Feb. 1897.] BOTANICAL SOCIETY OF EDINBUKGH 25 



The Bacteria of the Soil, with Special Eeference 

 TO Soil Inoculation. By R. Stewart MacDougall, 

 M.A., B.Sc. 



(Read 11th February 189 7.) 



Up till about twenty years ago the soil was looked upon 

 as made up merely of so many bits of dead material — stone 

 and lime, and clay, etc. Tillage was a mechanical opera- 

 tion, and changes that followed it were explained purely 

 on chemical and physical grounds. In these last years, 

 however, it has come to be recognised that the soil teems 

 with myriad minute forms of life — useful, and harmful, 

 and neutral ; in a word, for the tiller of the ground the 

 problem is not solely chemical, but also biological. 



Many workers in soil bacteriology have given their 

 attention to the exact numbers of germs present in the 

 soil, and in different layers of it. It would weary the 

 reader to give lists of the numbers which vary in a gramme 

 (just over 15 grains) of earth, according to the circum- 

 stances, from only a few up to some millions. Let me 

 rather give the general principles determining their number 

 as enunciated by Maggiora:^ — (1) The number of bacterial 

 germs, in otherwise resembling circumstances, is in forest 

 soils less than in arable land, and in these less than in the 

 soil of inhabited places. (2) In non-cultivated soils the 

 number of bacteria changes with {a) the geological formation 

 and the height above the sea ; (&) with the imperviousness 

 or the aeration of the soil, the germs being much less 

 numerous in the former than in the latter ; (c) with the 

 nature of the soil, — sandy soil is less rich in bacteria than, 

 say, humus soils. (3) In cultivated soils the number of 

 germs increases with the culture activity and with the 

 bringing of dung. Strongly dunged soils are much richer 

 than poorly dunged or imdunged. The greatest bacterial 

 richness is at a depth of 8 to 20 inches ; below this the 

 number quickly decreases. This is true for both cultivated 

 and non-cultivated soils. 



If I say that Kramer, as an average of three experi- 



^ Maggiora, Journal de la Agricole du Brabant, 1888. 



