| Crop
Yields
Crop
yields
Crop yield is the most
widely used parameter
in assessing the desirability
of an alley cropping
system (Nair 1993).
There is increasing
information on the effect
of alley cropping with
various hedgerow species
on crop yields in various
climatic and edaphic
conditions. Results
of studies on Alfisols
and other high base
status soils in the
humid and subhumid tropics
have usually been more
promising in terms of
crop yield. Soil nutrient
status is better and
tree-crop competition
in the topsoil is less
severe. For example,
Kang et al. (1990) showed
in an alley cropping
study in Nigeria that
maize yield, after eight
years of rotation with
cowpea, could be maintained
at 2 ton ha-1 using
Leucaena leucocephala
prunings alone, improved
to over 3 ton ha-1 using
prunings supplemented
by fertilizer (80 kgN
ha-1), comparing to
0.66 ton ha-1 in the
no-pruning, no-fertilizer
control. An earlier
study by Kang et al.
(1981) showed that applying
10 ton ha-1 of fresh
L. leucocephala
prunings had the same
effect on maize yield
as applying 100 kg ha-1
of nitrogen fertilizers;
however, such high amount
of pruning material
had to be supplemented
by external sources.
Some results are less
successful. For example,
in a study of alley
cropping with G.
sepium and L.
leucocephala in
Nigeria, Lal (1989a)
found a large decline
in the yields of maize
(60% of the initial
year) and cowpea (31%)
after six years.
Trials
in the humid zone on
acid and low base status
soils have given mixed,
often less successful
results. Nutrient availability
is usually low in the
soils, and root competition
likely intensified due
to acidity and toxicity
of subsoils. For example,
in an alley cropping
study with Inga
edulis on an infertile
acid Ultisol in Peru,
Fernandes et al., (1993a)
found grain yields of
rice under alley cropping
(with and without fertilizer)
declined considerably
over five rice crops
to less than 0.8 ton
ha-1, compared to the
steady increase and
stabilization of rice
yields of over 1.6 ton
ha-1 in the fertilized
control plot. The low
yields under the alley
cropping treatment were
attributed to tree-crop
root competition, where
reduced crop yields
due to root competition
between hedgerows and
crops were detected
at 11 months after hedgerow
establishment.
Similar
low yields have been
reported on a highly
weathered Oxisol in
Sumatra by Evensen et
al. (1995), where yields
of rice and cowpea alley-cropped
with P. falcataria,
C. calothyrsus and G.
sepium declined over
four years to unacceptable
low levels (cowpea)
or to crop failure (rice),
until fertilizer inputs
were increased and G.
sepium replaced with
F. macrophylla because
of poor growth. Thereafter,
crops yields increased
and returned to original
levels. Soil cations
showed a similar pattern
of change. Evensen et
al. (1995) concluded
that there was little
build-up of nutrient
cations due to recycling
by trees, and that successful
alley cropping on acid
soils required maintenance
of soil fertility with
external inputs.
Selection
and husbandry of hedgerows
Three aspects of alley
cropping provide the
most opportunity for
management to improve
efficiency and profitability
of the system: tree
species selection,
hedgerow husbandry,
interhedgerow spacing.
As mentioned earlier,
fast-growing leguminous
perennials with deep
and non-spreading
rooting pattern are
often preferred to
attain higher pruning
biomass, soil nitrogen
status enhancement,
nutrient uptake from
subsoils, and reduced
tree-crop root competition.
Depending on the growth
vigor and decomposition
characteristics of
prunings obtained
from selected hedgerow
species, the shoot
pruning regime can
be adjusted to minimize
shading and achieve
better synchrony between
nutrient release from
mulch/green manure
and peak crop nutrient
demand, and hence
nutrient use efficiency.
Decomposition rate
and nutrient release
pattern can also be
adjusted by modifying
the method of application
of the prunings as
mulch or green manure.
Nutrient
release is not the
only concern when
selecting the decomposition
characteristics of
hedgerow species.
In crop lands where
weed infestation or
soil erosion presents
a particularly serious
problem, providing
a good ground cover
for a longer period
might be more important
than the quick provision
of nutrients. In such
cases prunings with
relatively rapid decomposition
rates from species
such as Leucaena
leucocephala, Gliricidia
sepium and Erythrina
spp.
might not be as desirable
as slower-decomposing
mulches of species
such as Cassia
siamea, F. macrophylla
and Dactyladenia
(syn. Acioa)
barteri.
MacLean (1992) suggested
that diversified hedgerows
with function-specific
species may be more
effective than hedgerows
with single "multipurpose"
tree species, especially
in situations where
erosion control and
weed suppression are
of primary concern.
Interhedgerow
spacing
Several conflicting
objectives need
to be considered
when determining
the optimal interhedgerow
spacing. Closer
interhedgerow spacing,
i.e. more hedgerows,
can provide higher
pruning biomass
and nutrient yields
per unit area in
an alley cropping
system. The nutrient
cycling benefits
of trees can also
be more prominent.
Weed suppression
between cropping
cycles is more effective
with narrower alley
where rapid canopy
closure is possible.
However, tree-crop
competition for
resources (solar
radiation, moisture
and nutrients) is
intensified with
smaller interhedgerow
spacing. Higher
proportion of tree-crop
interface -- areas
with the most pronounced
depression on crop
yield -- exists
on crop land with
more hedgerows.
Planting double
hedgerows can reduce
interface while
providing pruning
biomass at a level
similar to that
of single hedgerows
with smaller interhedgerow
spacing. Based on
the results of various
on-station and on-farm
trials, and on farmers'
acceptance (mainly
based on ease of
manual cultivation),
an interhedgerow
spacing of 4 to
6 m was observed
to be optimal for
species such as
G. sepium
and L. leucocephala
in the humid tropics
(Kang 1993).
Where
alley cropping is
used for erosion
control on sloping
lands, there is
less scope for adjustment
of interhedgerow
spacing. The introduction
of trees on arable
land also reduces
the land area available
for crop production.
This reduction of
land devoted to
producing food crops
may or may not translate
into reduction of
total crop yields,
depending on the
effectiveness of
the system to improve
yield per land cropped
through nutrient
provision and weed
suppression. Studying
yields of taro alley-cropped
with Calliandra
calothyrsus
and G. sepium,
in Western Samoa,
Rosecrance et al.
(1992) found that,
even though 4-m
interhedgerow spacing
was most effective
in suppressing weed
and a slight yield
increase over the
control was found
on a per plot basis
in the alleys, total
taro yields were
significantly reduced
in the 4-m alleys
(6 ton ha-1) compared
to the 5-m, 6-m,
and control plots
(8 ton ha-1). These
yield reductions
were primarily a
result of more land
taken out of taro
production and put
into hedgerow growth.
External
inorganic fertilizer
input
In alley cropping
systems, the annual
removal of nutrients
is usually greater
than in most agroforestry
systems, resulting
in greater demand
on the system to supply
and/or recycle nutrients.
Nitrogen fixation
by hedgerow leguminous
trees accounts for
the only significant
nutrient addition
from external source
in an alley cropping
system. Otherwise,
prunings applied to
alley crops are recycling,
not augmenting, nutrients
within the system
and thus do no offset
the nutrients lost
via crop harvest.
External inputs to
the system from precipitation
are small. Some nutrients,
otherwise unavailable
to crops because they
are below the rooting
zone of the crops,
might be brought into
the system from deeper
layers in the soil
by deep-rooting trees,
but the magnitude
of this "input"
is not known (Nair
1993, Palm 1995).
Much
of the research on
alley cropping has
addressed the ability
of this system to
maintain productivity
with or without inorganic
fertilization, in
particular the capacity
of leguminous species
to substitute for
nitrogen fertilizer
application. A review
of alley-cropped maize
show that the application
of inorganic nitrogen
fertilizers considerably
increases (up to 1000
kg ha-1) yields of
maize alley cropped
with L. leucocephala,
but gives much smaller
increases with G.
sepium, F.
macrophylla,
or other alley cropping
species (Szott and
Kass, 1993). In another
study on acidic soils,
fertilizer additions
(50 kg N, 25 kg P,
20 kg K, 35 kg Ca
and 16 kg Mg ha-1)
to rice and cowpeas
alley cropped with
Inga edulis
resulted in significantly
greater grain yields
in the fertilized
treatment in the fourth
through the seventh
crops of a seven-crop-long
sequence (Fernandes
1990).
These
results from fertilization
studies indicate that
nutrient availability
can be a limiting
factor in unfertilized
alley crop systems.
This is especially
likely on acidic,
infertile LAC soils
in the humid tropics
where the ability
of hedgerow leguminous
trees to recycle nutrients
or fix nitrogen appears
to be restricted by
the intrinsically
low levels of available
nutrients and high
levels of elements
toxic to plant growth
(Szott and Kass 1993).
For example, nodulation
by legumes can be
prevented by deficiency
of phosphorus, which
results in low levels
of nitrogen fixation
and increased competition
between trees and
crops for available
phosphorus. Balances
of phosphorus additions
and removals in alley
cropping systems on
infertile and acidic,
or fertile but phosphorus-
deficient soils suggest
that phosphorus may
eventually limit the
productivity of these
systems (Szott and
Kass 1993), supporting
the argument that
pruning alone, especially
on infertile soils,
cannot sustain the
productivity of continuous
alley cropping because
even if nitrogen demand
can be met by biological
nitrogen fixation,
other nutrients, notably
phosphorus, will ultimately
become limiting after
continuous exports
(Szott 1991a, b, Evensen
1995).
Hedgerow
perennials can be
affected by the addition
of inorganic fertilizers
as well. For example,
on an acidic soil
in Rwanda, Yamoah
et al. (1989) observed
that growth of Sesbania
sesban, Calliandra
calothyrsus, Leucaena
leucocephala
and Markhamia
lutea increased
with manure additions
of up to 5 Mg ha-1
in the first four
months after transplanting,
but by the eighth
month there was no
response to manure.
Fernandes (1990) noted
that pruning biomass
and nutrient contents
were greater in Inga
edulis hedgerows
bordering plots fertilized
with N, P, K, Ca and
Mg than those bordering
unfertilized plots.
Nutrient uptake from
applied fertilizers
by hedgerows can be
seen as a benefit
of more efficient
nutrient cycling and
retention, since less
added nutrients will
be leached and lost
from the system; however,
it also represents
undesirable competition
between trees and
crops for nutrients
intended for crop
production. Nitrogen
fertilization may
also decrease the
amount of nitrogen
fixed by leguminous
trees.
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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