384 MXPEBIJilENTS WITH PLANTS 



page 147) ; these bacteria, it should be said, are not 

 confined to the manure -heap but are found almost 

 everywhere in soil which contains organic matter. ^ 



Occurring in soil and water, along with the bacteria 

 just mentioned, are found the nitrogen -fixing bacteria, 

 which have the power of fixing the free nitrogen of the 

 air and converting it into compounds which eventually 

 become available to the plant. Our knowledge of 

 these bacteria is as yet scanty, but several of them 

 have been isolated and studied, and one of them is 

 offered for sale as an almost pure culture under the 

 name of "alinit" (its practical value is still in dispute 

 and probably depends a good deal on local conditions) . 

 Since these bacteria need abundance of air, good 

 tillage is important in promoting their activity. 



Another kind of bacteria which also possess the 

 power of fixing free nitrogen and making it available 

 to the plant inhabits the root -tubercles of certain 

 plants (principally members of the Pea family) , which 

 are thus able to draw supplies of nitrogen directly from 

 the air. On this account they are of inestimable value 

 as green manures (see page 149). 



It is found that if these plants are cultivated in 

 sterilized soil no tubercles appear; furthermore, such 

 plants begin to suffer after a time from nitrogen 

 hunger ; if now they are watered with soil infusions 



1 See an article in the Year-Book of the U. S. Department o£ Agriculture 

 for 1895 by Wiley; for 1902 by Moore, 



