Vol. XIV Washington, D. C., August 12, 1918 No. 7 
SOIL REACTION AND THE GROWTH OF AZOTOBACTER 
[PRELIMINARY PAPER] 
By P. L. Gainey 
Soil Bacteriologist, Kansas Agricultural Experiment Station 
INTRODUCTION 
It has frequently been observed in this and other laboratories that, 
when sbils are examined for Azotobacter,some give, on a mannite nutrient 
solution, a characteristic dark-brown film composed almost wholly of 
Azotobacter cells. Others give no visible surface growth. During the 
summer of 1917 a preliminary survey was conducted to ascertain to 
what extent soils in the vicinity of this Station exhibited the above 
variations. In all, 90 soils were collected within a radius of 2 miles of 
the laboratory. These samples were taken from as widely varying con¬ 
ditions as could be located. Some were collected from the highest hills 
and others from the lowest overflow bottom land, one even from a 
sand bar in the Kansas River. Samples of soil were taken from all of 
the following soil conditions: Cultivated, permanent alfalfa, pasture, 
roadsides, hedges, river and creek banks, and forests. Some of the 
spots from which samples were obtained were very fertile, while others 
were practically barren. 
EXPERIMENTAL WORK 
In collecting the soil for examination the ordinary precautionary 
methods used to prevent contamination were observed. The soil 
sample examined was a well-mixed composite of six or more smaller 
samples collected within a few yards of each other. When convenient, 
soil was taken to a depth of approximately 6 inches. As soon as possible 
the samples were brought to the laboratory, and sterile Erlenmeyer flasks 
containing 50 cc. of the following cultural solution were immediately 
inoculated. The composition of the cultural solution was: Di-potassium 
phosphate (K 2 HP 0 4 ), 0.2 gm., magnesium sulphate (MgS 0 4 ), 0.2 gm., 
sodium chlorid (NaCl), 0.5 gm., mannite, 20 gm., ferric chlorid (FeCl s ), 
trace, distilled water, 1,000 gm. After all the salts had been dissolved, the 
solutioif was rendered slightly alkaline to phenolphthalein with sodium 
(265) 
Journal of Agricultural Research, 
Washington, D. C. 
ot 
Vol. XIV, No. 7 
Aug. 12, 1918 
Key No. Kans.-i4 
