January 2, 1903.] 



SCIENCE. 



19 



from measured plots of fields from whicli 

 the crops had been harvested. His re- 

 sults are given in kilos per hectare and 

 refer to dry matter. The nitrogen of the 

 refuse was also determined. His figures 

 are given in the following table: 



Nitrogen of 

 Crop. Refuse. Refuse. 



Wheat 1,002 518 2.1 



Oats 1,608 650 2.6 



Clover 1,975 1,547 27.9 



If it be considered that the essential ash 

 ingredients of plant food are equally high 

 in the clover refuse, it will be seen that the 

 manurial value of the clover refuse is out 

 of all proportion to that of the two cereals, 

 and consequently that clover must be a 

 better forerunner for a grain crop than a 

 grain crop itself. But Boussingault did 

 not stop here. He also collected the refuse 

 matter, roots and leaves from a crop of 

 mangolds, and found that not only the dry 

 matter, but also the nitrogen contained 

 therein, was in excess of that of the clover. 

 Here was a dilemma ; for it was well known 

 that, compared to legumes, root crops were 

 poor forerunners for grain crops. The 

 explanation for this apparent contradic- 

 tion was found in extensive experiments 

 made at Rothamstead. Laws and Gilbert 

 raised root crops on the same field for 

 years in succession without the application 

 of manures, and found that they rapidly 

 exhausted the surface soil. On the other 

 hand, they showed that with clover, even 

 after the removal of a highly nitrogenous 

 crop, the soil was left richer in nitrogen 

 than it was before. It is but fair to state 

 in this connection that other investigators 

 found much larger yields with clover than 

 Boussingault. Thus, to take the other ex- 

 treme, Heiden obtained from measured 

 plots of clover, after it had become fully 

 ripe, and by removing the whole aerial 



portion of the crop, the following results, 

 expressed in kilos per hectare: 



Aerial Portion. Roots. 



Dry matter 14,548 8,469 J5 



Nitrogen 381.5 275.3 



Laws and Gilbert, Heiden, and in faet • 

 all who investigated this subject explained- 

 this large accumulation of nitrogen prin- 

 cipally by the assumption that clover, on . 

 account of its deep roots, had the power, 

 in a marked degree, of obtaining a large 

 portion of its food from the subsoil ancJ 

 bringing it to the surface. Furthermore, 

 it was assumed that on account of the great 

 leaf surface of clover, its more succulent 

 nature and its longer period of growth, it 

 was capable of collecting more ammonia 

 from the air than was the case with grasses 

 and cereals. Another peculiarity which, 

 the legumes were thought to possess was 

 their ability to assimilate, in a higher de- 

 gree than other crops, the reserve nitrogen 

 of the soil. This assumption would ex- 

 plain, of course, why these plants should 

 make a luxuriant growth on soils on which, 

 for lack of available nitrogen, other crops 

 failed to make a good stand, but it would 

 not throw any light upon the fact, estab- 

 lished by general observation, that the 

 total fixed nitrogen of the soil was so ma- 

 terially increased. 



It may be truthfully said that all these 

 explanations taken together were not en- 

 tirely satisfactory to those who were en- 

 gaged in the teaching of agricultural chem- 

 istry, but, in short, this was the status of 

 the nitrogen question for a generation or 

 more, when Hellriegel announced befOTie 

 the section of agricultural chemists of the 

 (■ erman Association of Men of Science and. 

 Physicians, at their meeting iu 1886, that 

 the leguminous plants could assimilate the 

 free nitrogen of the air, and that this as- 

 similation was intimately connected with 

 the nodules appearing upon the roots of 



