CHEMICAL MANURES. 27 



LECTUEE THIKD. 



/GENTLEMEN: You know that the minerals entering into the 

 VJ composition of plants' are ten in number namely, phosphorus, 

 sulphur, chlorine, silicium, calcium, magnesium, potassium, sodium, 

 iron and manganese. But you will be surprised to find that we are 

 almost entirely ignorant as to what form these enter into the organ- 

 ization of vegetable tissues. We know that it is in the form of 

 binary or ternary compositions, without being able to exactly deter- 

 mine their nature and composition. The imperfectness of our know- 

 ledge on this head will astonish you less if I add that, to acquire the 

 least idea of their presence, we must begin by burning the tissues 

 which contain them. But if science shows a lamentable gap in this 

 respect, we at least know with certainty under what form and what 

 conditions the minerals can be made extremely efficacious agents of 

 fertility. If phosphorus is in question, we employ it in the form of 

 phosphate of lime ; potash, in the form of a carbonate, a nitrate or 

 a silicate ; and lime, in that of a carbonate or a sulphate. We are, 

 then, perfectly fixed on this second point, which is much more import- 

 ant than the first namely, the form most favorable to the good 

 effects of minerals as agents of fertility. But here a most unex- 

 pected question presents itself. 



I have just told you that ten different minerals enter into the 

 composition of plants, and now I am forced to add that three of 

 them, with the aid of some azotic matter, are sufficient to increase 

 and maintain the fertility of the soil, and that the agriculturist need 

 not occupy himself with the seven others. 



Does that mean that these last do not affect vegetation ? No ; 

 they are not less necessary than the first three, and if practice can 

 pass them over, it is only because poor soils are abundantly provided 

 with them. j 



If the facts which I have just shown are exact, the conclusion is 

 forced : we ought by their aid to obtain from burnt sand, inert of 

 itself, as prosperous a growth as from the most fertile alluvial soil. 

 For that we only need ten minerals and azotic matter. From these 

 fundamental facts it equally results that from a natural soil we 

 should obtain the same growth by the addition of azotic matter and 

 three minerals phosphate of lime, potash and lime. Experience 

 confirms these two provisions of theory. 



We may go still farther in the same train of ideas. If it is true 

 that each mineral fulfills a duty proper to itself, and that the useful 

 effects of the whole be in a measure dependent upon the presence of 

 each of these elements in particular, we ought, by the suppression of 

 one or several of the parts of this fertilizing mixture, to determine a 

 series of gradations running from the most doubtful to the highest 

 yield. Experience again confirms this new anticipation of theory ; 

 but as this is touching a very grave question, we will put our con- 



