68 



SCIENCE 



[isr. S. Vol. XLIII. No. 1098 



titled, " The Determination of Nitric Nitrogen 

 in Soils," in whicli several of the older meth- 

 ods for determining this elusive radical re- 

 ceived extended and probably deserved criti- 

 cism. 



Among those receiving its full share was 

 the aluminum reduction method proposed by 

 the writer a little over two years ago. The 

 title of the article proclaiming this method 

 was, " The Aluminum Eeduction Method as 

 applied to the Determination of Nitrates in 

 ' Alkali ' Soils." It was at that time put for- 

 ward by the writer, not as the best possible 

 method that the future might develop for this 

 purpose, but as one which, in the presence of 

 the soluble chlorides, sulfates and carbonates 

 abounding in the " alkali " soils of the arid 

 west, would give far more reliable results than 

 the phenol disulfonic acid method of Gill then 

 commonly used in soil work. Comparisons 

 with this latter method are given. Another 

 reason for developing the method in question 

 was to accurately determine nitrates in nitri- 

 fication cultures in soils containing one or 

 more of the " alkali " salts. Occasionally 

 large amounts of nitrates are here encoun- 

 tered, and, as was shown, when more than 

 twenty or twenty-five milligrams of nitrogen 

 as nitrate are present, the colorimetric method 

 previously mentioned is of questionable value 

 even in the absence of " alkali." 



Briefly, the criticism of the method as made 

 by Allen is that very high amounts of soluble 

 humus and organic matter cause incomplete 

 reduction, the results running low. 



As all soil scientists know, the " alkali " 

 soils of California and other arid sections are 

 very low in soluble organic matter commonly 

 termed humus.^ They vary from almost noth- 

 ing to, in some few cases, 3 per cent. The 

 average for the surface soils of California is 

 1.28 per cent. It was for these soils, and not 

 for those high-humus soils of the middle west, 

 that the aluminum reduction method was orig- 

 inally intended. It was satisfactorily tried 



1 See ' ' Humus and Humus-nitrogen in Cali- 

 fornia Soil Columns, ' ' University of California 

 Publication in Agricultural Science, Vol. 1, pp. 

 173-274, by E. H. Loughridge. 



out with varying amounts of the " alkali " 

 salts singly and combined, also with soluble 

 organic matter in the forms of glucose and 

 soluble humus, in amounts far in excess of 

 that ever leached from " alkali " soils with 

 distilled water. 



The writer admits that some of the state- 

 ments made for the accuracy of his method 

 were possibly too broad and far-reaching, but 

 they were made more especially with reference 

 to its application to " alkali " soils, as the title 

 should suggest. The method, as proposed, has 

 been successfully used by others in arid sec- 

 tions, and the author still recommends it for 

 use under such conditions. 



In conclusion the writer wishes to state that 

 he will be the first to welcome any method for 

 determining nitrates in soils which combines 

 accurate and reliable results with a minimum 

 of time expended. 



Note. — Since the above was written (July 

 last) an article entitled " The Determination 

 of Nitrates in Soil," by E. S. Potter and R. S. 

 Snyder- has appeared in which the alumimim 

 reduction method, proposed by the writer and 

 criticized by Allen, is shown to be far superior 

 to the colorimetric methods even where the 

 high humus soils of Iowa were used. 



P. S. Burgess 



The Experiment Station op the 

 SuGAE Planters Association, 

 Honolulu, H. I. 



A SIMPLE METHOD FOR THE ELIMINATION 



OF PROTOZOA FROM MIXED CULTURES 



OF BACTERIA 



Protozoa, particularly various flagellates and 

 eiliates, often hamper the study of the higher 

 bacteria in mixed cultures. 



Such cultures may be readily and eSectively 

 freed from these undesired animals by centrif- 

 ugalization. By this means protozoa are 

 quickly thrown to the bottom of the tube, while 

 the bacteria for the most part remain in sus- 

 pension. If subcultures are then inoculated 

 from the supernatant fluid they will be found 

 entirely freed from protozoa. 



Doubtless this " fractional " centrifugali- 



2 Jo-ur. Ind. and Eng. Chem., Vol. 7, No. 10, p. 

 863. 



