46 
THIRTEENTH REPORT. 
SOIL AND SOIL PROBLEMS FROM THE STANDPOINT OF THE 
MICROBIOLOGIST. 
OTTO RAHN. 
Soil is to the physicist a mixture of particles of various sizes, as 
gravel, coarse sand, fine sand, silt, and clay. These particles have cer- 
tain physical qualities, as absorption, heat conduction, water capacity, 
and pore space. The physical qualities of a soil are determined by the 
amounts and kinds of particles constituting it. 
The chemist looks at soil problems in a different way. Soil appears 
to him as a mixture of soluble and insoluble compounds, of organic 
and inorganic matter. Some of the soil constituents are known to be 
plant foods, and to them the special attention of the chemist is di- 
rected. 
To the microbiologist, soil is a medium for the development of a 
large variety of microorganisms. The domain of the soil bacteriologist 
is increasing rapidly, and the term bacteriology seems hardly broad 
enough for a science that includes, besides the bacteria, yeasts and 
molds, even the single-celled animals, — protozoa. 
In order to discuss the soil problems from the standpoint of the micro- 
biologist, it seems advisable to first state briefly the facts that are al- 
ready established in soil bacteriology. Like the chemist and the 
physicist, the microbiologist studies soil problems ultimately from the 
viewpoint of soil fertility or crop production. From this viewpoint, 
microorganisms may be divided into those that are beneficial and into 
those injurious to crops. Of course, this distinction is not an absolute 
one, but it is helpful in classifying soil microorganisms according to 
their significance. 
Microorganisms can be useful to plants in various ways, either di- 
rectly by symbiosis, or indirectly by preparing plant food. Symbiosis 
of higher plants with microorganisms is limited practically to the roots, 
but there it is a very common occurrence. The root-tips of the evergreens 
and of many other forest trees show quite regularly a growth of mold 
mycelium which is called mycorrhiza. Many other plants show mycor- 
rhiza growth frequently, while with some it occurs only very rarely or 
not at all. There is no doubt that the mycorrhiza is of benefit to the 
plants, though the relations between mold and host are not entirely ex- 
plained as yet. It seems that the mold helps the plant to obtain food, 
either mineral matter or nitrogenous compounds. Perhaps, they ful- 
fill different purposes in different plants. 
Much attention has been given to another case of symbiosis, namely 
that of legumes with Pseudomonas radicicola, the bacterium of the root 
nodules. This symbiosis is so well understood that pure cultures of 
such bacteria are now grown commercially for inoculating the seed. 
It is at present the only case of a direct application of soil bacteriology. 
The subject is so generally known that it would be wasted time to 
discuss any details before this audience. 
