Effect of Bacteria on Minerals of the Soil. 185 



<ier such conditions, the question of fertility may not be 

 affected so much by the addition of more phosphate as by 

 the incorporation of organic matter that will render 

 available the phosphorus which is already present in the 

 soil. 



Potassium. The content of soils in this important 

 element is usually very high. A soil may contain many 

 thousand pounds of potassium per acre and yet so little 

 of it be made available to the plant that the soil will re- 

 spond to the addition of a potash fertilizer. Potassium 

 is rendered soluble by bacterial action in the same man- 

 ner as are calcium and phosphorus. The growth of the 

 nitrogen-fixing bacteria is possible only when potassium 

 is available. , 



Sulphur. The sulphur that is taken from the soil by 

 the green plant in the form of calcium, potassium or so- 

 dium sulphate is rendered available to the plant once 

 more through the action of bacteria. When organic 

 matter, either of plant or animal origin, undergoes de- 

 composition, the sulphur is changed into hydrogen sul- 

 phide, a gas that is one of the causes of the offensive 

 odors coming from putrefying materials. This gas is 

 poisonous to the green plant. It is, however, utilized by 

 the sulphur bacteria which are found most abundantly 

 in sulphur springs. They also occur in the soil where 

 they use the hydrogen sulphide formed from decompos- 

 ing matter, changing it to sulphates, the form in which 

 it can be again used by the plant. 



The characteristic odor of the soil has been shown to 

 "be due to a compound formed by the growth of a certain 

 <jlass of bacteria in the soil. 



