New York Agricultural Experiment Station. 205 



Each sample of soil has been plated in two different dilutions. 

 These dilutions have varied somewhat with the different soils used; 

 but in each test listed in this bulletin, the figures for the different 

 media have invariably been obtained from plates of the same dilu- 

 tion. The dilution chosen for counting has usually permitted about 

 one hundred colonies to develop per plate. Plates have always been 

 made in triplicate. The counts given in the tables represent the 

 average of the three plates, except in cases where one of the three 

 has been lost by liquefaction or otherwise. In any case where 

 only one of the three triplicate plates has given a reliable count, 

 the results have been discarded or else the figures in the table have 

 been marked doubtful. 



THE SOIL-EXTRACT GELATIN. 



DESCRIPTION of the medium. 



The soil-extract gelatin has been used in routine work by the 

 writer for five years. When first used, it was thought to be merely 

 a makeshift that would quickly be superseded by other media upon 

 which soil bacteria could grow more readily, but it has proved so 

 satisfactory that it has been kept in routine use in this laboratory 

 up to the present time, and has been included in such a large majority 

 of the tests reported in this bulletin that it serves as a basis of com- 

 parison between the media that are not compared together directly. 21 

 As already mentioned it is to be recommended for qualitative work 

 because it meets the requirement of showing distinctions in appear- 

 ance between the colonies of different kinds of bacteria. The data 

 published in this bulletin will show whether it is also adapted to the 

 growth of as large a number of soil bacteria as are the other culture 

 media that have been proposed for soil work. 



The preparation of this gelatin is as follows: Soil, heated in an 

 autoclave for an hour at 20 to 25 pounds pressure, is extracted by 

 mixing with an equal weight of distilled water, allowing the mixture 

 to stand cold for twelve hours and then boiling half an hour, re- 

 storing the water lost by evaporation, and filtering. In making up 

 each batch of the medium, the soil-extract is diluted with distilled 

 water to one-tenth its natural, strength and used for dissolving the 

 gelatin. (Gold Label Gelatin has always been used.) It is probably 

 unnecessary to carry out in detail the whole of this procedure for 

 obtaining soil-extract, but it was followed carefully throughout the 

 present work in the hope that the composition of the soil-extract 

 might be more nearly constant than it would be if the method of 

 preparation were allowed to vary. After dissolving the gelatin in 

 the diluted soil-extract, the medium is clarified by the use of the white 



21 The only caution necessary in employing this gelatin is the use of an incubation 

 temperature as low as 18° C. See p. 28. 



