This
observation
led
to
a
better
understanding
of
the
practice
of
crop
rotation
involving
legume
crops.
Direct
evidence
however,
to
show
that
pea
plants
cannot
grow
in
sterile
soils
in
the,
absence
of
an
inorganic
nitrogen
source
came
from
the
experiments
of
Hellriegal
and
Wilfarth
in
1888.
Subsequently,
Beijerinck
identified
the
bacteria
that
associate
with
the
roots
of
leguminous
plants
as
rhizobia.
Although
since
then,
inoculating
leguminous
crops
with
the
bacteria
that
fix
nitrogen
(rhizobia)
has
been
widely
accepted,
it
is
only
in
the
last
few
years
that
there
have
been
serious
attempts
to
understand
the
process
of
biological
nitrogen
fixation.
By
the
end
of
19th
century
in
addition
to
the
rhizobia,
many
free
living
aerobic
and
anaerobic
bacteria
were
found
to
have
the
ability
to
fix
atmospheric
nitrogen.
It
was
another
25
years
before
the
blue
green
bacteria
(blue
green
algae)
were
also
reported
to
have
the
ability
to
fix
nitrogen:
The
various
groups
of
bacteria
that
are
today
known
to
fix
atmospheric
nitrogen.
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