MICHIGAN ACADEMY OF SCIENCE. 
47 
Far more important than the above mentioned direct relations be* 
tween plants and microorganisms are the indirect relations, especially 
the preparation of plant food. Most soils contain plenty of plant food, 
bnt it is largely in a form unavailable to plants. Microorganisms attack 
and decompose many of these compounds and especially nitrogenous 
compounds are made available almost exclusively by microorganisms. 
Thus, plant growth depends upon microbial activity in the soil. 
Most attention has been paid to the changes in nitrogenous matter 
in soils. The nitrogen comes largely from dead leaves, plant roots, 
excreta, and remains of animals. These compounds are not directly 
assimilated by plants. They are readily decomposed by bacteria which 
break up the large molecules into smaller and smaller ones, until finally 
the nitrogen is in the simple form of ammonia. This may be assimi- 
lated by many plants, many, however, prefer nitrates, and they are ac- 
commodated by the nitrate-forming bacteria which oxidize ammonia 
to nitric acid. All of this microbial activity is named briefly “mineral- 
ization.” The organic matter is, by the exclusive activity of micro- 
organisms, changed to mineral compounds which alone can be used by 
plants. 
But the activity of soil organisms is not limited to the mineralization 
of organic matter. They also increase the nitrogen content of soils. 
Two groups of bacteria, the Azotobacter and the Clostridium are known 
to assimilate nitrogen gas from the air in order to build up their own 
protoplasm. It is believed that these organisms are necessary to keep 
constant the nitrogen content of soils which would otherwise be de- 
creased continually by crops, seepage, and denitrification. 
The decomposition of nitrogen-free organic matter is also of consider- 
able influence upon the availability of plant food. Cellulose, starch, 
sugars and similar constituents of the plants are transformed by micro- 
organisms to organic acids and carbonic acid, which will act upon in- 
soluble minerals as calcium phosphate or magnesium carbonate. In- 
soluble plant food thus becorhes available to plants. 
A very important product of the decomposition of organic matter 
has not been mentioned as yet, that is the humus. Very little is known 
about humus formation and it is mentioned here only because it is 
certain that humus is formed by microorganisms. How this is done and 
which microorganisms are essential is not known. 
Bacteria are further involved in the oxidations of hydrogen sulphide 
and of sulphuric acid. Iron compounds are also occasionally changed 
by microorganisms. 
So far, only the beneficial organisms have been mentioned. They 
constitute by far the majority of soil organisms. But there are also 
harmful organisms in soil which injure crops either directly or indi- 
rectly. Direct injury is caused by all organisms of plant diseases. 
The rot of vegetables, the potato blight are a few examples of diseases 
caused by soil organisms. 
The indirect injuries to crops caused by microorganisms are in con- 
nection with plant food. One group was much feared some time ago. 
namely the denitrifying bacteria which decompose the nitrates of the 
soil to nitrogen gas. Such denitrification means a direct loss of nitrogeu 
to the field, but it is known now that it is rather unusual in a well 
ventilated soil. Another group of harmful organisms are the protozoa, 
