162 THE SOIL FLORA 



organic matter usually ranges from three to forty million. Tight, 

 non-porous soils, as well as alkali and soils low in humus and 

 moisture, yield low bacterial counts. 



The number of microorganisms in light, sandy, very tight clay, 

 desert and forest soils is usually much smaller, other things being 

 equal, than in normal cultivated soils. Cultivation of soils tends 

 to increase greatly the number of bacteria. The average of several 

 hundred determinations made on cultivated soil of the arid regions 

 gave a bacterial count of 4,452,000, whereas the average of a similar 

 number of samples taken from adjoining virgin soils was 2,270,000. 



Factors Influencing Number. Optimum moisture, organic matter 

 and aeration tend to increase the number as does also 'the addition 

 of sugar and certain antiseptics : 



Bacterial number after 50 days. 



Soil treated with antiseptics, 

 Control soil. millions per gram. 



Cane sugar (p. 25 per cent.) ... 21 51 



Amyl alcohol (0.1 per cent.) ... 30 85 



Phenol (m/200 per kilo) .... 27 101 



Hydroquinone (m/200 per kilo) 16 55 



There is a direct relation between the number of bacteria found 

 in the soil and the quantity of organic manure added. This is 

 illustrated in results obtained by the author and given in tabular 

 form below. The unmanured soil is taken as 100 per cent. 



Treatment. Fallow soil. Cropped soil. 



No manure .... 100 100 



5 tons of manure 144 123 



1 5 tons of manure 177 129 



The aeration of the soil often increases manyfold the number of 

 bacteria, and according to Conn the greatest numerical increase 

 occurs in the group of non-spore-forming bacteria. 



The number of bacteria found in the soil varies considerably 

 with the season of the year. A very interesting phenomenon, noted 

 by Conn and confirmed by Brown, was that the number of bacteria 

 in soil increase on freezing. This fact is illustrated in the following 

 table from the work of Conn. In it are listed a series of gelatin- 

 plate counts made from a single soil plate at intervals throughout 

 the course of three years. 



During the three years over which the sampling was conducted 

 the plate count of non-spore-formers varied from 5,000,000 to 

 44,000,000 per gram, whereas the spore-forming bacteria and 

 Actinomycetes varied only from 3,200,000 to 10,500,000. The 

 increase which occurs during the winter is in the slow-growing 

 bacteria and not in those which liquefy gelatin rapidly or in the 

 Actinomycetes. Conn tries to account for the noted phenomenon 

 by assuming two groups of bacteria winter and summer bacteria. 

 The latter, he thinks, prevent the former from multiplying rapidly 

 in warm weather. Hence, the increase in the frozen soil is due to the 

 depressing effect of the cold upon the summer bacteria. There is, 



