ACTION OF SALTS. 103 



stituents of plants ; others consist of chlorine ; oth- 

 ers of sulphur and oxygen, and others of carbon 

 and oxygen. In other words, the acids are compos- 

 ed of elements, which form food for plants, or of 

 elements 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, are fed, by the second, plants 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 con- 

 verted by the plant, well, good effects follow ; if on 

 the other hand, the elements of the acid are such as 

 the plant demands not, its presence acts on the 

 plant, like poison on the animal economy. 



158. Let salts be divided into two classes, on this 

 principle of the peculiarity of action depending upon 

 the acid of the salts, the first nourishes, the second 

 poisons plants. The first class contains, first, car- 

 bonates — second, nitrates — third, phosphates. 



159. The action of the first class is to be stud- 

 ied under its three divisions. First, the 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, 



