370 INORGANIC ELEMENTS OF MANURES AND CROPS. 



It was at one time asserted, that in order to ensure to a crop of 

 wheat the necessary quantity of phosphates, its cultivation was pre- 

 ceded by one of roots or tubers, or leguminous plants, which were 

 supposed to contain a much less proportion of these salts. By ref- 

 erence, however, to the table of mineral substances, removed from 

 the soil by different crops, the absurdity of such reasoning becomes 

 evident. For example, beans and haricots take 20 and 13.7 lbs. of 

 phosphoric acid from every acre of land ; potatoes and beet-root 

 from the same surface take but 11 and 12.8 lbs. of that acid, exactly 

 what is found in a crop of wheat. Trefoil is equally rich in phos- 

 phates with the sheaves of corn which have gone before it, and this 

 large dose of phosphoric acid whhdrawn from the soil, will nowise 

 diminish the amount which will enter into the wheat that will by 

 and by succeed the artificial meadow. It may be readily under- 

 stood, that if the ground contains more than the quantity of mineral 

 substances necessary for the total series of crops in a rotation, it is 

 a matter of indifference whether the crops draw upon the soil in 

 any particular order, and these succeed according to rules generally 

 adopted for quite different reasons. It suUs well, for instance, to 

 begin a rotation with a drill crop sown in spring, and which, conse- 

 quently, follows in our system the oats which closed the preceding 

 rotation ; it is a great advantage to be able to collect and cart out 

 the manure during winter. Besides, the order is quite at the farm- 

 er's discretion, and there are places where, from particular reasons, 

 quite another course is pursued. One part of the produce returns, 

 as has been shown, to manure, after having served as fodder for the 

 animals belonging to tiie farm. The inorganic matters are restored 

 to the eartii from which they came, deducting the fraction assimi- 

 lated in the bodies of the cattle. Lastly, the whole of the wheat, 

 and a certain amount of flesh will be exported, and with these a no- 

 table quantity of inorganic matter. Thus, in the above described 

 rotation of live years, the minimum exportation of saline substances 

 which must be removed from every acre of land, may be represent- 

 ed by 27\ lbs. of phosphoric acid, and from 30 to 45 lbs. of alkali ; 

 this is just so much lo.st for the manure, and as there is definitively 

 found at the end of tlie rotation a quantity of manure equal and 

 nearly similar to that disposed of at the commencement, it is essen- 

 tial that the loss of mineral substance be made up from without, 

 unless it be naturally contained in tiie soil. 



In my first researches on the rotation of crops,* I stated that 

 wherever there are exportable products, it becomes indispensable to 

 keep a large proportion of meadow land, quoting, as an extreme 

 case, the triennial rotation with manured sununer-fallow. It is, in 

 fact, the meadow which restores to the arable land the principles 

 which have been carried off. This point, advanced upon analogy, 

 is amply confirmed by the results of analysis. 



1 have examined, in reference to this question, the ashes of the 

 hay of our meadows of Durrcnbach, irrigaleil by tiie Saner. The 



• Memoir couiniuiiicatrd to the Acaiii-iiilc des .^rience.", in 183d 



