300 Dr Davy's ^Agricultural Discourse. 



plants, are obvious. There is another source of fertility in 

 the soil of a more obscure kind. There are some soils, for 

 example, capable of yielding nitre, and that for successive 

 years. Nitre, you know, is a compound of nitric acid and 

 potash. When formed in a soil, there is reason to infer that 

 invariably the alkali is furnished by the soil, derived from a 

 compound mineral, and that the elements of the acid, azote 

 and oxygen, are furnished by the atmosphere, and that in con- 

 sequence of certain ingredients in the soil, favouring the 

 union of these elements, especially lime, the acid is formed 

 and the salt is produced. So there are other conditions of 

 soil in which clay, and perhaps oxide of iron, act a part, pro- 

 moting the production from its elements, viz. azote and hy- 

 drogen, of ammonia, or the volatile alkali. And, as the sub- 

 stances first mentioned, lime, magnesia, potash, silica, are 

 fertilizers as inorganic elements ; these latter, nitre and the 

 volatile alkali, are probably useful in administering to the 

 formation of the organic parts. In taking a view of the ca- 

 pabilities of a soil, the attention should be given to the under 

 portion, to the subsoil as well as the surface soil. If the lat- 

 ter be neglected and never turned up and brought into use, 

 great may be the loss to the agriculturist. Water holding 

 carbonic acid in solution, I have endeavoured to shew, is the 

 principal agent or menstruum by which the sap is formed 

 and plants are fed. This is the result when it is absorbed 

 by the roots, and is transmitted by their ascending to be 

 distributed to the various parts where required ; but when, 

 instead of being thus absorbed, it passes from the soil to the 

 subsoil, it impoverishes the former and enriches the latter, 

 removing out of it more or less of the soluble constituents, 

 so that you may have at the same time an exhausted surface 

 soil and a rich subsoil, requiring only a change of place, it 

 may be, a mixing of the two, to become highly productive. 



Gentlemen, — I must now bring this discourse to a conclu- 

 sion. As I began it with drawing your attention to the ana- 

 logy between plants and animals, I do not know how I can 

 better finish it than by reverting to that analogy. All I have 

 hitherto said, has been on the subject of manures generally, 



