INOCULATION OF SOIL WITH MICROORGANISMS 231 



they are by no means ubiquitous. Artificial cultivation of the 

 legumes over considerable areas widely separated from regions 

 where legumes have been previously grown necessitates special 

 treatment to insure the presence of the organisms. 



For the purpose of inoculation, some soil in which the particular 

 crop has been grown before may be used. One disadvantage of 

 this practice is the likelihood of also introducing plant parasites 

 and weed seeds at the same time that the bacteria are added. 

 The practice of inoculating with specially prepared cultures of the 

 specific legume bacteria has found most satisfactory use. Pure 

 cultures of the specific organisms grown in liquid or solid media 

 have proved to be effective. Other useful inoculating material is 

 prepared by mixing pure cultures with ground peat or similar sub- 

 stances. Either the seeds themselves or the soils are inoculated 

 with these special cultures of the specific legume bacteria or with 

 soil which is known to carry the organisms; this inoculation insures 

 the presence of the proper bacteria at the time and place where the 

 seeds develop. Inoculation of the seed is most effective and 

 requires the least amount of effort. A definite quantity of the 

 culture is mixed with the seed in a place not exposed to the sun. 



Once a soil has been inoculated with the specific organism and 

 has had a crop of the specific plant grown on it, it need not be rein- 

 oculated. However, in view of the fact that different strains of 

 the same organism vary in their vitality, in inoculating power and 

 in the amounts of nitrogen that they are able to fix, it has been sug- 

 gested that it is fully worthwhile to reinoculate the soil with fresh 

 cultures when a new crop is grown. Even a slight increase in crop 

 yield and in nitrogen content may more than cover the small addi- 

 tional cost of the culture. This is especially true of acid soils, in 

 which the nodule-forming bacteria deteriorate more rapidly than 

 in neutral and alkaline soils, so that within a very few years they 

 may completely disappear in the acid soils. 



Among the other processes which necessitate the artificial 

 introduction of microbes, it may be sufficient to mention the bac- 

 terial oxidation of suKur to sulfuric acid. Although there is no 

 doubt that most soils harbor organisms capable of oxidizing 

 sulfur to some extent, few soils contain organisms capable of per- 

 forming the oxidation very rapidly. The introduction of such an 

 organism as Thiobacillus thiooxidans into the soil with the 

 sulfur may considerably hasten the formation of acid. The use 



