SOIL FERTILIZERS 315 



kind doing a different work, until the proper nitrogen 

 compounds are formed. When these are dissolved in 

 soil water, they are ready to be taken up for food by the 

 plant. 



The bacteria of decay do not add to the nitrogen of the soil ; 

 they simply work over the nitrogen compounds that they 

 encounter. Without their activities, the growing plant would 

 die for want of properly prepared food. In the course of 

 decay, various acids and gases 

 are formed. The acids help to 

 decompose certain minerals into 

 soluble forms that the plant 

 can use. 



If the acids become too abun- 

 dant, they make the soil " sour," 

 thus preventing the growth of 

 needful bacteria. Such soil can 

 be readily "sweetened" by the 



17 > f J SOIL BACTERIA 



addition of sufficient lime. It is 



very easy to test whether a soil is sour or not, by placing 

 a piece of blue litmus paper in a hole in the ground a few 

 inches deep, and allowing it to remain there for several 

 hours. If the blue litmus paper turns red, the soil is sour. 

 When lime, which is a base, is mixed with sour soil, it 

 unites with the acids of the soil to form salts that are not 

 injurious to the needed bacteria. 



Soil Fertilizers. So rapidly do the growing plants use 

 up soluble compounds of nitrogen that the nitrogen would 

 soon be removed from most soils if it were not in some way 

 replaced. There are two other substances that are much 

 needed by plants and that are soon exhausted from the soil 



