i 3 6 THE POPULAR SCIENCE MONTHLY 



nitrogen was a matter of controversy for a long time afterwards. The 

 experiments of some investigators showed that with sterilized soil and 

 with all sources of combined atmospheric nitrogen cut off, the free 

 nitrogen takes no part in the food supply of the plant. Other investi- 

 gators arrived at just the opposite conclusion. These opposite views led 

 to a great deal of discussion, and it was not until 1888 that Hellrigel was 

 able to account for these conflicting results by growing leguminous 

 plants in nitrogen-free soils. One set of plants was watered with dis- 

 tilled water, while to the other set was added in addition small amounts 

 of leachings, containing only a trace of nitrogen, from a cultivated field. 

 The plants watered with distilled water made but a small growth and 

 soon died of nitrogen starvation, but those watered with the leachings 

 reached a full growth and were found to contain about one hundred 

 times more nitrogen than the seed sown. It was observed that the roots 

 of the latter plants were covered with swellings, or nodules, which con- 

 tained characteristic organisms while those which were watered with dis- 

 tilled water only had none. Furthermore, no nodules appeared and the 

 plants did not develop when the soil leachings were sterilized before 

 using. The experiments thus showed that the plants which were pro- 

 vided with nodules must have obtained nitrogen through the agency of 

 the microorganisms ; that these must have come from the soil leachings ; 

 and that they must have the property of fixing the nitrogen of the air. 

 For some unknown reason these bacteria, to which the name Bacillus 

 radicicola has been given, do not develop on the roots of non-leguminous 

 plants ; consequently, when plants of this kind are grown in the soil and 

 harvested the total quantity of nitrogen present gradually becomes less. 

 The advantage of rotating leguminous plants with crops of this kind thus 

 becomes clear, because when a crop of the former is grown or plowed 

 under as green manure the total nitrogen in the soil is increased. As the 

 plants decay a part of the protein nitrogen of the plant again passes into 

 the elementary state, part changes into ammonia, and a third part 

 changes into nitrates. These changes are brought about by different bac- 

 teria, those responsible for the formation of nitrates being called nitrify- 

 ing bacteria. 



The amount of protein nitrogen which is converted into nitrates in 

 the soil by these bacteria varies with conditions and depends on the 

 physical condition of the soil, the quantity of organic matter present, the 

 moisture content and the temperature. A basic element as potash, soda 

 or lime must also be present with which the nitric acid formed may 

 unite. On a limited scale these conditons may be so controlled that large 

 quantities of protein nitrogen may be converted into nitrates, as is still 

 done in India for the production of potassium nitrate to be used in the 

 manufacture of gun-powder. The action of the nitrifying bacteria in 

 thus leading to the formation of nitrates does not bring about any in- 



