846 ROTATION. 



of the Andes wheat fields, which had yielded excellent cr:ps annaa]- 

 iy for more than two centuries. Maize may likewise be continually 

 reproduced upon the same ground without inconvenienc-e : this fact 

 is well known in the south of Europe ; and the greater portion of 

 the coast of Peru has produced nothing else, from a date long ante- 

 rior to the discovery of America. Further, potatoes may come again 

 and again upon the same soil ; they are incessantly cultivated at 

 Santa-Fe and Quito, and nowhere are they of better quality. In- 

 digo and sugar-cane may be brought under the same category. In 

 Europe the Jerusalem artichoke produces constantly in the same 

 place.* It must be conceded, that if all these plants excrete from 

 their roots, their excretions are not of such a nature as to interfere 

 with the progress of vegetation of the species producing them. 



But the capital objection to the hypothesis of De Candolle is this, 

 that it would be very remarkable indeed did any soluble organic 

 matter, like such secretions, not putrefy when lying in the ground 

 In a word, it is difficult to understand how it should resist for years, 

 as is pretended, the decomposing influence of heat and moisture to« 

 gether. 



That there is no absolute necessity for alternation of crops when 

 dung and labor can be readily procured, is undeniable. Never- 

 theless, there are certain plants which cannot be reproduced upon 

 the same soil advantageously except at intervals more or less re- 

 mote. The cause of this exigence on the part of certain vegetables 

 is still obscure, and the hypotheses propounded for clearing it up far 

 from satisfactory. 



One of the marked advantages of alternate cultures, is the periodic 

 cultivation of plants which improve the soil. In this way a sort ol 

 compensation is made for exhaustion. The main thing to be secur- 

 ed in rotation of crops is such a system as shall enable the husband- 

 man to obtain the greatest amount of vegetable produce with the 

 least manure, and in the shortest possible time. This system cao 

 be alone realized by employing in the course of rotation those plants 

 which draw largely upon the atmosphere. 



The best plan of rotation in theory, is that in which the quantity 

 of organic matter obtained most exceeds the quantity of organic 

 matter introduced into the soil in the shape of manure. This does 

 not hold quite in practice. It is less the surplus amount of organic 

 matter over that contained in the manure, than the value of this 

 same matter which concerns the agriculturist. The excess required, 

 and the form in which it should be produced, must vary widely ac- 

 cording to locality, commercial demand, and the habits of people, 

 considerations wholly apart from theoretical provisions. One point 



* To this list might be added, according to the recent researches of M. Braconnot, 

 ihe bay-rose with double flowers, and Papaver somniferum. That distinguished 

 chemist terminates his memoir as follows : " My experiments are unfavorable, as may 

 be perceived, to the theory of rotation of cops based on the excretions of the roots. 

 These excretions if really occurring in the lormal state are so obscure and little known 

 as to lead to the inference that the general system of rotations must be referred ta 

 some other source." (Recherches sur I'influence des plantes sur le sol, Annales d« 

 Chimie t Ixxii. p. 27.) 



