554 ABSTRACTS 



or almost wholly checked by this process, especially if salts are present 

 prior to leaching. Investigations are being carried on to determine if 

 the injury done is or is not an ephemeral one which may entirely disap- 

 pear in a few months under field conditions. This and other experi- 

 ments in progress may lead to the discovery of the cause or causes of 

 damage done to relatively new soils by irrigation flooding. — Z. N. 



Quantitative Method for the Estimation of Bacteria in Soils. R. C. Cook. 



(Soil Science, 1912, 1, 153-163.) 



A synthetic medium is most desirable so that results may be compar- 

 able at different times and places, and for quantitative work the me- 

 dium which will permit the development of the maximum number of 

 colonies is usually most satisfactory. 



Twelve media were at first compared on four soils, Lipman and 

 Brown's modified synthetic agar being taken as a basis for comparison, 

 the only difference being in the methods of sterilization. In the third 

 series of experiments seven media were eliminated and in the last two 

 series, Lipman and Brown's agar was eliminated. Results indicate 

 that sodium asparaginate agar, albumen agar, and urea ammonium 

 nitrate agar will in most cases give a greater colony development for 

 soil bacteria than other media in common use in bacteriological work. 

 A five-day count gives much higher bacterial counts than a three-day 

 count. — Z. N. 



Stimulating Influence of Arsenic upon the Nitrogen-fixing Organisms of 

 the Soil. J. E. Greaves. (J. Agr. Res., 1916, 6, 389^16.) 

 The author has previously shown that arsenic stimulates the nitri- 

 fying and ammonifying powers of soil. The present investigation is 

 to show whether it has a similar effect upon nitrogen-fixation. Results 

 show that all forms of arsenic tested, except par is green, stimulate the 

 nitrogen-fixing power of soil, but not so greatly as the nitrifying power. 

 These results are obtained only when the tests are made in soil (either 

 untreated soil or sterilized soil reinoculated) ; for when solutions adapted 

 to nitrogen-fixation are inoculated with soil, the addition of arsenic 

 always proves toxic to the organisms concerned. One culture of Azoto- 

 bacter was obtained that was directly stimulated by the arsenic; but in 

 general, increase in nitrogen-fixation is observed only when a mixed 

 flora is used. If the soil infusion used for inoculating sterile soil is pre- 

 viously filtered or heated as high as 55°C., no stimulation is brought 

 about by the arsenic. 



The author does not favor any one theory to account for these results; 

 but considers the suppression of harmful microorganisms to be at least 

 a partial explanation. He points out that the results are almost exactly 

 what might be expected on the assumption that these harmful organ- 

 isms were protozoa. — H. J. C, 



