MICROORGANISMS AS A FACTOR IN SOIL FERTILITY 355 



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 bacteria 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 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 is a marked removal of lime and of other basic materials from the 

 soil as cultivation and the use of commercial fertilizers become more 

 thorough. Knisley shows, for instance, that 38.75 per cent of the 

 Oregon soils examined were acid, and that 16.25 per cent were strongly 

 acid. Similarly, Blair found that of 189 samples of different Florida 

 soils and subsoils examined, 68.22 per cent of the former and 51.35 

 per cent of the latter were acid. He also found that virgin soils were 

 less acid than cultivated soils. 



CAUSES OF SOIL ACIDITY. Soil acidity may be due to acids or acid 

 salts, both inorganic and organic. Under ordinary conditions the 

 latter are of much greater importance than the former as a cause of 

 soil acidity. This is demonstrated by the extremely acid conditions 



