SPECIFIC FOOD IN SOILS FOR PLANTS. 303 



wheat, and vice versa. The second theory by which 

 these facts are accounted for is that of the justly 

 celebrated De Candolle, which supposes that plants, 

 in growing, secrete substances injurious to, and not 

 required by them ; which substances, strictly excre- 

 mentitious, are thrown off by the roots into the soil, 

 and, by repetitions of the process, finally render it 

 unfit for the growth of the plant. This excrementi- 

 tious matter is, however, supposed to affect no plants 

 except those of the same kind with that from which 

 it has been ejected : thus that from the pine would 

 not prevent the growth of the oak, nor would that 

 which rendered the soil unfit for wheat injure in the 

 least a crop of rye. 



It is clear that most of the phenomena alluded to 

 above as attending a succession of crops on the 

 same soil, or the changes which the forests of a 

 country undergo, may be explained by either of 

 these theories; and it is not improbable that both 

 may be more or less influential at the same time. 

 To us the weight of testimony has appeared to be in 

 favour of the theory which attributes these changes 

 to an exhaustion of the specific food of the plant ra- 

 ther than to the excretion of poisonous matter from 

 the preceding vegetation. It must be admitted, how- 

 ever, that the experiments of De Candolle and Ma- 

 caire go far to prove such an exudation or secretion 

 form plants ; and, this fact once established, it cannot 

 be unreasonable to suppose that the matter thus re- 

 jected must be unsuitable for the succeeding plant, 

 should it be one requiring the same kind of nutri- 

 ment. 



Thus, admitting the truth of either of these the- 

 ories, or of both, we arrive at the conclusion that 

 change of plants is a law of nature ; and the manner 

 in which we can imitate her, and thus avoid the in- 

 evitable consequences of continued successions of 

 the same kind of plant, is distinctly pointed out. 

 Nature restores the original constituents of the soil 



