18 CHYMISTRY APPLIED TO AGRICULTURE. 



the concurrence of air, heat, and water, is completed only 

 at the end of some years. 



Land owes its fertility mostly, if not wholly, to the 

 presence, in a greater or less abundance, of principles 

 analogous to those constituting mould. These principles 

 are furnished by manures, and by the decomposition of 

 plants ; but each harvest causes a diminution of them, a 

 part being washed away by rains, and a part absorbed by 

 the crops which are raised ; thus the soil is deprived by 

 degrees of its nutritive qualities,* till at length nothing re- 

 mains but an earthy residuum, deprived of its nourishing 

 juices, and completely barren ; it is to restore its fertility 

 that land must be manured afresh, after having yielded 

 several crops. 



ARTICLE IL 



Of the Nature of Soils. 



The question which we are now about to treat, is one 

 of the most difficult in agriculture, but as it is perhaps 

 one of the most important, we ought to give it the greatest 

 attention, and to direct all our researches to proving the 

 difference existing amongst arable lands, and their various 

 properties. 



The earth furnishes support to nearly all plants ; and 

 as each species of these requires a soil suited to its par- 

 ticular organs, we find that different portions of the earth 

 differ widely amongst themselves in character. An ac- 

 quaintance with the nature of soils is especially necessary, 

 as it serves to throw light upon the cultivation of vegeta- 

 bles, which are principally nourished by them, and upon 

 the suitable adaptation of which most of their properties 

 depend. 



Arable soils, which are the only ones of which I shall 

 here speak, are generally composed of silica, lime, alumi- 

 na, magnesia, oxide of iron, and some saline substances. 

 The various characters of soils arise from the different 

 proportions in which their component parts are combined ; 

 and the name given to each is according to that of the pre- 

 dominating portion of earth found in it, as siliceous, cal- 

 careous, argillaceous, &c. It is necessary that they should 



