THE CONSTITUTION OF THE SOIL 



57 



Some ions, however, are not precipitated in the soil, including 

 CO 3 , SO 4 , NO 3 , Cl, Mg, Ca, Na ; l these are, therefore, the chief con- 

 stituents of drainage water. 



Organic substances, particularly those of high molecular weight, 

 are also withdrawn from their solutions, but the reaction is apparently 

 of a different type, since nothing appears to be given up from the soil 

 in exchange. The result is of extreme importance ; practically the 

 whole of the organic matter added to the soil by plant residues or 

 manure remains near the surface unless carried down mechanically by 

 some agency such as earthworms. Even when heavy dressings of dung 

 are annually supplied at Rothamsted there is after fifty years no ap- 

 preciable enrichment of the subsoil in nitrogen (Table XX VI.). 



TABLE XXVI. NITROGEN IN BROADBALK WHEAT SOILS, 1893. 

 Per cent, of dry soil. 



Ib. per acre. 



The purification of sewage by land treatment affords further illus- 

 trations of the absorptive power of soil for organic matter. 



The mechanism of the absorption has recently been carefully 

 studied. Way's purely chemical hypothesis held the field for many 



1 From the time of Aristotle it has been known that sea water could be " desalted " by 

 filtering through sand or soil. But it has recently been shown by Von Lippmann and 

 Erdmann (Chem. Zeit., 1911, xxxv., 629) that the water first running through the sand 

 filter is not desalted sea water, but displaced water. When this has all gone the salt 

 water runs through unchanged. 



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