Soil Changes Produced By Micro-organisms. 37 
phate and a number of organic salts of potassium. Once more we 
see that the micro-organisms of the soil are the responsible agents 
in restoring potassium to the soil in a form which plants can use. 
Sulphur. 
In our arid soils, sulphur abounds in calcium sulphate as gyp¬ 
sum, and in sodium sulphate and magnesium sulphate, two of our 
common white alkalies. 
When protein compounds undergo decomposition in the ab¬ 
sence of oxygen, quantities of hydrogen sulphide are evolved. This 
gives rise to the disagreeable odors so characteristic of putrefaction. 
The oxidation of the hydrogen sulphide, liberated in this manner, 
furnishes energy to a group of organisms known as the “sulphur 
bacteria.” As a result of this oxidation, free sulphur is produced 
which may often be seen within the bodies of the bacteria as minute 
red granules. The sulphur may be further oxidized to sulphur diox¬ 
ide which later may be changed to sulphuric acid in the presence of 
oxygen and water. Such a thing as a free sulphuric acid in our 
soil, however, would be a rare occurrence, for it would combine 
immediately with a lime or magnesium base to form calcium or 
magnesium sulphate. Plants obtain the sulphur for their proteids 
from some of the soluble sulfates and animals feed upon these 
plants. When plants and animals die, the sulphur present in their 
tissues is restored to the soil through the combined action of de¬ 
composing and sulphur bacteria. 
Iron. 
Among the larger bacteria, we find certain groups which are 
characterized by a deposit of iron rust or iron oxide in the sheath 
surrounding the cells. Attention has been called to these, particu¬ 
larly because of the trouble which they have caused in municipal 
water supplies by plugging the pipes with their growth. These or¬ 
ganisms are said to possess the power of precipitating iron oxide out 
of a dilute solution of iron salts; and through the exercise of this 
function, great masses of a slimy, gelatinous, rusty growth are pro¬ 
duced in the water mains which give no end of annoyance to those 
who have charge of the distribution of such water. 
In considering the relation of bacteria to the iron compounds, 
the deposits of bog iron in swamps and wet places are of consider¬ 
able interest since it was believed at one time that they owed their 
origin to the reduction of *ron sulphate by bacteria. Later authori¬ 
ties, however, claim that the precipitation of the iron oxide is a me¬ 
chanical rather than a chemical process, but whether chemical or 
