ATMOSPHERIC NITROGEN 103 



amount in the subsoil remained nearly the same. All 

 of these facts plainly indicated that crops like clover 

 had the power of gaining nitrogen from unknown 

 sources. The results of prominent German agricul- 

 turists led to the same conclusion. It was known 

 that wheat grown after clover gave as good results as 

 the use of nitrogenous manures for the wheat, but for 

 many years this fact was unexplained. 



Laboratory experiments with sterilized soils do not 

 represent the normal conditions of growing crops 

 where all of the bacteriological agencies of the soil, 

 the air, and the plant, are free to act. Experiments 

 have shown that these agencies have an important 

 bearing upon plant growth. 



In the work of the different investigators prior to 

 1888, plants were grown mainly in sterilized soils, and 

 under such conditions they were unable to make use 

 of the free nitrogen of the air, except when subse- 

 quently innoculated from the air. 



121. Hellriegel's Experiments. Hellriegel grew 

 leguminous plants in nitrogen-free soils. One set of 

 plants was watered with distilled water, while another 

 had in addition small amounts of leachings from an 

 old loam field. The plants watered with distilled 

 water alone, made but little growth, while those 

 watered with the loam leachings reached full matur- 

 ity and contained something like a hundred times 

 more nitrogen than was in the seed sown. The dark 

 green color was also developed, showing the presence 

 of a normal amount of chlorophyl. The roots of the 



