MICROORGANISMS IN THE SOIL 43 
Sulphates.—These salts are also needed in small amounts only. 
Iron Salts.—Needed in small quantities only 
Lime and magnesia should also be in the soil for reasons that 
will be given later, but as foods 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 prob- 
blem, 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 hundred 
thousands to many millions per gram. In sandy soil there may be 
comparatively 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 certain depth, which varies according 
to soil conditions, they almost disappear. 
The numbers present vary with the moisture content of the 
soil, moderately moist soils having more than fairly dry soils. 
Strange to say, larger numbers have apparently been observed in 
frozen soil than in unfrozen soil. To explain this rather surpris- 
ing fact, the theory has been advanced that certain kinds of soil 
bacteria are capable of growing at rather low temperatures and 
find enough moisture present fin {the unfrozen portions of frozen 
soil and that these same bacteria would grow to as high numbers 
