SOIL EXHAUSTION AND ROTATION OF CROPS. 149 



Let US consider how the structure and habits of two typi- 

 cal crops, wheat and clover, stand in relation to their power 

 of assimilating atmospheric nourishment. In respect of 

 foliage we cannot certainly say that the wheat plant or 

 the wheat crop when full grown, exposes a less surface to 

 the air than full grown clover, but we know that the leaves 

 of wheat, as of all our cereals, maintain their green color 

 and succulence during a much shorter time than is true of 

 clover. In case of winter grain the period of leaf-activity 

 usually begins in October and ends shortly after heading out, 

 in June, some weeks before the crop is harvested. Clover, on 

 the other hand, is not arrested in its growth by any crisis of 

 seed-production, but, when cut for hay, sends up new shoots, 

 unfolds new leaves, and shortly yields an aftermath, its 

 growth going on uninterruptedly all the summer and late into 

 autumn, until checked by heavy frosts. 



That the actual leaf surface of the clover crop, taking its 

 duration into account, is much greater than that of the wheat 

 crop, I do not doubt, because although the total weight of 

 the harvested crops is, on the averaojp, not very unlike when 

 clover is cut for hay,* the total amount of vegetable matter 

 organized is much greater in case of clover than in that of 

 wheat, as appears from the table on page ninety-six, where 

 clover roots are seen to constitue two-fifths (equal to six- 

 fifteenths) of the entire plant, while the roots of rye, which 

 doubtless do not differ much from those of wheat, are but 

 one-fifteenth of the entireplant. 



You see that the foliage and mode of life of these two 

 classes of plants are very different for the purposes of gather- 

 ing food fi-om the atmosphere, and they must therefore be ex- 

 pected to leave the soil in a very different condition, because 



* Corresponding crops are, according to 



Winter Wheat. Clover. 



Lbs. per acre. Lbs. per acre. 



E. Wolff, . 6.230 5,340 



Lippc-Weisenfcld, 5,760 6,330 



Rohde, ....... 4,270 to 6,400 3,480 to 5,230 



