44 tHE CflEMiSTRt 0^ THE FARM 



the humic constituents of soil, but it may be brought 

 into combination by living organisms in the soil. The 

 bacillus isolated from soil by Winogradsky assimilates 

 nitrogen only when supplied with fermentable organic 

 matter, and in the absence of oxygen and ammonia ; 

 the absence of oxygen is apparently sufficiently assured 

 by the presence of some organism making a large 

 use of that gas. The bacterium-like organism which 

 forms the tubercles on leguminous plants inhabits the 

 soil, it assimilates nitrogen freely when in union with 

 the roots of the host plant, but it has not been proved 

 that it assimilates nitrogen in the soil when not in 

 union with the plant. A soil bacterium (probably one 

 of those already mentioned) is also capable of living 

 in union with certain green algae, and under these cir- 

 cumstances is capable of assimilating nitrogen. In all 

 these cases it appears that the supply of carbohydrates 

 furnished by a green plant is needed if nitrogen is to 

 be assimilated. Whether nitrogen can be assimilated 

 when humus is the only organic matter available is at 

 present doubtful. The practical conclusions are thus 

 at present limited. The enrichment of the soil with 

 nitrogen when leguminous plants hold possession of the 

 land is, however, a fact well assured and of the highest 

 importance. With the exception of this fact, it is well 

 to remember that with land under tillage the losses 

 of nitrogenous matter by oxidation and drainage 

 generally greatly exceed any gains, and a necessity 

 thus exists for restorative cropping and manuring. 



Attempts have been made to increase the fertility of 

 soils by adding to them the living organisms which 

 assimilate nitrogen from the air ; such applications 

 have generally failed, probably because soils are already 

 sufficiently provided with these organisms. 



