SCIENTIFIC AGRICULTURE. 185 



seeds, tho same result would follow as in other crops. And 

 with regard to forest trees, it has been observed that where an 

 oak forest has been cut down, a growth of pine will succeed; 

 and where a pine forest has been cleared away, a growth of 

 oak will spring up in its place : where beech and maple are 

 cut, poplar and basswood often succeed them. Thus it appears 

 that the soft and hard woods alternate with each other. 



The reasun formerly given for the necessity of rotation was, 

 that all plants throw off certain matters or excrements by their 

 roots, which prove injurious to another crop of the same kind 

 of plants j but which are beneficial rather than injurious to 

 crops of a different kind. 



This beautiful theory originated with the distinguished bota- 

 nist, Decandolle, and explains, apparently, in an easy and satis- 

 factory manner, all the reasons for the necessity of rotation 

 of crops. The simplicity and high authority of this theory 

 obtained for it, for many years, an unquestioned assent; and 

 the only objection which lies against it now is, that it is not 

 supported by a. single fact 



The objections to it are, 1. That plants do not excrete so 

 great an amount of noxious matters as supposed by Decan- 

 dolle. 2. No evidence exists of their injurious effects upon 

 the plants from which they are excreted. 3. There has been 

 no demonstration of their nutritive effects on other plants. 



This theory, then, must be abandoned, and we must look 

 for one which is supported by facts: and if one cause be found 

 adequate to explain all the effects produced, we are not bound 

 to seek for another. 



The necessity of rotation does not depend upon there being 

 too much, but too little, of some particular elements in the 

 soil. (Johnston.) All plants require certain elements for food, 

 and these are indispensible to their growth and maturity : one 

 plant requires them in certain proportions and another requires 

 these and others besides, in quite different proportions. 



*16 



