B. P. I.— C12. 



LEGUME INOCULATION AND THE LITMUS 



REACTION OF SOILS. 



INTRODUCTION. 



A better understandin<j; of the conditions which affect the ability 

 of certain nitrogen-fixing bacteria to produce nodules upon the roots 

 of leguminous plants is recognized as of great importance, and is 

 especially desired in connection with the practical inoculation and 

 cultivation of leguminous crops. Following the indications previ- 

 ously secured concerning soil conditions affecting legume inoculation" 

 a more extended investigation along these lines has been in progress. 

 The attempt has been made to examine man}" different soils both in 

 the field and by samples, with a view to correlating, if possible, cer- 

 tain of their characteristics with their behavior toward the growth 

 of legumes and the nodule bacteria of legumes. Though no broadly 

 comprehensive results liave as yet been obtained, and perhaps are 

 not to be expected, some interesting data have been secured. The 

 more fundamental problems having to do with the associative action 

 of the nodule-forming organism with the different soil bacterial 

 floras must be reserved for further study. 



THE LITMUS REACTION FOR SOILS. 



Some planters believe that for growing legumes the best soils are 

 those which either turn litmus blue or have no action upon it. 

 There has been some criticism, however, of the old-style plan of 

 testing soils by firmly squeezing strips of litmus paper against samples 

 of the soils, and there are undoubtedly many sources of error in this 

 crude technique entirely aside from the question of the proper inter- 

 pretation of the color of the litmus. In this office a method has been 

 developed that is free from some of the objections to the old method, 

 and it secures a fair degree of consistency in technique. 



Samples of soil are taken from representative areas of the fields 

 under consideration, and their reaction is tested in the following man- 

 ner: Strips of neutral litmus paper are placed in the bottom of a 

 number of petri dishes; over these are laid four thicknesses of filter 

 paper and on the filter paper are placed the soil samples to be tested. 



aKellerman, Karl F., and Robinson, T. R. Conditions Affecting Legume Inocu- 

 lation. lUillctiii 100, pi. 8, Bureau of Plant Industry, U. S. Dept. of Agriculture, 1907. 



60524°— Cir. 71-10 3 



