LECTURE XV. 



Improvement of the soil by chemical means.— Principles on which all manuring depends. 

 —Mineral, vegetable, and animal manures.— Saline manures.-Carbonates.— Pearl-ash. 

 —Sulphates.— Glauber salts.— Chlorides.— Common Salt.— Nitrates.— Nitrate of soda.— 



Phosphates.— Phosphate of lime.— Sihcates. SiUcate of potash.— Saline mixtures 



Vegetable ashes.— Prepared granite.— Use of lime. 



The mechanical methods of improving the soil, described in the pre- 

 ceding section, are few in number and simple in theory. They are so 

 important, however, to the general fertility of the land, that were they 

 judiciously employed over the entire surface of our islands, they would 

 alone greatly increase the average produce of the British and Irish soils. 

 1 may, indeed, repeat what was stated in reference to draining (p. 308), 

 that the full effect of every other means of improving the soil will be 

 obtained in those districts only where these mechanical methods have 

 already been had recourse to. 



The chemical methods of improving the soil are founded upon the 

 following principles, already discussed and established : — 



1°. That plants obtain from a fertile soil a variable proportion of their 

 organic food ; — of their nitrogen probably the greatest part. 



2°. That they require inorganic food also of various kinds, and that 

 this they procure solely from the soil. 



3°. That different species of plants require a special supply of dif- 

 ferent kinds of inorganic food, or of the same kinds, in different pro- 

 portions. 



4°. That of these inorganic substances one soil may abound or be 

 deficient in one, and another soil in an()ther ; and that, therefore, this or 

 that plant will prefer to grow on the me or the other accordingly. 



On these few principles the whole art of improving the soil by che- 

 mical means — in other words, of beneficially manuring the soil — is 

 founded. 



It must at the same time be borne in mind, that there are three dis- 

 tinct methods of operation by which a soil may be improved : — 



1°. By removing from it some noxious ingredient. The only method 

 by which this can be effected is by draining, — providing an outlet by 

 which it may escape, or by which the rains of heaven, or water applied 

 in artificial irrigation, may wash it away. 



2°. By changing the nature or state of combination of some noxious 

 ingredient, which we cannot soon remove in this way ; or of stjme inert 

 ingredient which, in its existing condition, is unfit to become food for 

 plants. These are purely chemical processes, and we put them re- 

 spectively in practice when we add lime to peaty soils, or to such as 

 abound in sulphate of iron (p. 212), when by admitting the air 

 into the subsoil we change the prot-oxide into the per-oxide of iron, 

 (p. 210,) or when by adding certain known chemical compounds we 

 produce similar beneficial chemical alterations upon other compounds 

 ^Iready existing in the soil. 



