METHODS EMPLOYED IN BACTERIOLOGICAL INVESTIGATIONS. 11 
baking experiments it was noticed that the nearer a set of samples 
was baked at 100° C. the better the subsequent filtermg, probably 
indicating that the clay is siliceous. 
The Griess method is the standard for determining nitrites, but 
owing to the delay in gettmg chemicals at Fallon the potassium- 
iodid-starch method was used for a large part of the work. This 
method, while primarily a qualitative one, was found to be fairly 
reliable for quantitative determinations if a large quantity of reagent 
was used when the nitrites were high, as indicated by a rapid develop- 
ment of the blue-black color. The Grandval-Lajoux phenol-sulphonic 
acid method as modified by Syme! was used for estimating nitrates; 
before determining nitrates the nitrites were removed by urea in acid 
solution in accordance with Piccini’s method. 
Chlorids were frequently high in soil solutions in which nitrates were 
to be determined, and it was necessary to remove them when present 
in concentrations greater than 50 or 70 parts per million. This was 
accomplished by the use of silver sulphate. 
Chlorids? were determined by the Mohr method, titrating the 
neutral solution with N/10 silver nitrate and using potassium chromate 
as an indicator. Sulphates? were determined by the turbidity 
method described by the Bureau of Soils.® , 
DENITRIFICATION. 
Studies of denitrification were made by inoculating Dunham’s 
peptone solution containing 0.2 per cent potassium nitrate with soil 
and with a Frost scale measuring roughly the quantity of free nitrogen 
evolved. Either ordinary fermentation tubes or test tubes inverted 
in salt-mouth bottles were used. The latter method is preferred, as 
it permits the use of larger quantities of soil for inoculations. 
NITROGEN FIXATION. 
Leguminous plants were examined for the presence of nodules, and 
Azotobacter cultures were isolated from soil samples. 
‘Syme, W. A. The Colorimetric Determination of Nitrates in Soil Solutions Con- 
taining Organic Matter. Thirty-first Annual Report of the North Carolina Agricul- 
tural Experiment Station, for the Year Ending June 30, 1908, pp. 64-65. 
2 Both of these salts were determined by Mr. C. A. Jensen, of the Office of Western 
Agricultural Extension of the Bureau of Plant Industry. 
3 Schreiner, Oswald, and Failyer, George H. Colorimetric, Turbidity, and Titration 
Methods Used in Soil Investigations. Bulletin 31, Bureau of Soils, U. S. Dept of 
Agriculture, 1906. 
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