THEIR EARTHY CONSTITUENTS. 



189 



time to time until these soluble substances 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 with one which, like clover, 

 although it takes up 77 pounds of alkali per acre, may be consumed 

 on the iield, so as to restore most of this alkali in the manure for the 

 succeeding crop. 



341. It has been asserted that the advantage of preceding a 

 wheat crop by one of Leguminous plants (such as Peas, Clover, 

 Lucerne, &c.), or of roots or tubers, is owing to the fact, that these 

 leave the phosphates, &c. nearly untouched for the wheat which is 

 to follow, and which largely abstracts them. The results of Bous- 

 singault's experiments and analyses show that these products are 

 far from having the deficiency of phosphates wliich was alleged. 

 " For example, beans and haricots take 20 and 13.7 pounds of 

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

 take 11 and 12.8 pounds of that acid, exactly what is found in a 

 crop of wheat. Trefoil is equally rich in phosphates with the 

 sheavas of corn that have gone before it." * His further re- 



cessive 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 foi-m 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." — p. 503. 



* Boussingault, I. v., p. 497. — Subjoined is a table, from the same work, of 

 the percentage of Mineral Substances taken tip from the soil hy various jylants grown 

 at Bechelbronn. 



