374 INORGANIC ELEMENTS OF MANURES AND CROPS, 



in operation. At least, it would be difficult to assign any other office 

 to chalk in the marling or liming of land intended for corn, when 

 we know how little lime corn absorbs. If, indeed, gypsum promotes 

 the vegetation of trefoil, lucerne, sainfoin, &c., by furnishing the 

 needful calcareous element, it could not fail to exercise an equally 

 farorable agency upon wheat and oats, did they require it. The ex- 

 periments adduced prove it not to be so, and their results are in 

 some measure corroborated by analysis. Thus, if we compare the 

 different quantities of lime withdrawn from the soil by trefoil arvd 

 corn, we find them as follows : 



The clover crop takes from 1 acre of ground nearly 70 lbs. of Ume. 

 Wheat " " " 16 



Oat ^ " " " 6.4 " 



With this comparison before us, it seems evident that if the marl- 

 ing and liming of corn lands had no other object than the introduc- 

 tion of the minute portion of lime which is encountered in the crops, 

 it would be difficult to justify the enormous expenditure of calcare- 

 ous carbonate which is proved by daily experience to be advan- 

 tageous. 



It may be inferred from the foregoing, that in the most frequent 

 case, namely, that of arable lands not sufficiently rich to do without 

 manure, there can be no continuous cultivation without annexation 

 of meadow ; in a word, one part of the farm must yield crops with- 

 out consuming manure, so as to replace the alkaline and earthy salts 

 that are constantly withdrawn by successive harvests from another 

 part. Lands enriched by rivers alone permit of a total and contin- 

 ued export of their produce without exhaustion. Such are the fields 

 fertilized by the inundations of the Nile ; and it is difficult to form 

 an idea of the prodigious quantities of phosphoric acid, magnesia, 

 and potash, which in a succession of ages have passed out of Egypt 

 with her incessant exports of corn. 



Irrigation is, without doubt, the most economical and efficient 

 means of increasing the fertility of the soil, out of the abundant for- 

 age which it produces, and the resulting manure. Plants take up 

 and concentrate in their organs the mineral and organic elements 

 contained in the water, sometimes in proportions so minute as to es- 

 cape analysis ; just as they absorb and condense, in modified forms, 

 the aeriform principles which constitute but some 10,000th parts in 

 the composition of the atmosphere. It is thus that vegetables col- 

 lect and organize the elements which are dissolved in water, and 

 disseminated through the earth and the air, as a preparative to their 

 being assimilated by animals. 



