THEIR EARTHY CONSTITUENTS. 



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with one which, like clover, although it takes up 77 pounds of al- 

 kali per acre, may be consumed on the field, 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 which 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 

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

 searches seem to show that these crops exhaust the soil less than 

 the cereal grains, in part at least, on account of the large quantity 

 of organic matter, rich in nitrogen, which they leave to be incor- 

 porated with the soil. The theory of rotation in crops, founded by 

 De Candolle on the assumption that excretions from the roots of a 

 plant accumulated in the soil until in time they became injurious 

 to that crop, but furnished appropriate food for a different species, 

 is entirely abandoned as an explanation ; and even the fact that 



* Boussingault, 1. c., p. 497. Subjoined is a table, from the same work, of 

 the percentage of Mineral Substances taken up from the soil by various plants 

 grown at Bechelbrunn. 



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