492 THEORY OF THE ROTATION OF CROPS. 



kept continuously than before, but that in addition a crop of corn could be 

 reaped every second year. On the other hand, those which had been 

 cropped with corn alone, or which after two white crops had usually been 

 left to naked fallow, yielded more corn in a given number of years than 

 before, while a green crop every second year was raised on them besides. 

 It cannot be doubted, therefore, that a change of crojpjnng influences, in 

 a great degree, the amount of food which the same piece of land is fitted 

 to produce. 



§ 4. Of the theory of the rotation of crops. 



Upon what principles do the beneficial effects of this change of crop- 

 ping depend ? What is the true theory of a rotation of cipps ? 



It was supposed by Decandolle — 



1°. That the roots of all plants gave out or excreted certain substan- 

 ces peculiar to themselves — and, 



2°. That these substances were unfavourable to the growth of those 

 plants from the roots of which they came, but were capable of promo- 

 ting the growth of plants of other species — that the excretions of one 

 species were poisonous to itself, but nutritive to other species. 



Upon these suppositions he explained in a beautiful and apparently 

 simple and convincing manner the beneficial effects of a rotation or al- 

 ternation of different crops. If wheat refused to grow after wheat, it was 

 because the first crop had poisoned the land to plants of its own kind. 

 If after an intervening naked fallow a second wheat crop could be safely 

 grown, it was because during the year of rest the poisonous matter had 

 time to decompose and become again fitted to feed the new crop. And 

 if, after beans or turnips, wheat grew well, it was because the excretions of 

 these plants were agreeable to the young wheat, and fitted to promote 

 its growth. 



Thus easily explained were the benefits both of a rotation of crops 

 and of naked and other fallows — and supported at once by its own beauty 

 and by the great name of Decandolle, this explanation obtained for 

 many years an almost universal reception. 



But though there seems reason enough for believing (p. 82) that the 

 roots of plants really do give out certain substances into the soil — there is 

 no evidence that these excretions take place to the extent which the 

 theory of Decandolle would imply — none of a satisfactory kind that 

 they are noxious to the plants from which they are excreted* — and none 

 that they are especially nutritive to plants of other species. Being un- 

 supported by decisive facts and observations therefore, the hypothesis of 

 Decandolle must, for the present, be in a great measure laid aside, and we 

 must look to some other quarter for a more satisfactory theory of rotation. 



Tlie true general reason why a second or third crop of the same kind 

 will not grow well, is — not that the soil contains too much of any, but that 

 it contains too little of one or more kinds of matter. If, after manuring, 

 turnips grow luxuriantly, it is because the soil has been enriched with 

 all that the crop requires. If a healthy barley crop follow the turnips, 

 it is because the soil still contains all the food of this new plant. If 

 clover thrive after this, it is because it naturally requires certain other 

 kinds of nourishment which neither of the former crops has exhausted. 

 If, again, luxuriant wheat succeeds, it is because the soil abounds still in 

 • See page 81, note. 



