NITROGEN, NITRIFICATION, NITROGENOUS MANURES 123 



elusion. It was known that wheat grown after clover 

 gave as good results as when nitrogenous manures were 

 used, but for many years this 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 show 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 the soils were 

 subsequently inoculated from the air. 



132. Hellriegel's Experiments. Hellriegel grew le- 

 guminous 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 maturity and contained 

 something like a hundred times more nitrogen than was 

 in the seed sown. The dark green color also was 

 developed, showing the presence of a normal amount of 

 chlorophyll. The roots of the plants had well-formed 

 swellings or nodules, while those that were watered with 

 distilled water alone had none. The loam leachings 

 contained only a trace of nitrogen. 42 



