MICROORGANISMS IN THE SOIL. 39 
Lime and magnesia should also be in the soil for reasons that 
will be given later, but they are only slightly used by most plants. 
There are still other materials used by plants in very minute 
quantities, but they hardly fall in the scope of our study. All 
the foods above mentioned are commonly called inorganic foods, 
since they come chiefly from the soil and the air. Organic foods 
on the other hand refer to the more highly organized products, which 
are the immediate remains of living things, like roots, starches, 
fats, wood, cellulose and other similar bodies. Our problem, then, 
is to explain nature's methods of keeping the soil supplies of these 
various inorganic ingredients from diminishing. 
MICROORGANISMS IN THE SOIL. 
The upper layers of the soil are exceedingly rich in bacteria, 
the number varying according to conditions, from a few thousands 
to many millions per gram. In sandy soil there may be very few, 
while in soil polluted with organic matter, as in the vicinity of 
manure heaps, there may be as many as 100,000,000 per gram or 
even more, 1,600,000,000 per gram having been found in some soils. 
They rapidly diminish in numbers, as we pass to the lower layers, 
and at a depth of from four to six feet, they have almost disappeared. 
Below this they are rarely found, except in places where drainage 
currents carry them downward. The microorganisms thus found 
in the soil include bacteria in the greatest abundance, and also 
quantities of the higher fungi and yeasts. Each of these classes is 
represented by many varieties, and each has an important share in 
the complex activities going on in the soil. These functions and 
the relation of the soil microorganisms to them may best be under- 
stood by noticing in succession their relation to the various soil 
ingredients that constitute plant foods. 
ORIGIN OF SOIL. 
The ingredients in the soil may be divided into two classes: i. 
The purely mineral matters. 2. The organic ingredients consti- 
tuting the humus. 
