138 BACTERIA 



or nearly so, virgin soils contain much fewer than cultivated 

 lands, and these latter, again, fewer than made soils and 

 inhabited localities. In cultivated lands the number of 

 organisms augments with the activity of cultivation and the 

 strength of the fertilisers used. In all soils the maximum 

 occurs in July and August. 



But the condition which more than all others controls the 

 quantity and quality of the contained bacteria is the degree 

 and quality of the organic matter in the soil. The quantity 

 of organic matter present in soil having a direct effect upon 

 bacteria will be materially increased by placing in soil the 

 bodies of men and animals after death. Dr. Buchanan 

 Young two or three years ago performed some experiments 

 to discover to what degree the soil bacteria were affected by 

 these means. ** The number of micro-organisms present in 

 soil which has been used for burial purposes,'* he concludes, 

 " exceeds that present in undisturbed soil at similar level, 

 and this excess, though apparent at all depths, is most 

 marked in the lower reaches of the soil." * The numbers 

 were as follows : — 



Virgin soil, 4 ft. 6 in. = SZAZ^ "^-o. per gram of soil. 

 Burial soil (8 years), 4 ft. 6 in.= 363,411 m.o. per gram of soil. 

 " (3 '' ),6ft.6in.= 722,7Si 



Methods of Examination of Soil, Two simple methods are 

 generally adopted. The first is to obtain a qualitative esti- 

 mation of the organisms contained in the soil. It consists 

 simply in adding to test-tubes of liquefied gelatine or broth 

 a small quantity of the sample, finely broken up with a 

 sterile rod. The test-tubes are now incubated at 37° C. and 

 22° C, and the growth of the contained bacteria observed 

 in the test-tube, or after a plate culture has been made. 

 The second plan is adopted in order to secure more accurate 

 quantitative results. One gram or half-gram of the sample 



' Proc. Royal Soc. of Edin., xxxvii., pt. iv., p. 759. 



