162 THE ALTERNATION OF CROPS. 



tain constituents to the soil, which are removed with 

 the roots, fruit, or grain, or entire plants grown 

 upon it. 



But it has been observed, that the crops are not 

 always abundant in proportion to the quantity of 

 manure employed, even although it may have been 

 of the most powerful kind ; that the produce of 

 many plants, for example, diminishes, in spite of the 

 apparent replacement by manure of the substances 

 removed from the soil, when they are cultivated on 

 the same field for several years in succession. 



On the other hand it has been remarked, that a 

 field which has become unfitted for a certain kind of 

 plants was not on that account unsuited for another; 

 and upon this observation, a system of agriculture 

 has been gradually founded, the principal object of 

 which is to obtain the greatest possible produce with 

 the least expense of manure. 



Now it was deduced from all the foregoing facts, 

 that plants require for their growth different con- 

 stituents of soil, and it was very soon perceived, 

 that an alternation of the plants cultivated main- 

 tained the fertility of a soil quite as well as leaving 

 it at rest or fallow. It was evident, that all plants 

 must give back to the soil in which they grow differ- 

 ent proportions of certain substances, which are capa- 

 ble of being used as food by a succeeding generation. 



But agriculture has hitherto never sought aid from 

 chemical principles, based on the knowledge of those 

 substances w^hich plants extract from the soil on 

 which they grow, and of those restored to the soil 

 by means of manure. The discovery of such prin- 

 ciples \\'ill be the task of a future generation, for 

 what can be expected from the present, which recoils 

 with seeming distrust and aversion from all the 

 means of assistance offered it by chemistry, and 

 which does not understand the art of making a 

 rational application of chemical discoveries ? A 

 future generation, however, will derive incalculable 

 advantage from these means of help. 



