SOIL EXHAUSTION AND ROTATION OF CROPS. J 99 



always chiefly supplied by the atmosphere and may be supplied 

 by the atmosphere exclusively. It is uot necessary that it should 

 be in the soil. 



The nitrogen of the plant, which forms indeed a small propor- 

 tion — two per cent, perhaps, as an average — of the dry plant, is 

 an important ingredient, for without it vegetation cannot exist. 



Some crops have the power of gathering nitrogen without any 

 difficulty; and they not only supply themselves with it but they 

 even cause its accumulation in the soil. There are other crops 

 which are dependent upon artificial supplies of nitrogen, unless 

 the soil be naturally very rich in this element — crops which, if we 

 undertake to raise them continuously on the same field, presently 

 begin to show tljat they lack something, while if we apply nitro- 

 genous compounds as fertilizers, the growth is ensured. We do 

 not know in full detail how plants acquire a sufficient supply of 

 nitrogen from the atmosphere, but we conclude, with great prob- 

 ability, from the results of practice, that different plants draw on 

 the natural supplies of nitrogen in a different way. 



Let us consider how the structure and habits of two t}*pical 

 crops, wheat and clover, stand in relation to their power of assim- 

 ilating 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, main- 

 tain 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 dura- 

 tion 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 average, not very unlike when clover is cut for hay,* 



* Corresponding crops arc, according to Winter Wheat. Clover. 



Lbs. per acre. Lbs. per acre. 



E.Wolff 6,230 5,340 



Lippe-Weiscnfdd 5,700 6,330 



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



