336 HOW CROPS FEED. 
are no longer a solution of nitrate of potash, but one of 
nitrates of lime, magnesia, and soda. The potash has 
disappeared from solution* and become a constituent of 
the soil, while other bases, chiefly lime, have been dis- 
placed from the soil, and now exist in the solution with 
the nitric acid. 
If we operate in a similar manner on a fresh tube 
of soil with solution of salt (chloride of sodium), we 
shall find by chemical examination that the soda of the 
salt is absorbed by the soil, while the chlorine passes 
through in combination with lime, magnesia, and potash. 
In case of sulphate of magnesia, magnesia is retained, and 
sulphates of lime, etc., pass through. With phosphates 
and silicates we find that not only the base, but also these 
acids are retained. 
Law of Absorption and Displacement.—From a great 
number of experiments made by Way, Liebig, Brustlein, 
Henneberg and Stohmann, Rautenberg, Peters, Weinhold, 
Kiillenberg, Heiden, Knop, and others, it is established 
as a general fact that all cultivable soils are able to de- 
compose salts of the alkalies and alkali earths in a state 
of solution, in such a manner as to retain the base together 
with phosphoric and silicic acids, while chlorine, nitric 
acid, and sulphuric acid, remained dissolved, in union with 
some other base or bases besides the one with which they 
were originally combined. The absorptive power of the 
soil is, however, limited. After it has removed a certain 
quantity of potash, etc., from solution its action ceases, it 
has become saturated, and can take up no more. If, 
therefore, a large bulk of solution be filtered through a 
small volume of earth, the liquid, after a time, passes 
through unaltered. 
* The absence of potash may be shown by aid of strong, cold solution of 
tartaric acid, which will precipitate bitartrate of potash (cream of tartar) from 
the original solution, if not too dilute, but not from ‘that which has filtered 
through the soil. The presence of lime in the liquid that passes the soil may be 
shown by adding to it either carbonate or oxalate of ammonia: 
