect of earth can- 

 not be relied upon for the permanent protection of wells from 

 the surface-drainage from barnyard or house refuse. Even if 

 fissures or layers of sand or gravel should not intervene so as 

 to permit of the direct communication of surface-drainage with 

 wells, it is certain that in the course of a few years at most, 

 the intervening earth will become so far saturated with the 

 noxious ingredients that the latter will pass through unhin- 

 dered, and may contaminate to a considerable extent the 

 domestic supply of drinking water. 



Waste of Fertilizers. The same, of course, holds true in 

 regard to manure-water, or soluble fertilizers of any kind used 

 on the soil of a field. The soil will retain them to a certain 

 extent ; but beyond that limit any surplus added will be quickly 

 washed through into the country drainage by the rains. More- 



