CHEMICAL ABSORPTION BY SOILS. 243 



that date says: "Earth, especially cla}*, seizes upon the sol- 

 uble matters intrusted to it, and holds them back, in order 

 that it may gradually furnish them to plants according to their 

 needs." 



653. When dilute solutions of a salt are slowly filtered through 

 sand which contains a good admixture of clay, the water passes 

 out for a time without more than a trace of the salt, and in 

 some cases all the salt is retained by the soil. Even sewage 

 liquids can by this method be freed from their offensive ingre- 

 dients. This phenomenon of filtration is due to adhesion (that 

 is, the attraction which the surface of one kind of matter has 

 for another kind of matter). The substances which are removed 

 by the particles of soil are so fastened to them that even 

 when the soil is washed in pure water only traces of them are 

 removed. 



654. Chemical absorption by soils. Besides this physical ad- 

 hesion, there are exhibited by many soils certain chemical phe- 

 nomena also, which have been collectively termed chemical 

 absorption. If a solution of potassic nitrate is filtered through 

 a well-pulverized clay soil containing an admixture of insoluble 

 compounds of magnesium and calcium, such as are met with in 

 almost any ordinary soil, the water which drains off will con- 

 tain very little if indeed airy potassium ; but it will have, in- 

 stead, magnesium and calcic nitrate in appreciable amount. But 

 this absorptive power of a soil is soon satisfied ; for after 

 a certain amount of potassium has been removed no more is 

 taken up. 



The strength of the saline solution affects the amount of 

 absorption, more of the base being absorbed from strong solu- 

 tions. Different substances are absorbed by the soil in different 

 amounts ; thus in the experiments by Peters the bases were 

 absorbed in the following order: (1) Potassa, (2) Ammonia, 

 (3) Soda, (4) Magnesia, (5) Lime. Different soils absorb the 

 same substance in different amounts, depending upon the physi- 

 cal condition of the soil, but chiefly, it is believed, upon the 

 mode in which the substance is combined ; thus, more potassa is 

 absorbed from the phosphate than from the carbonate, and more 

 from the latter than from the sulphate. 



In general it may be said that the salts of the alkalies and 

 the alkaline earths are so absorbed by rich soils that the bases 

 are retained in new combinations, while the acids pass off, 

 having also, of course, formed new combinations. The phos- 

 phates and silicates are retained undecomposed. The case of 



