EIGHTEENTH ANNUAL YEAR BOOK— PART VHI 439 



duced into soil deficient in nitrogen, legumes would grow satisfactorily 

 on that soil and actually increase the amount of nitrogen in it. They 

 not only take enough nitrogen from the air for their own growth, but 

 store a surplus in the soil. Without the presence of the bacteria, however, 

 the legumes do not thrive and they are not able to secure their supply 

 of nitrogen from the air. 



It was also demonstrated that not only must the necessary bacteria 

 be present in the soil for this important work of the legumes, but they 

 must enter the roots of the plants and form swellings or nodules there. 

 Thru this means a state of mutual helpfulness is set up, called symbiosis. 

 The plant supplies the bacteria with certain food materials and in 

 return the bacteria draw nitrogen from the air and furnish it directly 

 to the plant. 



Later investigations led to the discovery that the nodules on any 

 legume contain only one kind of bacteria; they are therefore called 

 "pure cultures." The organisms which grow thus with all legumes 

 have since been found to belong to the same species, and have been 

 named Bacillus radicicola. They include, however, v/ell defined strains 

 or varieties, each especially adapted to grow with certain legumes, 

 and it is more or less difficult if not impossible for them to adapt 

 themselves to certain other legumes. Such adaptation apparently does 

 occur thru long periods of tiyne, but farm practice demands immediate 

 results and hence the proper species of bacteria must be present in the 

 soil if inoculation is to occur. 



WHEN SOIL IXOOULATION IS >"ECESSARY 



There is no easy test for the presence of a particular kind of bac- 

 teria in a soil. The only- way to be positive about the inoculating power 

 of soil is to grow a legume and note whether there is an abundance of 

 nodules on its roots. However, certain facts indicate rather definitely 

 when inoculation should be practiced. In the first place, if the soil is 

 poor and it has never borne a legume crop previously, the necessary 

 bacteria are probably not present and they should be introduced. If one 

 legume has been grown on a soil and has been inoculated, that does not 

 give assurance that the proper bacteria are present for any other legume 

 and inoculation should be made. The bacteria which grow on the roots 

 of certain legumes will not grow on roots of certain others except in one 

 case. Alfalfa and sweet clover do cross inoculate and the same bacteria 

 grow in the roots of both these legumes. Successful inoculated crops 

 of either of these plants will therefore insure inoculation of the other 

 on the same soil. 



If a legume has been grown successfully but its roots have no nodules, 

 the soil should be inoculated in order that the plant may secure its proper 

 supply of nitrogen from the air. On fertile soils legumes many times 

 make entirely satisfactory growth without inoculation, drawing their 

 nitrogen from the soil just like other crops. 



If a legume made unsatisfactory growth even tho some nodules were 

 found on its roots, inoculation should be practiced. The poor crop 

 production indicates the absence of suflacient vigorous bacteria of the 



