290 



On the Rotation of Crops. 



proofs of the excretory functions of plants ; and, therefore;, we 

 have no occasion to call in aid the analytic powers of chemistry. 



Hie only plausible argument that has been adduced against the 

 theory is that of the exhaustion of the soil. It is supposed that 

 each individual plant selects its own peculiar aliment, and there- 

 fore that rotation economises manure, and provides the supply of 

 the several crops in due and successive order. It is quite certain 

 that plants elaborate a certain proper juice, and effect specific 

 characteristic secretions ; but all these secretions, be they farina, 

 starch, sugar, gum, or odorous resin, are merely modifications of 

 the four grand elements, oxygen, hydrogen, carbon, and azote ; 

 and there is not perhaps one solitary fact which leads to the belief 

 that the common or raw sap imbibed by the roots contains any 

 characteristic principles. This is usually supposed to be little 

 more than water, holding carbonic acid in solution ; a fact which, 

 assuredly, cannot be determined by experiment, but at the same 

 time is more in accordance with electro-chemical principles than 

 the opinion that each individual plant deprives the soil of some 

 sort of aliment peculiar to its own habits and constitution. Ex- 

 haustion is certainly effected by vegetable action, but to understand 

 the term aright we must consider that the earths proper undergo 

 little or no change ; that all manures (or humus) are resolvable 

 into the elements of vegetable substance ; and, therefore, are ela- 

 borated by the vital energy of plants into sap, and in that state are 

 absorbed by their roots. All decomposable matters can be and 

 are thus gradually removed from the earths with which they have 

 been incorporated by the labour of man, and, so far, the land may 

 become exhausted ; but any one individual plant will cease to 

 thrive if repeatedly planted in the same spot long before this state 

 of exhaustion shall be brought about ; the ground itself becoming 

 at the very time imbued with that peculiar odour which indicates 

 the excretory power of the roots."^ 



* Although it is not improbable that the observations lately made on 

 this subject by M.M. De Candolle, Macaire, and other vegetable physiolo- 

 gists and. chemists, may be attended with future benefit in regard to a more 

 regular course of cropping in the alternate system, yet the main object of 

 cultivation, besides the necessary attention to the management of the soil, 

 must ever consist in supplying the land with a sufficiency of manure to 

 prevent it from being impoverished. "The main object of all rotations 

 should therefore be, to establish such a series of crops as, by preventing 

 the too frequent recurrence of any one of those which are considered ex- 

 hausting, shall guard against the dissipation or loss of those component 

 parts, or qualities of the soil, which seem peculiarly adapted to the growth 

 of each, and in the abundance of which consists its fertility. The precise 

 natm'e of those qualities, or rather the causes which influence their peculiar 

 effect on plants of different species, has not been ascertained ; indeed, has 

 been only vaguely conjectured ; and all the researches of chemical science 

 on the subject have ended in proving little more than what was already 

 known by experience; viz., that certain plants can only be grown with ad- 



