34 CHEMICAL MANURES. 



have drawn between the different elements of which the soil is com- 

 posed. 



Let us suppose a soil containing both quartz sand and feldspar 

 sand among its mechanical elements. For vegetation these two 

 sands are equivalent, although the first is from silica and nothing but 

 silica, while the second is a silicate based upon lime, potash and soda, 

 besides containing phosphate of lime in very feeble but very appre- 

 ciable quantities. 



Here, then, are two bodies whose composition, in spite of similitude 

 of exterior, have no analogy, and which, however, are equivalent in 

 an agricultural point of view, because the feldspar being insoluble 

 in water its role in regard to vegetation descends to that of the quartz 

 sand ; that is to say, to a simple mechanical element. But for the 

 chemist there are no insoluble bodies, so he confounds in one whole 

 the potash, lime and phosphate of lime that the feldspar sand con- 

 tains, though they are of no use in vegetation, with the products of 

 the same nature which we have ranged under the class of active as- 

 similable elements. Thus is explained the insufficiency of the signs 

 with which chemistry can furnish us. 



We have a striking example of the dangers of the confusion into, 

 which we are often drawn in the farm at Vincennes. After an anal- 

 ysis which I made with the greatest care of this soil, in 3,555,555.05 

 pounds which pretty nearly represents the vegetable layer spread 

 over the surface of an acre there were : 



Phosphoric acid 1596.66 Ibs. 



Potash 2045.33 Ibs. 



Lime 34,991.66 Ibs. 



This constitutes a considerable fund of fertility. Now, if we culti- 

 vate wheat on this land for four successive years, employing azotic 

 matter as a fertilizer, at the end of the four years the return will not 

 be more than from 7.11 to 8.44 bushels. 



The soil shows a penury of minerals, and these harvests have 

 shown but 



Phosphoric acid 74.55 Ibs. 



Potash 81.77 Ibs. 



Lime 35.45 Ibs. 



from the land. Very different quantities from those shown by the 

 chemical analysis. 



Was there an error, then, in my analysis ? No, gentlemen : the 

 soil contains just what I reported ; but this indication cannot be of 

 any practical use, because in the mixture of these minerals we have 

 not distinguished between those which are active in regard to plants 

 and those which are inert. 



Doubtless you find this conclusion very unsatisfactory. 



To what good giving ourselves the trouble to discover the agents 

 to which plants owe their formation, and to define the conditions of 

 their efficacy, if at last we are not able to recognize their presence in 

 the soil in the especial forms which assure their good effects ? 



