334 
Journal of Agricultural Resaerch 
Vol. XIV, No. 8 
organism was studied for each strain and in every case the organism 
used to inoculate plants grown under sterile conditions effected inoculation. 
In all of the tests the organisms were inoculated into 50-cc. portions 
of mannitol solution in 200-cc. Erlenmeyer flasks, the reaction changed 
by the addition of sulphuric acid or sodium hydroxid, and the cultures 
incubated for four weeks at 28° C. At the end of the period of incu¬ 
bation the presence or absence of the bacteria was determined by plate 
counts, microscopical mounts, and by inoculation of mannitol-agar slants. 
Aside from the total acid or alkali, the hydrogen-ion content in these 
cultures was measured by the colorimetric method. 
The results of these experiments show clearly that sulphuric add in 
culture solutions is far more injurious to alfalfa bacteria than to lupine 
bacteria. In other words, the nodule bacteria from different plants 
behave differently toward add. The legume bacteria may be divided 
into groups about as follows: 
1. Critical Ph 4*9.Alfalfa and sweet clover. 
2. Critical Ph 4-7.Garden pea, field pea, and vetch. 
3. Critical Ph 4* 2 .Red clover and common beans. 
4. Critical Ph 3 3.Soybeans and velvet beans. 
5. Critical Ph 3.15.Lupines. 
The alfalfa organism is the most sensitive of the legume bacteria to 
acidity, and, conversdy, the lupine organism is the most resistant to 
acidity. 
The toxidty of sodium hydroxid toward legume bacteria is not notice¬ 
able until the alkali is added in large amounts; approximately 10 times 
as much normal alkali as normal add is required to produce a similar 
injury. The organisms from the nodules of different legumes failed to 
show any dedded difference in respect to alkali. For instance, it ap¬ 
pears that the alkali limit of growth is the same for Rhizobium legumi- 
nosarum from lupine or from alfalfa. 
One striking fact noted in the data of these experiments is the 
extreme sensitiveness of Azotobacter to slight changes in reaction. 
As compared with the legume bacteria, this organism is far more sen¬ 
sitive. The add limit of growth in mannitol solution for Azotobacter is 
about N/i )t 333.3 and the alkaline limit about N/i,ooo , or the critical P H 
add value 6.5 and the alkaline value 8.6. 
In rdation to hydrogen-ion concentration of medium the nodule 
bacteria from different legumes show a very dedded difference. The 
evidence supports the condusion that a correlation exists between the 
add resistance of the bacteria and the acid resistance of the higher 
plant. 
