188 THE FOOD AND NUTRITION OF PLANTS. 



numerous weeds which grow chiefly around dwellings, and follow the 

 footsteps of man and the domestic animals, flourish only in a soil 

 abounding in nitrates (their ashes containing a notable quantity 

 either of nitrate of potash or of lime) ; why the Vine requires alka- 

 line manures, to replace the lai-ge amount of tartrate of potash Avhich 

 the grapes contain ; and why Pines and Firs, the ashes of which 

 contain very little aUcali, will thrive in thin or sterile soils, while the 

 Beech, Maple, Elm, &c., abounding with potash, are only found in 

 strong and fertile land. 



340. Where vegetation is undisturbed by man, all these needful 

 earthy materials, which are drawn from the soil during the growth 

 of the herbage or forest, are in time restored to it by its decay, 

 in an equally soluble form, along with organic matter which the 

 vegetation has formed from the air. But in cultivation, tlie prod- 

 uce is carried away, and with it the materials which have been 

 slowly }'ielded by the soil. " A medium crop of Wlieat takes from 

 one acre of ground about 12 pounds, a crop of Beans about 20 

 pounds, and a crop of Beets about 11 pounds, of phosphoric acid, 

 besides a very large quantity of potash and soda. It is obvious that 

 such a process tends continually to exhaust arable land of the 

 mineral substances useful to vegetation which it contains, and that a 

 time must come, Avhen, without supplies of such mineral matters, the 



land would become unproductive from their abstraction In 



the neighborhood of large and populous towns, for instance, where 

 the interest of the farmer and market-gardener is to send the largest 

 possible quantity of produce to market, consuming the least possible 

 quantity on the spot, the want of saline principles in the soil would 

 very soon be felt, were it not that for every wagon-load of greens 

 and carrots, fruit and potatoes, corn and straw, that finds its way 

 into the city, a wagon-load of dung, containing each and eveiy one 

 of these principles locked up in the several crops, is returned to the 

 land, and proves enough, and often more than enough, to replace all 

 that has been carried aAvay from it." * The loss must either be 

 made up by such equivalent return, or the land must lie fallow from 



* Bousgingaiilt, Economie Rurale : from the Engl. Trans., p. 493. Furtlior : 

 "It may' be infeiTcd tliat, in the most frequent case, namely, that of aralilc 

 lands not sufficiently rich to do without manure, there can be no continuous 

 [independent] cultivation without annexation of meadow ; in other words, one 

 part of the farm must yield crops without consuming manure, so that this may 

 replace the alkaline and earthy salts which arc constantly withdrawn by sue- 



