MICROORGANISMS AS A FACTOR IN SOIL FERTILITY. 235 



of numerous investigators on the multiplication of individual 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 ac- 

 tivities become more intense as gauged by the formation of carbon dioxide. 



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

 at 12 nitrification is already quite perceptible; that urea bacteria grow 

 slowly at 5; Ps. radicicola at 4; members of the B. subtilis group at 6 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 temperatures are 

 approached. The organic matter itself, apart from serving 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 magnesium. 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 rea- 

 son, 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 bacteria and of higher plants. More food is, therefore, assimi- 

 lated 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 decomposition 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 lime 

 content has been restored repeatedly by the application of wood ashes, 

 marl, oyster and clam shells, and various grades of burned or crushed 

 limestone. In the United States acidity is becoming prevalent in many 

 of the cultivated soils, as is shown by the investigations of the Rhode 

 Island, Ohio, Illinois, Oregon and Florida experiment stations. These 

 investigations, confirmed by experiments in other states, show that there 



