LEGUME INOCULATION AND THE LITMUS REACTION OF SOILS. 5 



held on filter paper. The resulting? filtrate was nearly clear. Tlie 

 extracts were then tubed and sterilized. In the case of the soil 

 which reddened litmus, additional extracts were similarly prepared 

 after adding to the soil before leaching a small quantity of (1) air- 

 slaked lime and (2) shell marl. The tubes of sterilized soil extracts 

 were then inoculated with a vigorous culture of the vetch organism 

 and ])latings were made from these tubes two days later. The 

 results are presented in Table I. 



Table I. — Growth of vetch (Pseudomonas radicicola) culture inoculated into soil extracts 

 without treatment and also after the addition of lime and marl. 



The bottom land was considered the more fertile, and its extract 

 was certainly not less rich in food material for the bacteria than the 

 extract from the upland soil, but it would seem that the nature of 

 the latter soil extract w^as more favorable to the growth of the vetch 

 organisms. 



With alfalfa the red reaction and inhibition of soil-extract cultures 

 is commonh' obtained where no nodules are found, while spots in 

 the same field where nodules formed and the plants thrived are 

 invariably found to give either a blue reaction or to leave the color 

 of the litmus paper unchanged, and the extracts are favorable to the 

 growth of si:)ocific cultures. 



In three alfalfa fields in the same region as the vetch })reviously 

 referred to, examinations were made of spotted areas, soil samples 

 of the good and poor portions being tested by the litmus method. 

 A red litmus reaction was shown in every case where the growth 

 was poor, and the plants upon these areas were always devoid of 

 nodules, while no reaction was obtained from the good portions of 

 the field where nodules were found upon the roots. 



Samples of soil from similar "spotted" fields have been received 

 from widely separated regions, such as New Jersey, North Carolina, 

 and Oregon; those from the successfully inoculated portions invari- 

 ably proved neutral or blued litmus, while the soil where inoculation 

 "failed to take" reddened litmus in every case. The fact that such 

 spots exhibiting difl"erent reactions to litmus occurred in close prox- 

 imity in an aj)parentiy homogeneous soil suggests that the microflora 

 associated with the organisms which form nodules upon legumes 

 may bear some direct or causal relation to the reaction. The occur- 

 rence of these adjacent areas giving such diverse reactions to litmus 



ICir. 71] 



