FUNCTION OF ACTINOMYCETES IN SOIL 201 



and in order to avoid as many as possible of the other factors 

 that might be involved in a comparison of miscellaneous soils 

 two samples were always collected on the same date, from spots 

 in the same soil not more than a few yards apart, one in old sod, 

 the other in a cultivated field. In this series of tests thirty- 

 eight pairs of samples were taken. Also a second shorter series 

 of tests was made to compare the Actinomyces flora of three 

 neighboring spots in a single soil type (Dunkirk silty clay loam), 

 one spot in fallow soil, one in old sod and the third in a field 

 which had been lq grass for two or three years only. 



All of the counts in these tests were made by means of gelatin 

 plates, because in the earlier work gelatin had been found the 

 best of the various media used for distinguishing Actinomyces 

 colonies from those of the lower bacteria. The gelatin used 

 sometimes contained soil-extract and sometimes tap-water 

 alone.^ Plates were always incubated for seven days at 18°C. 

 before counting. 



The results of the first series of tests are given in Table II. 

 It will be seen that the average number of Actinomycetes in sod 

 soil is nearly twice as high as the average number in cultivated 

 soil and that they averaged 37.5 per cent of the total flora in 

 sod soil but only 20.5 per cent of the flora of cultivated soil. 

 These general averages are much like those given in Table I, 

 but they tell only a part of the story, as it is possible for individual 

 exceptions to obscure the differences in the average. In order 

 to show the differences more plainly, the individual ratios were 

 determined and averaged. In the sixth column of Table II 

 is given the ratio of the actual number of Actinomycetes in the 

 sod soil to the number in the corresponding samples of cultivated 

 soil; in the last column of the table is given the ratio of the per- 

 centage of Actinomycetes in sod soil to the percentage in the 

 corresponding samples of cultivated soil. A study of these 

 ratios brings out some information not shown by the general 

 averages. 



* For the composition of these media see : Conn, H. J. Culture Media for 

 Use in the Plate Method of Counting Soil Bacteria. N. Y. Agric. Exper. ta. 

 Tech. Bui. 38, 1914. 



