2^8 MICROBIOLOGY OF SOIL ' 



already been made by Ebermayer, Schloesing and others, that carbon 

 dioxide production in the soil is greater in summer than it is in winter. 

 These facts, taken together with the early observations of Forster on 

 the inxiltipUcation of photo-bacteria at o°, and the more recent ob- 

 servations of numerous investigators on the multiplication of in- 

 dividual species, or of mixtures of species in milk, water, soil, butter, 

 . etc., at o°, or even below that, make it evident that bacterial activities 

 are not entirely suspended at relatively low temperatures. As the 

 latter rises these activities become more intense as gauged by the 

 formation of carbon dioxide. 



Coming down to specific groups of soil bacteria, it may be noted that 

 ati 2° nitrification is already quite perceptible; that urea bacteria grow 

 slowly at 5°; Pi. radicicbla at 4"^; members of the B. suUilis group at 

 6° to 10°, etc. At 15° the breaking down of organic matter is fairly 

 rapid, and at 25° the optimum is reached for many species. It follows, 

 thus, that the production of plant food — namely, ammonia, nitrates, 

 sulphates, phosphates, etc. — gains rapid headway as the optimum tem-' 

 peratures are approached. The organic matter itself, apart from serv- 

 ing as a source of plant food, furnishes carbon dioxide and various 

 organic acids that help to attack 'the rock fragments and to render 

 available compounds of phosphorus, potassium, calcium and mag^ 

 nesium. It is likewise evident that in warm countries bacterial 

 activities are not only more intense at any one time, but they continue 

 through a longer period. For this reason, the soils of the South can 

 furnish both relatively and absolutely a greater amount of available 

 plant food than the soils of the North. 



The production of plant food is necessarily followed by more 

 vigorous growth of baj:teria and of higher plants. More food is, there- 

 fore, assimilated and more moisture used up until the very rank growth 

 of the crops hastens the depletion of the soil moisture. In this manner 

 the soil may be dried out sufficiently to retard seriously the growth of 

 soil bacteria and to retard thereby the decompositon of organic matter; 

 under such conditions, moisture, rather than temperature, becomes' 

 the controlling factor of growth. 



Reaction 



Range of Soil Acidity. — Acid soils are very common in humid 

 regions. The older soils of Europe include extensive areas whose Hme 



