842 ROTATION. 



cropping is proportioned to the amount of nutriment in the crops 

 estimating the nutritive value according to Einhof s determination 

 But the above deduction is founded upon error. 



In fact, to adopt the above principle is tacitly admitting that the 

 whole organic matter of plants originally comes from the soil. This, 

 no doubt, contributes in a certain proportion to the development of 

 plants, but so also do air and water. On the other hand, physiolo- 

 gists, in opposition to the ideas of the school of Thaer, have perhaps 

 exaggerated the material withdrawn from the air. Thus, M. de 

 Saussure reckons that a sun-flower derives from the ground during 

 its growth not more than ^oth of its weight, supposing the plant dry. 

 The reasoning upon which he formed his conclusion is based, on the 

 one hand, upon a knowledge of the extractive matter of garden- 

 mould ; on the other, upon the quantity of water a plant like sun- 

 flower may absorb in a given time, to return it again to the air by 

 transpiration.* 



Little objection could be urged against the above conclusion, did 

 not the experiments of M. Gazzeri tend to prove that roots virtually 

 exercise, by their contact with solid organic matter, an incontesta- 

 ble absorbent action in imparting solubility.f I might refer to an 

 observation of M. de Saussure, in which he states that plants grown 

 in garden-mould deprived of its soluble components by repeated 

 washing, reached, nevertheless, perfect maturity, although the pro- 

 duce in seed was less abundant than it might have been. J It is 

 most probable that both parties have promulgated extreme opinions. 

 Plants possibly draw from the atmosphere more than agriculturists 

 commonly suppose, and the soil furnishes, independently of saline 

 and earthy substances, a proportion of organic matter larger than 

 certain physiologists admit. There is every reason to believe, from 

 what I could learn respecting guano during my sojourn on the coast 

 of Peru, that the greater part of the azotized principles of plants 

 originates in the ammoniacal salts which exist or are formed in ma- 

 nure.^ 



In discussing the advantage of one course of crops over another, 

 the question always hinges upon that of exhaustion. Wherever an 

 unlimited supply of dung and of handiwork can be procured, there 

 is no absolute necessity for following any regular system of rotation. 

 Under such favorable circumstances, it is expedient to ascertain 

 what kind of cultivation is, commercially speaking, best suited to 

 the climate and the soil. There is little to fear that by a continued 

 succession of similar crops, the fields will get infested with noxious 

 weeds, because this inconvenience may be obviated by labor. Nor 

 is impoverishment of the soil to be dreaded, since that can be re- 

 medied by the purchase of manure. The whole craft of agricuhure 

 is reducible to comparison of the probable value of the crop with 

 the cost of manure, labor, &c. Farming of this sort excludes the 



* Saussure, Recherches Chimiques sur la V6g6tation, p. 2681 

 t Annales de rAj^TicuUnrc Franfaise, No. iii. p. 57 

 i Saussure, Recherches Chimiques, p. 171. 

 ^ Annales de Chimie, t. Ixv. ann^ 1837. 



