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Soil
Organic
Matter
&
Nutrients
Maintenance
of
soil
organic
matter
Soil
organic
matter
(SOM)
supplies
most
of
the
nitrogen
and
sulfur
and
half
the
phosphorus
taken
up
by
unfertilized
crops.
Phosphorous
fixation
by
amorphous
oxides
is
lowered
by
organic
radicals
blocking
the
fixation
charges.
Living
trees
add
organic
matter
to
the
soil
mainly
through
the
natural
process
of
litter
and
dead
root
decay.
In
addition,
during
active
root
growth
significant
amount
of
organic
matter
can
enter
the
soil
in
the
form
of
sloughed-off
root
tissue,
representing
a
steady
release
of
carbohydrate-rich
organic
material
(Giller
and
Wilson
1991,
Nair
1993).
In
alley
cropping,
where
nutrient
removal
through
crop
harvest
places
a
greater
demand
on
SOM
availability,
this
transfer
of
living
plant
tissue
to
SOM
is
facilitated
by
actively
removing
biomass
from
the
hedgerows
(i.e.,
pruning)
to
soils
in
the
adjacent
alleys
(i.e.,
green
manuring
or
mulching).
Decomposition
of
litter,
green
manure
and
mulch
by
soil
microfauna
usually
occurs
at
a
high
rate
in
the
humid
tropics.
Nutrient
yield
from
prunings
The
nutrient
contribution
of
hedgerow
woody
perennials
(i.e.,
nutrients
made
available
from
the
decomposition
of
green
manure
and
mulch
applied
to
soil)
is
the
most
important
source
of
nutrients
for
crops
in
unfertilized
alley
cropping
systems.
The
amount
of
nutrient
yield
largely
corresponds
to
the
biomass
yield
of
trees,
which
in
turn
depends
on
the
species,
management
and
site-specific
edaphic
and
climatic
factors
(Nair
1993).
A
large
number
of
biomass
production
screening
trials
and
alley
cropping
trials
have
been
conducted
in
a
range
of
climate
and
soil
conditions
with
a
variety
of
tree
species.
Fast-growing
nitrogen-fixing
leguminous
perennials
are
generally
preferred
as
hedgerow
species
since
they
have
the
potential
to
yield
more
and
higher
"litter-quality"
prunings.
The
term
"litter
quality"
commonly
refers
to
nutrient
content
and
comparative
rate
of
decomposition
of
plant
residues
(Nair
1993).
Nutrient-rich,
fast-decomposing
plant
litter
is
traditionally
considered
to
be
of
high
quality.
The
nutrient
content
of
the
prunings
depends
on
many
factors,
including
tree
species,
the
relative
proportions
of
leafy
and
woody
components
in
the
prunings
and
their
respective
nutrient
concentrations
(Palm
1995).
A
number
of
studies
on
biomass
production
have
shown
that
leguminous
trees
in
alley
cropping
systems
generally
produce
enough
pruning
biomass
and
nutrients
to
meet
most
crop
requirements,
especially
nitrogen
(Budelman
1989,
Kang
et
al.
1989,
Palm
and
Sanchez
1990,
Szott
et
al.
1991,
Haggar
et
al.
1993),
even
though
as
little
as
14%
and
often
less
than
50%
of
the
nitrogen
in
prunings
is
from
nitrogen
fixation
(Giller
and
Wilson
1991).
Phosphorus
is
a
notable
exception.
Comparison
of
nutrients
added
in
prunings
and
nutrients
removed
in
crop
harvest
from
alley
cropping
systems
on
both
fertile
and
infertile
soils
shows
an
insufficient
quantity
of
phosphorus
in
prunings
(Palm
et
al.
1991,
Salazar
et
al.
1993).
Introduction
|
Definition
|
Hedgerow
Species
I
Species
Selection
Criteria
|
Soil
Organic
Matter
&
Nutrients
|
System
Management
|
Crop
Yields
|
Soil
Conservation
|
Weed
Dynamics
|
Tree-Crop
Competition
|
References
BACK
Funded
by a grant from the
Cornell Agroforestry
Working Group (CAWG)
and the Distance Learning
Program of the Cornell
International Institute
for Food, Agriculture
and Development (CIIFAD).
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