8 



the world. But in all parts of the world it is the same bacterium 

 which is responsible for the production of nitrates. 



In America the living bacteria of suitable soil are actually culti- 

 vated and distributed for trial in poor soils, by the U.S. Depart- 

 ment of Agriculture. 



In the laboratory, where pure cultures can be obtained, these two 

 organisms can be demonstrated separately, but in nature they work 

 simultaneously. They are most active a few inches below the 

 surface, for, like most bacteria, they work in the dark and object to 

 the light of day. This nitrification of the soil by means of bacteria 

 can take place only under certain conditions, namely, the presence 

 of small amounts of easily oxidisable organic matter. In other 

 words, imperfectly rotted manure is detrimental to their activity. 

 Hence the importance of applying well-rotted manure to crops. They 

 will not tolerate acid condition, hence the need of a sufficiency of 

 carbonate of lime. In this manure the ammonia arising from 

 the action of putrefactive bacteria is changed into carbonate, which 

 is rapidly converted by ' ' Niirosomonas " to nitrites, and then by 

 " Nitrobacte r " to nitrates, the changes proceeding so rapidly that only 

 traces of ammonia or nitrite are ever found in normal arable soil. 

 Absence of air puts an end to the activity of the bacteria heiioe 

 the importance of harrowing. 



From these lacts it seems that if and when the vast deposits of 

 nitrate in Chili and Peru become exhausted, it would be possible to 

 make nitre artificially by the aid of bacteria. f But that is for the 

 future. 



In addition to these nitrifying bacteria there are in the soil 

 numerous organisms that are able to nx atmospheric nitrogen, and 

 of these a bacterium, "Azotobacter," is most important. For the 

 action to be rapid the bacteria must be sufficiently provided with 

 carbonate of lime, potash, phosphates, organic matter, and moisture. 

 A suitable temperature must be maintained. 



It has long been known that leguminous plants will nourish on soils 

 that are very poor in nitrogen, on sandy soils for instance, and it 

 was for many long years a mystery as to how they managed to do 

 this. It was known as long ago as 1866 that the root nodules that 

 occur on lucerne, beans, clover, vetches, and other leguminous plants 

 contain bacteria^ and it is now known that these have the power of 

 tixing free nitrogen; these plants are independent of nitrogenous 

 manures. These facts are perfectly familiar to everyone, and form 

 an important factor in the rotation of crops. These plants can be 

 used in increasing the supply of nitrogen in cultivated soils and re- 

 claiming barren sands and clays. Wherever leguminous plants grow 

 they enrich the soil in organic nitrogen through the co-operation of 

 nodular bacteria. When the host plant dies or is ploughed into the 

 'land the nitrogen compounds are speedily changed into plant food. 

 Few improvements in agriculture have produced more marked effects 

 than the extension of leguminous cropping. Yet how were these 

 facts discovered? By countless experiments lasting over many 



tBurnet, Etienne. "Microbes and Toxins," 1912, p. 15. 



