CHAPTER XIII.

BACTERIOLOGIC EXAMINATION OF SOIL.

ALMOST all soil contains bacteria in its upper layers.
Their number and character, however, depend some-
what upon the surrounding conditions. Near the hab-
itations of men, where the soil is cultivated, the ex-
crement of animals, largely made up 'of bacteria, is
spread upon it to increase its fertility, this being a treat-
ment which not only adds new bacteria to those already
present, but also enables those present to grow very much
more luxuriantly because of the increased
amount of organic matter they receive.

The researches of Fliigge, C. Frankel,
and others show that the bacteria of the
soil do not penetrate very deeply—that
they gradually decrease in number until
the depth of a meter is reached, then
rapidly diminish until at a meter and a
quarter they rather abruptly cease to be
found.

Many of the soil-bacteria are anaerobic,
and for a careful consideration of them
the reader must be referred to monographs
upon the subject The estimation of their
number seems to be devoid of any dis-
tinct practical importance. C. Frankel
has, however, originated a very accurate
method of determining it. By means
of a special boring apparatus (Fig. 49)

FIG. 49.—Fran-
kel's instrument for
obtaini ng earth from
various depths for
bacteriologic study.

earth can be secured from any depth without digging and
without danger of mixing that secured with that of the
superficial strata. With sterile liquefied gelatin a definite

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