METHODS EMPLOYED IN BACTERIOLOGICAL INVESTIGATIONS. 11 



baking ex})eriiiioiits it was noticed that the nearer a set of samples 

 was baked at 100° C. the better the subsequent filtering, probably 

 indicating that the clay is siliceous. 



The Griess method is the standard for determining nitrites, but 

 oAving to the delay in getting chemicals at Fallon tlie potassium- 

 iodid-starch method was used for a large part of tlie woik. This 

 method, while primarily a cpialitative one, was found to be fairly 

 rehable 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. 



^ Both of these salts were determined by Mr. C. A. Jensen, of (he Office of Western 

 Agricultural Extension of the Bureau of Plant Industry. 



^ Schreiner, Oswald, and Failyer, George If. ( 'olorimetric. Turbidity, and Titration 

 Methods Used in Soil Investigations. Bulletin 31, Bureau of Soils, U. S. Dept of 

 Agriculture, 1906. 

 211 



