The sun., 1919-10-05

growing of a single crop may become
difficult or Improbable. Perhaps the
most striking instance Of this sort is
the growing of red clover. This crop
Is recognized as one of the best of our
soil improving crops. Through its
ability to assimilate atmospheric nj
trogen it increases the store of this
constituent in the soil and leaves
heterogeneous residues for use of sue
ceeding crops. Unfortunately, how
ever, the clover fails to "catch" as
regularly as is desired by the farmer.
Hence farmers in the New as well az in
the Old World have come to feel that
a good catch of clover once In six or
seven years is about as much as may
be expected on the older soils. Evil
IDENTIFY something is left in the land
which interferes with the growth OF
the clover plants and this something
disappears only gradually. The fall
ure of clover to grow makes 1t diff
cult to maintain a satisfactory supply
OF nitrogen in the soil at reasonable
cost and forces farmers to modify
their rotations and to use other le-
gumes or to purchase larger quanti-
ties Of heterogeneous fertilizers. In
the same way the inability to grow GER
tain money crops continuously an
wolves serious loss, since 1t compels
farmers to diversity their crop meth
DDS and to produce crops which In
themselves are much less profitable.
Various explanations have been Of
fered to account for soil sickness. In
many instances It is obvious that such
sickness is due to the accumulation of
fungi or injurious insects. Flax sick
ness is due to fungus disease which
produces fax wilt. Tobacco sickness,
COW pea sickness and potato scab are
likewise brought about by fungi.
Profitable corn growing often becomes
impossible on account Of plant lice
distributed on the roots of the corn
plant by certain species Of ants. There
are, therefore, numerous instances OF
crop failure attributable either to the
prevalence of fungi or of injurious in
sects. But apart from crop failures
due to causes Just mentioned there
are frequent failures that are clearly
due to other causes. In the words of
practical growers, soils simply become
tired of certain crops and diversified
ton becomes Imperative Various ex-
plantations have been offered to ac-
count for instances of soil tiredness
where injury by fungi or insects is
excluded. 1t has been suggested that
different crops utilize plant food in
gradients in different proportions.
Hence the continuous growing of any
one crop may bring about the partial.
or at least temporary, exhaustion of
one or another of the important plant
food constituents.
Another explanation offered for the
inability of certain crops to grow con-
tlnuously ON the same land presup-
poses the accumulation in the soil of
certain poisonous or toxic plant re-
mains or eXcreta. At one time the
bureau Of soils of the United States
Department OF Agriculture was In
dlined to regard crop rotations as the
cure for all soil ills. 1t was claimed
by the bureau of soils that plants Of
any one kind may be injured by their
own excreta, but not by those of
plants of another kind. 1t has been
shown, however. by long continued
rotation and fertilizer experiments,
notably at Rothamsted in England,
that the mere rotating Of crops will
not suffice TO maintain satisfactory
production. Nevertheless, there is a
very considerable amount of evidence
at hand to show that certain products
pass out OF the roots of plants into
the surrounding soil, and that these
products may have an injurious effect
on other plants.
1t has been shown that soils may
h.N O..
practically all crops. Very often such
soils contain adequate amounts of
plant food. Many instances are on
record of gardens whose soil. in spite
OF the application of very large guam
titles of animal manure and of COM
post, has ceased to be productive.
The presence of specific fungi or of
! insects cannot be offered as an ex-
PLANTATION OF the unproductveness of
the soil. 1t must be assumed, there
fore, that in such soil there is an &C
emulation Of something which is in
! furious to all vegetables and other
crops commonly grown in the garden.
Recent research in the bacteriology
Of soils seems to justify at least two
possible explanations of the unwhole-
some condition of generously MANFRED
garden soils. There is no doubt that
| soils containing a large proportion of
vegetable and animal matter favor
the development of enormous numbers
OF bacteria and Of other micro-organ-
isms. Where in an ordinary fertile
soil the number of bacteria may range
from 100000 to 100000 per gram,
their number in garden soils may
reach the amazing total of 10000. 900
or even 100000 per gram; In other
words there may be as many as So
100000 to 100,000.000 of bacteria in a
thimbleful OF soil. In view of the fact
that bacteria are after all microscopic
plants and that like higher plants they
are in need of certain constituents for
the building of their body substance il
Is evident that bacteria and other
tnicro-organisms may compete with
crops for plant food and may deprive
the crops Of the necessary nourish-
ment. 1t is probably true, likewise.
that in richly MANFRED soils certain
types OF bacteria and fungi may accu-
mulate whose activities lead to the
formation and accumulation of com
pounds which serve to poison the
higher plants.
Recent studies in a number of son
laboratories show that certain micro
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