98 



Rotation of Crops. 



Vol. X. 



cially in Hungary, successive crops of grain 

 plants may be grown year after year contin- 

 ually, on the same soil, without disadvantage. 

 In meadows and forests, also, we see the 

 same species of plants succeed each other 

 for ages, and suffer no injury from the accu- 

 mulation of the secretions of preceding gene- 

 rations. To explain such cases as these 

 would require a new theory to be added to 

 the first, and without the aid of chemistry, 

 this would be as weak and unsatisfactory as 

 we have shown the former theory to be. 

 We must, therefore, reject the hypothesis of 

 the secretions of plants being the cause of 

 the advantage or necessity of the rotation 

 of crops, and endeavour to discover another, 

 capable of affording a satisfactory explana- 

 tion of the known facts, perfectly consistent 

 with true science, and especially with chem- 

 istry; and if such a theory be thus estab- 

 lished, it cannot fail to be of great use in 

 practice. 



If we assume that the cause of the utility 

 of the rotation of crops depends exclusively 

 upon the circumstance that cultivated plants 

 withdraw from the soil unequal amounts of 

 certain ingredients for their nutrition, all the 

 observed facts are at once and satisfactorily 

 explained, and the possibility of determining 

 the rotation of crops, or of avoiding it alto- 

 gether, if desirable, rendered evident. 



I need not here repeat what I have al- 

 ready told you, respecting those constituents 

 of plants which they derive from the soil, 

 but I must remind you that plants of various 

 species differ very much with respect to the 

 nature as well as to the quantity of mineral 

 or saline constituents which they require 

 for their growtii and developement. 



Bearing this in mind, it is obvious that 

 the growth of a plant may be impeded, sim- 

 ply because the mineral constituents princi- 

 pally needed, indeed essential to their proper 

 developement, have already been drawn from 

 the soil by the previous cultivation of another 

 plant, requiring nearly or altogether the same 

 constituents. If, for example, we take a field 

 the soil of which contains the mineral and 

 saline materials required to produce wheat, 

 and yet only in a quantity exactly sufficient 

 to produce a single crop, it follows, of course, 

 that a second crop of wheat cannot be reared 

 upon the same field. The soil is completely 

 exhausted for the moment, and will remain 

 so for ever, if it does not contain substances 

 which may by disintegration and decompo- 

 sition furnish a new supply of the ingredi- 

 ents necessary to the growth of plants, or it 

 these essential matters are not artificially 

 supplied. 



Such a complete exhaustion of the soil as 

 we have supposed, for the sake of illustra- 



tion, to be effected by a single crop, is not 

 very likely ever to happen in fact. But 

 what really happens, and that commonly 

 enough, is, that altiiough all the salts are 

 not exhausted, yet being present in the soil 

 in relative proportions very different to the 

 amounts required by various plants, a single 

 crop of wheat may deprive the soil so com- 

 pletely of one of its mineral constituents, 

 that another crop of wheat would not grow 

 upon it, and yet this soil may still contain 

 abundant mineral constituents for the pro- 

 duction of a good crop of clover or turnips. 



It will now be obvious that it is possible to 

 grow three, four, or more successive crops 

 of the same grain upon the same fields, 

 whenever the soil contains a sufficient 

 amount of the necessary mineral constitu- 

 ents, and that if a soil possessed an illimit- 

 able amount of these substances, or received 

 a constant and sufficient supply of them, it 

 would be able to produce successive crops of 

 the same cereals continually and for ever, 

 and moreover that a rotation of crops would 

 be in such cases wholly unnecessary. 



What we have stated with respect to the 

 cereals, applies equally to all other culti- 

 vated plants; so that any plant may be 

 grown upon the same field continually, and 

 good crops obtained, if the ingredients of the 

 soil which the plant requires either are pre- 

 sent originally to an unlimited amount, or 

 the farmer furnishes the field with a con- 

 stant an3 sufficient supply of these sub- 

 stances. 



Viewed in this light, the subject will be 

 clearer to you than perhaps has hitherto 

 been the case. You will now understand 

 that an exhausting plant must be one which 

 in comparison with other cultivated plants 

 requires many inorganic constituents, and 

 consequently requires for its successful cul- 

 tivation a soil rich in those constituents. 

 We need by no means wait for the perfect 

 developement of a plant, and subsequent 

 trials upon the same soil where it is grown, 

 in order to know whether it is an exhaust- 

 ing plant or not; we can arrive at a positive 

 conclusion upon this point immediately by 

 burning the plant and examining the ashes. 

 The case for example may occur that some 

 hitherto unknown plant is recommended for 

 cultivation, and tried in a soil equally un- 

 known, as to its amount of the constituents 

 which that particular plant may require. 



Practical experience, arising from the 

 growth of this plant in one field or soil, 

 may pronounce it to belong to the class of 

 exhausting plants, whilst in another soil it 

 may be found to be a non-exhausting crop. 

 Thus, the most contradictory conclusions 

 may be drawn from practical experience, 



