DR. DAUBENY ON THE ROTATION OP CROPS, ETC. 209 



It appears then that out of the whole series, there are only four cases in which the 

 average amount of the permanent crop was equal or superior to that of the shifting 

 one. 



In the first of these, the Tobacco, the fact may be accounted for, from the condition 

 x)f the ground being more favourable to the permanent than to the shifting crop on 

 the year of its introduction, the former being obtained from soil which had been re- 

 cently manured, the latter from what had been partially exhausted by preceding crops. 



The second, the Beet, was scarcely continued for a sufiicient length of time to lead 

 to any certain conclusions. 



The two others, namely, the Endive and Mint, present results so nearly agreeing 

 in the amount of their permanent and shifting crops, that the slight disparity may be 

 fairly referred to contingent circumstances, and an uniformity in the products obtained 

 may in consequence be inferred. 



Setting aside then the above four cases as exceptional, the general tenor of the 

 experiments would seem to indicate a manifest advantage on the side of the shifting 

 crops, varying from 1 to 75 per cent., but more generally approaching to the latter. 



Yfit it by no means follows that this difference is to be attributed to the influence 

 of root excretions. Were such the cause, we ought to perceive a more regular, as 

 well as a more rapid, diminution in the permanent crop than is indicated in these 

 Tables ; we should not find, for instance, the crop of potatoes equalling in the fifth year 

 the produce of the first ; the Turnips, after sinking to 37'0 lbs. in the third year, rising 

 in the sixth to 128 lbs. ; not to allude to other similar instances of oscillation. 



If De Candolle's theory too could be carried out, we might have expected to find 

 a more manifest improvement in the shifting crop occasionally occurring, owing to 

 the excretions of the family of plants which had preceded it proving congenial to its 

 constitution. 



But if nothing positively injurious be imparted to the soil by the crop, the gradual 

 falling off in the amount of the latter can only be attributed to the deficiency, either 

 of organic, or of inorganic matter fitted for its development, in the soil in which it 

 was reared. 



Of the two continental writers on chemical agriculture whose works have excited 

 the greatest interest in this country, the one would seem to favour the former, the 

 other the latter explanation, although it may be more correct to consider them, as 

 viewing the subject under two different aspects, rather than as laying down principles 

 irreconcileable one with the other. 



LiEBiG, for instance, although he regards the presence of certain inorganic matters 

 as the only condition essential to the existence of a plant, does not deny, that its growth 

 may be accelerated in proportion to the ready access to it of ammonia and carbonic 

 acid, and these, it is evident, would be supplied more abundantly by the presence in 

 the soil of organic matter in a readily decomposable condition. 



Nor, on the other hand, would Boussingault deny the necessity for a supply of the 



