192 THE FOOD AND NUTRITION OF PLANTS. 



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, the prod- 

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

 slowly yielded by the soil. " A medium crop of Wheat takes 

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

 20 pounds, and a crop of Beets about 1 1 pounds, of phosphoric 

 acid, besides a very large quantity of potash and soda. It is obvi- 

 ous 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 when, without supplies of such mineral 

 matters, the land would become unproductive from their abstrac- 

 tion In the neighbourhood 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 wag- 

 on-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 every 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 away from it." * 

 The loss must either be made up by such equivalent return, or the 

 land must lie fallow from time to time until these soluble substan- 

 ces are restored by further disintegration of the materials of the 

 soil : or meanwhile the more exhausting crops may be alternated 

 with those that take least from the soil and most from the air ; or 



* Boussingault, Economie Rurale ; from the Engl. Trans., p. 493. Further : 

 " It may be inferred that, in the most frequent case, namely, that of arable 

 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 may replace the alkaline and earthy salts which are constantly withdrawn by 

 successive harvests from another part. Lands enriched by rivers alone permit 

 of a total and continued 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 in- 

 cessant exports of corn." p. 503. 



