ACTION OF SALTS. 113 



which enter, in a small proportion only, into the composition 

 of plants. 



157. In the first case, the salts admit of a larger quantity 

 being applied, than in the second. By the first, plants are 

 fed, by the second, they are poisoned ; for the base of all 

 salts acting as has been explained, the acid is eliminated ; if 

 this is set free in large quantities, and its elements can be 

 taken up and converted by the plant, well, good effects 

 follow ; if, on the other hand, the elements of the acid are 

 such as the plant does not demand, they act like poison on 

 the animal economy. 



158. Let salts be divided, on this principle of the peculi- 

 arity of action depending upon the acid of the salts, into two 

 classes; the first nourishing, the second poisoning plants. 

 The first class contains, a. carbonates, b. nitrates, c. phos- 

 phates. 



159. The action of the first class is to be studied under its 

 three divisions, (a. 158), Carbonates. These include a 

 very large portion of all salts used in agriculture. It 

 includes limestone (14), marble, old mortar, shells, shell 

 marl. In all these cases, the base or lime let loose by the 

 action of the living plant, acts at once, as caustic lime upon 

 insoluble geine, and unconverted vegetable fibre, changing 

 these into soluble vegetable food ; while the carbonic acid 

 acts immediately upon silicates, decomposing these, and 

 upon tiiegeates in the soil, converting these into super-geates. 

 Carbonates of alkalies, as ashes, &;c., act at once. They are 

 soluble, their alkali acts immediately upon the geine. Their 

 carbonic acid acts upon silicates and geine. Immediate and 

 decided good effects follow their application ; while carbon- 

 ate of lime acts slower. It often requires many years to 

 bring out the good effects of carbonate of lime, and though 

 ultimately these effects, it is believed, have never failed of 



