134 ON THE PRINCIPLES OF 



be cultivated, the elements of the ashes of these roots are 

 wanting-, the plants bring' forth leaves, stalks, blossoms, and 

 seeds; but the roots and tubercles are imperfect. Every one 

 of the elements which the soil gives up to the plants is in a 

 direct quantitive proportion to the production of the separate 

 elements of the plants. Two fields, which, under otherwise 

 equal circumstances, are unequally rich in mineral elements 

 of the grain, produce unequal crops. One containing- them 

 in larg-er quantity produces more than another containing* 

 them in less. In the same manner, the capacity of a soil to 

 produce tuberculous plants, or such which have many leaves, 

 depends upon its amount of the elements of the soil which 

 are found in the ashes of those plants. 



It results from this with certainty, that the mineral sub- 

 stances which are furnished by the soil, and which are found 

 ag-ain in the ashes of plants, are their true food ; that they 

 are the conditions of veg'etable life. 



It is evident, that from a lield in which diiferent plants are 

 cultivated, we remove with the crop a certain quantity of 

 these elements ; in the seeds, those mineral parts which the 

 soil had to provide for their development ; and in the roots, 

 tubercles, stalks, and leaves, those elements which are neces- 

 sary for tJu'ir production. However rich the field may be in. 

 these elements, there can be no doubt that, b}' several cul- 

 tures, it becomes more and more impoverished; that for 

 every plant a time must arrive when the soil will cease to 

 furnish, in sufficient quantity, those elements which are 

 necessary for a perfect g-rowth. Even if such a field, during' 

 many subsequent years, produced twenty-five or thirty fold 

 the amount of the seed — for instance, of wheat — experience 

 shows that the crop g-radually decreases, until at last the 

 amount will be so small, that it approaches the plant in its 

 wild state, and would not repay the cost of cultivation. 



According* to the unequal quantity in which the mineral 

 elements of grain, tubercles, roots, seeds, and leaves, are con- 

 tained in a soil, or according- to the proportions in which they 

 may have removed in the crop, the land may have ceased to 

 be fertile for roots and tubercles, but it may 3'et produce 

 g'ood crops of wheat. Another may not produce wheat, but 

 potatoes and turnips may thrive well in it. The mineral 

 substances contained in a fertile soil, and serving* as food to 

 the plants, are taken up by them with the water, in which 

 they are soluble. In a fertile field they are contained in a 

 state which allows of their being- absorbed by the plant, and 



