Effects of urea fertigation of apple trees on soil pH, exchangeable cations and extractable manganese in a sandy loam soil in New Zealand


Belton, P.R.th; Goh, K.M.

Fertilizer Research 33(3): 239-247

1992


Changes in soil pH, exchangeable aluminium (Al), calcium (Ca), magnesium (Mg), and potassium (K) and extractable manganese (Mn) were investigated after urea fertigation of a sandy loam soil in an apple orchard in the new Zealand. Urea at three rates (0, 25, 50 kg N ha-1 yr-1 or 0, 16.9, 33.8 g N emitter-1 yr-1) was applied in 4 equal fertigations. Soil cores at 4 profile depths (0-10, 10-20, 20-40 and 40-60 cm) directly below and 20 cm from the emitter were sampled approximately 4 weeks after each fertigation and in the following winter. Results obtained showed that the largest changes in soil pH and cations occurred in soils directly below the emitter in the 50 kg N ha-1 yr-1 treatment where the soil pH decreased by 1.6 pH units at all soil depths. The lowest pH of 4.3 was observed at a depth of 27 cm. Exchangeable Al and extractable Mn levels increased to 11 meq kg-1 and 78 mu-g g-1 respectively. Estimated losses of Ca, Mg and K from the upper soil profile depth (0-10 cm) represented 23, 63, and 27% of their respective total exchangeable levels. At lower profile depths ( gt 20 cm), accumulation of displaced K was evident. Variable, and generally non-significant, chemical changes recorded in soils 20 cm from the emitter attributed to restricted lateral water movement, and therefore urea movement, down the profile. The present study showed that one season of urea fertigation by trickle emitters, applied to a sandy loam, at half the rate conventionally applied to apple orchards (50 kg N ha-1 yr-1) resulted in pH and mineral element imbalances which were potentially and sufficiently to inhibit three growth.

Fertilizer
Research
33:
239-247,
1992.
C)
1992
Kluwer
Academic
Publishers.
Printed
in
the
Netherlands.
239
Effects
of
urea
fertigation
of
apple
trees
on
soil
pH,
exchangeable
cations
and
extractable
manganese
in
a
sandy
loam
soil
in
New
Zealand
P.
Ruth
Belton
&
K.M.
Goh
Department
of
Soil
Science,
Lincoln
University,
Canterbury,
New
Zealand
Received
30
August
1991;
accepted
in
revised
form
10
December
1992
Key
words:
Al,
apple
orchard,
Ca,
Fertigation,
K,
Mg,
Mn,
sandy
loam,
soil
pH,
urea
Abstract
Changes
in
soil
pH,
exchangeable
aluminium
(Al),
calcium
(Ca),
magnesium
(Mg),
and
potassium
(K)
and
extractable
manganese
(Mn)
were
investigated
after
urea
fertigation
of
a
sandy
loam
soil
in
an
apple
orchard
in
New
Zealand.
Urea
at
three
rates
(0,
25,
50
kg
N
ha
-1
yr
-1
or
0,
16.9,
33.8
g
N
emitter
-1
yr
-1
)
was
applied
in
4
equal
fertigations.
Soil
cores
at
4
profile
depths
(0-10,
10-20,
20-40
and
40-60
cm)
directly
below
and
20
cm
from
the
emitter
were
sampled
approximately
4
weeks
after
each
fertigation
and
in
the
following
winter.
Results
obtained
showed
that
the
largest
changes
in
soil
pH
and
cations
occurred
in
soils
directly
below
the
emitter
in
the
50
kg
N
ha
-1
yr
-1
treatment
where
the
soil
pH
decreased
by
1.6
pH
units
at
all
soil
depths.
The
lowest
pH
of
4.3
was
observed
at
a
depth
of
27
cm.
Exchangeable
Al
and
extractable
Mn
levels
increased
to
11
meq
kg
-1
and
78
p.g
g
-1
respectively.
Estimated
losses
of
Ca,
Mg
and
K
from
the
upper
soil
profile
depth
(0-10
cm)
represented
23,
63
and
27%
of
their
respective
total
exchangeable
levels.
At
lower
profile
depths
(>20
cm),
accumulation
of
displaced
K
was
evident.
Variable,
and
generally
non-significant,
chemical
changes
recorded
in
soils
20
cm
from
the
emitter
were
attributed
to
restricted
lateral
water
movement,
and
therefore
urea
movement,
down
the
profile.
The
present
study
showed
that
one
season
of
urea
fertigation
by
trickle
emitters,
applied
to
a
sandy
loam,
at
half
the
rate
conventionally
applied
to
apple
orchards
(50
kg
N
ha
-1
yr
-1
)
resulted
in
pH
and
mineral
element
imbalances
which
were
potentially
and
sufficiently
severe
to
inhibit
tree
growth.
Introduction
In
recent
years,
there
has
been
increasing
inter-
est
in
applying
fertiliser
nutrients
through
the
irrigation
system
(fertigation)
as
a
means
of
providing
nutrients
to
orchard
fruit
trees
in
New
Zealand.
Urea
is
well
suited
to
fertigation
because
it
is
relatively
cheap
per
unit
of
N
and
is
highly
soluble.
However,
a
major
problem
asso-
ciated
with
urea
fertigation
is
soil
acidification.
Stoichiometrically,
during
the
rapid
microbial
transformation
of
urea
to
nitrate
in
soil,
the
concomitant
release
of
a
net
1
mole
of
H
+
per
mole
of
urea-N
occurs.
In
addition,
soil
acidifica-
tion
under
urea
fertigation
is
intensified
due
to
the
limited
soil
volume
of
application
compared
to
conventional
methods
of
fertiliser
application.
This
problem
is
further
accentuated
in
coarse-
textured
soils
due
to
low
soil
buffering
capacity
per
unit
volume
and
the
narrower,
basically
cylindrical
wetted
volume
under
the
emitter
[17].
Although
ammonium
based
fertilisers
have
twice
the
acidifying
effect
per
unit
N
as
urea,
urea
is
considerably
more
mobile
in
soil
[10].
Acidifica-
tion
resulting
from
ammonium
sulphate
fertiga-
tion
was
confined
to
the
top
20
cm
of
a
silt
loam
while
urea,
applied
under
the
same
conditions,
caused
acidification
to
a
depth
of
40
cm
[11].
The
240
mobility
of
fertigated
urea
down
a
soil
profile
can
therefore
result
in
acidification
occurring
at
a
soil
depth
that
is
difficult
to
ameliorate
by
the
surface-applied
liming
materials.
In
acidic
soils
at
pH
>
4,
it
is
not
the
low
pH
per
se
(high
H
±
concentration)
that
limits
plant
growth
but
the
toxicity
and
deficiencies
of
miner-
al
elements.
Trace
elements
Al
and
Mn
increase
to
potentially
toxic
levels
for
plants
at
pH
below
5.5
[16].
Also,
as
soils
become
acidic,
base
cations
Ca,
Mg
and
K
are
displaced
and
become
susceptible
to
leaching
[12].
In
New
Zealand
an
increasing
number
of
orchards
are
being
established
on
sandy
loam
or
similar
coarse-textured
soils.
Because
these
soils
are
susceptible
to
summer
drought,
trickle
irriga-
tion
is
usually
employed.
Fertigation
is
a
viable
option
for
reducing
the
cost
for
fertiliser
applica-
tion
and
increasing
the
efficiency
of
fertiliser
use.
It
has
been
demonstrated
that
nitrogen
(N)
fertigation
at
half
the
conventionally
applied
rate
did
not
reduce
the
N
content
of
temperate
fruit
trees
[14].
However,
there
is
limited
research
on
the
limitations
of
these
soils
which
are
highly
susceptible
to
both
acidification
and
leaching
to
the
effect
of
urea
fertigation.
This
study
examines
the
effects
of
one
season
of
urea
fertigation
at
half
the
conventional
rate
on
soil
pH,
exchangeable
Al,
Ca,
Mg
and
K
and
extractable
Mn
in
a
sandy
loam
apple
orchard
soil.
Materials
and
methods
Trial
site
and
design
The
site
was
situated
at
the
Ministry
of
Agricul-
ture
and
Fisheries
(MAF)
Research
Station,
Templeton,
Canterbury,
New
Zealand.
A
full
description
of
the
soil
type,
Waimakariri
sandy
loam
(Aquic
Dystric
Eutrochrept),
is
available
in
the
Soil
Bureau
Bulletin
[20].
The
soil
was
a
friable,
moderately
developed
sandy
loam
with
topsoil
of
about
30
cm
above
a
very
friable
loamy
sand
which
extends
to
beyond
60
cm.
Organic
carbon
levels,
determined
colorimetri-
cally
[4]
at
each
depth,
were:
2.8%
(0-10
cm);
2.5%
(10-20
cm);
1.4%
(20-40
cm);
0.7%
(40-
60
cm).
Apple
trees
(Malus
domestica
Borkh)
cultivar
`Gala'
on
rootstock
M9
were
planted
during
winter.
Each
of
the
three
tree
rows
represented
a
block
with
4
trees
per
treatment
(8
emitters
per
treatment)
and
a
terminal
guard
tree
at
each
end.
Annual
N
applications
were
split
into
4
equal
applications
which
were
fertigated
at
ap-
proximately
monthly
intervals
over
the
follow-
ing
summer.
Treatments
were
arranged
in
a
randomised
complete
block
design,
at
rates
of
0,
25
and
50
kg
N
ha
-1
yr
-1
(0,
16.9
and
33.8
g
N
emitter
-1
yr
-1
).
A
`Desmatic-plus'
venturi
fertigation
system
discharged
water
at
2.81h
-1
per
emitter
from
2
emitters
per
tree.
Fertigation
applications
range
for
approximately
1.5
h
per
application
followed
by
5
minutes
of
irrigation
with
water
except
for
the
first
applica-
tion
which
was
followed
by
2
h
irrigation
with
water.
Fertigation
was
applied
over
summer
at
the
period
of
maximum
crop
demand
for
water.
Sampling
and
chemical
analysis
Soil
cores
(25
mm
diameter)
from
the
3
blocks
x
3
treatment
plots
were
taken
with
a
soil
corer
immediately
below
and
20
cm
away
from
the
emitter
in
a
line
parallel
to
the
tree
row
on
the
emitter
side.
Cores
were
taken
at
approximately
4
weeks
after
each
fertigation
but
before
the
subsequent
treatment
and
once
during
winter.
There
were
a
total
of
five
sampling
times,
three
blocks,
three
treatments,
four
depths
and
two
emitter
distances.
Soil
core
sampling
dates
are
subsequently
referred
to
as
time
1
to
5
for
both
soil
cores
immediately
below
and
20
cm
away
from
the
emitter.
Core
holes
were
repacked
with
inter-row
soil
and
each
subsequent
core
was
extracted
from
below
a
different
emitter
within
each
respective
treatment.
Each
soil
core
was
divided
into
four
unbulked
profile
depths
(0-10,
10-20,
20-40
and
40-60
cm)
and
each
segment
was
analysed
separately
for
soil
pH,
exchange-
able
Al,
Ca,
Mg,
K
and
extractable
Mn.
Soil
pHs
were
determined
using
a
glass
elec-
trode
at
1:2.5
field
moist
soil
:
water
(w/v)
ratio
after
equilibrating
overnight.
241
Exchangeable
Al
was
extracted
from
1
g
air-
dried
soil
by
mechanical
shaking
with
20
mis
of
1M
KCI
for
2
h.
After
extraction,
samples
were
filtered.
Aliquots
of
filtrate
of
known
volume
were
made
up
to
25
ml
by
the
sequential
addition
of
1
ml
of
2M
HC1,
0.5
ml
of
0.1%,
1,10-phena-
throline,
1.1
ml
of
0.0375%
catechol
violet,
5.0
ml
of
30%
hexamine
buffer
and
deionised
water
to
volume
to
develop
the
colour
and
give
a
final
solution
pH
of
6.1
±
0.1.
Between
10
and
20
minutes
after
the
addition
of
the
hexamine
buffer,
Al
was
determined
colorimetrically
at
585
nm
using
a
Shimadzu
double
beam
spectro-
photometer
[8].
Results were
expressed
as
mil-
liequivalents
of
exchangeable
Al
per
kg
of
oven-
dried
soil.
Extractable
Mn
was
extracted
from
10
g
air-
dried
soil
by
25
ml
of
0.04
M
EDTA
at
pH
6.0
±
0.1
and
shaken
for
2
h.
After
extraction,
samples
were
centrifuged
and
filtered,
and
the
filtrate
analysed
for
Mn
by
flame
atomic
absorp-
tion
spectrophotometry.
Results
were
expressed
as
p.g
extractable
Mn
per
g
of
oven-dried
soil.
Exchangeable
Ca,
Mg
and
K
were
extracted
by
mechanically
shaking
2
g
air-dried
soil
with
30
ml
of
1M
ammonium
acetate
at
pH
7
for
2
h.
The
suspension
was
filtered
and
the
residue
was
leached
with
an
additional
50
ml
ammonium
acetate.
Cation
concentrations
were
determined
by
flame
atomic
absorption
spectrophotometry.
Results
were
expressed
at
milliequivalent
of
exchangeable
cation
extracted
per
kg
over-dried
soil.
from
the
SAS
computer
package
and
R
2
effective
values
calculated
[6].
Results
Soil
pH
Urea
fertigation
resulted
in
highly
significant
(p
0.0001)
changes
in
soil
pH
below
the
emit-
ter
at
all
soil
depths
from
time
2
to
time
5
(Table
1).
Soil
pH
decreased
linearly
with
increasing
urea
application
at
these
times.
The
gradient
of
pH
change
with
urea
fertigation
was
largest
at
time
5
where
a
pH
change
of
1.6
pH
units
estimated
according
to
the
regression
equation
(Fig.
1)
was
recorded
at
all
depths.
The
lowest
estimated
pH
of
4.3
occurred
at
27
cm
depth.
At
all
rates
of
urea
application,
soil
pH
followed
a
quadratic
relationship
down
the
profile
with
soil
pH
decreasing
then
increasing
with
soil
depth.
The
only
significant
response
to
urea
fertiga-
tion
detected
in
the
soil
cores
taken
20
cm
from
the
emitter
was
the
pH
at
time
5
(Table
2).
The
soil
pH
20
cm
away
from
the
emitter
decreased
significantly
with
increasing
rates
of
fertigation
particularly
at
the
lowest
soil
profile
depth
(Tables
1
and
2).
The
variation
in
pH,
as
indicated
by
the
standard
error
of
the
means,
was
greater
for
measurements
20
cm
from
the
emitter
compared
to
those
directly
below
the
emitter
(Table
2).
Statistical
analysis
Analysis
of
linear
functions
of
urea-N
treatment
levels,
depth
and
interaction
on
pH,
Al,
Mn,
Ca,
Mg
and
K
were
computed
using
a
Genstat
package.
As
sequential
observations
of
the
same
variable
were
recorded
from
each
experimental
unit,
statistical
analysis
of
contrasts
over
time
were
determined
as
described
by
Rowell
and
Walters
[19].
Equations
were
fitted
to
linear
functions
that
were
determined
to
be
significant.
Data
derived
from
fitted
equations
were
graphically
repre-
sented
using
the
3
dimensional
graphics
option
Table
1.
Mean
soil
pH
values
directly
below
the
emitter
at
different
sampling
times
and
at
different
soil
depths
in
the
50
kg
N
ha_'
yr
-1
fertigation
treatment
Time
of
sampling
Soil
depth
(cm)
T,
T,
T
2
T
3
T
4
T
5
0-10
5.83
5.67
5.10
5.00
4.70
4.73
10-20
5.80
5.20
4.53 4.73
4.57 4.27
20-40
5.91
5.17
4.57
4.77
4.60
4.30
40-60
6.30
5.90
5.00
4.90 4.70
4.63
Level
of
significance
compared
with
time
T
o
NS
***
***
***
***
=
p
0.0001, NS
=
not
significant.
3
Urea
2
(kg
N
h8
1
yr
1
)
5
0
0
242
Al
(meg
kg
1)
12
60
55
50
45
40
3
pH
3
2
Depth
25
15
(cm)
1
Fig.
1.
The
effect
of
urea
fertigation
on
soil
pH
directly
below
the
emitter
at
time
5.
(pH
=
6.51
-
0.032
urea
-
0.041
depth
+
0.00075
depth
x
depth;
R
2
=
87.2;
p
0.0001.)
Verti-
cal
bar
indicates
standard
error
of
the
mean.
Table
2.
Mean
pH
and
exchangeable
aluminium
at
time
5
after
application
of
urea
at
50
kg
N
ha
Soil
depth
(cm)
pH
Distance
from
emitter
(cm)
0
20
Al
(meq
g
Distance
from
emitter
(cm)
0
20
0-10
4.73
5.13
8.85
7.80
(0.13)
(0.46) (2.23)
(5.5-
7)
10-20
4.27
4.90
11.40
6.67
(0.12)
(0.38) (2.34)
(4.8-
7)
20-30
4.30
4.80
11.65
10.54
(0.20) (0.32)
(2.40)
(4.0-
2)
30-40
4.63
4.86
6.43
7.84
(0.21)
(0.33) (1.03)
(3.3-
5)
Level
of
significance
compared
with
time
T,
***
*** ***
NS
***
=
p
0.0001,
NS
=
not
significant,
numbers
in
paren-
thesis
indicate
standard
errors
of
the
mean.
Exchangeable
aluminium
A
highly
significant
(p
0.0001)
increase
in
soil
exchangeable
Al
with
increasing
rates
of
urea
0
5
(kg
N
hi
l
y7
1
)
Urea
2 $
2
15
20
Depth
25
0
3
10
(cm)
5
0
Fig.
2.
The
effect
of
urea
fertigation
on
soil
exchangeable
aluminium
directly
below
the
emitter
at
sample
time
5.
(Al
=
-2.18
+
0.168
urea
+
0.000451
urea
x
urea
+
0.281
depth
-
0.00532
depth
x
depth;
R
2
=
75%;
p
1
0.0001.)
Ver-
tical
bar
indicates
standard
error
of
the
mean.
application
was
only
detected
at
time
5
directly
below
the
emitter
(Fig.
2).
High
exchangeable
Al
levels
directly
below
the
emitter
were
re-
corded
from
time
2
but
there
was
considerable
variance,
and
trends
were
not
significant
except
at
time
5.
Exchangeable
Al
was
negligible
at
low
or
zero
urea
application.
As
soil
depth
increased,
exchangeable
Al
levels
showed
a
quadratic
re-
sponse
(Fig.
2).
Exchangeable
Al
reached
high
levels
20
cm
from
the
emitter.
However,
there
was
considerable
variance
and
the
relationship
was
not
significant
(Table
2).
A
significant
(p
0.005)
quadratic
relation-
ship
between
soil
pH
and
extractable
Al
at
all
soil
depths
was
recorded
at
time
5
(Al
=
146-
49.3
pH
+
4.18
pH
x
pH,
R
2
=
87.3%).
At
pH
below
5.6,
extractable
Al
increased
rapidly
and
levels
greater
than
10
meq
kg
.1
were
recorded
at
pH
<
4.5.
Extractable
manganese
Directly
below
the
emitter,
extractable
Mn
in-
creased
linearly
with
increasing
rates
of
urea
1.0
6.5
6.0
5.5
5.0
4.5
4.0
4 60
4
50
55
45
3
40
Ca
(meg
kg
1)
90
80
70
60
I
50
40
30
20
10
O
s
6
55
0
50
4
35
40
25
Depth
(cm)
3
30
2
Urea
(kg
N
htiT
l
yr
i
)i
20
1
10
Fig.
4.
The
effect
of
urea
fertigation
on
soil
exchangeable
calcium
directly
below
the
emitter
at
time
5.
(Ca
=
81.31
0.352
urea
0.946
depth;
R
2
=
77.6%;
p
0.0001.)
Vertical
bar
indicates
standard
error
of
the
mean.
0
50
60
50
Mn
g
1
)
90
60
30
55
35
30
45
40
35
2
Urea
2
(kg
N
ha
l
yr
1
)
IS
15
25
20
Depth
(cm)
10
13
5
30
243
fertigation
in
the
upper
soil
profile
at
time
5
(Fig.
3).
Calculated
extractable
Mn
levels
increased
to
78
lig
g
-1
under
the
50
kg
N
ha
-1
yr
fertiga-
tion.
Extractable
Mn
levels
decreased
linearly
with
depth,
and
levels
were
negligible
at
lower
profile
depths
at
all
sample
times
(Fig.
3).
Exchangeable
calcium
Exchangeable
Ca
decreased
linearly
with
in-
creasing
rates
of
urea
fertigation
and
soil
depth
at
time
5
directly
below
the
emitter
(Fig.
4).
Estimated
Ca
losses
under
50
kg
N
ha
-1
yr
-1
at
all
soil
depths
were
17.6
meq
kg
-1
representing
23
and
52%
of
exchangeable
Ca
from
the
upper
and
lower
soil
profile
depths
respectively.
Exchangeable
magnesium
Levels
of
exchangeable
Mg
decreased
linearly
with
increasing
rates
of
urea
fertigation
and
soil
depth
at
times
4
and
5.
At
time
5
(Fig.
5)
the
effect
of
urea
fertigation
on
exchangeable
Mg
60
35
2
15
10
5
5
4
55
50
3
25
0
20
Depth
(cm)
3
45
40
Urea
(kg
N
1
)
hely7
I
Mg
(meg
kg
10.0
7.
5
50
2.5
0.0
5
4
0
Fig.
3.
The
effect
of
urea
fertigation
on
soil
extractable
manganese
directly
below
the
emitter
at
sample
time
5.
(Mn
=
55.04
+
0.46
urea
1.112
depth
0.0093
urea
x
depth;
R
2
84.9%;
p
0.0001.)
Vertical
bar
indicates
standard
error
of
the
mean.
Fig.
5.
The
effect
of
urea
fertigation
on
soil
exchangeable
magnesium
directly
below
the
emitter
at
time
5.
(Mg
=
9.41+
0.120
urea
0.106
depth
+
0.0017
urea
x
depth;
R
2
=
82.6%;
p
0.0001.)
Vertical
bar
indicates
standard
error
of
the
mean.
244
was
most
pronounced
in
the
upper
soil
profile
(Fig.
5)
resulting
in
a
loss
of
63%
of
topsoil
exchangeable
Mg
(8.9
to
3.3
meq
kg
-1
at
5
cm
depth).
The
linear
decrease
in
exchangeable
Mg
down
the
profile
was
most
pronounced
at
the
lower
rates
of
urea
fertigation.
Exchangeable
potassium
Exchangeable
K
decreased
linearly
with
increas-
ing
rates
of
urea
fertigation
in
the
upper
soil
profile
but
increased
linearly
with
increasing
rates
of
urea
fertigation
in
the
lower
soil
profile
depths
(20-40,
40-60
cm)
at
time
5
(Fig.
6).
At
50
kg
N
ha
-1
applied
exchangeable
K
losses
in
the
upper
soil
profile
were
27%
of
total
ex-
changeable
K,
representing
a
decrease
of
2.0
meq
kg
-1
.
Exchangeable
K
decreased
linear-
ly
with
depth.
60
55
50
45
40
35
2
Urea
(kg
N
he
l
yi
1
)
20
10
5
Fig.
6.
The
effect
of
urea
fertigation
on
soil
exchangeable
potassium
directly
below
the
emitter
at
time
5.
(K
=
7.83
0.048
urea
0.103
depth
+
0.00161
urea
x
depth;
le
=
65.8%;
p
0.0001.)
Vertical
bar
indicates
standard
error
of
the
mean.
Discussion
The
concomitant
net
acidification
associated
with
urea
transformations
in
soil
as
obtained
in
the
present
study
is
well
documented
[11].
Acidifica-
tion
is
further
magnified
under
trickle
emitter
fertigation
due
to
the
confined
area
of
applica-
tion
compared
to
broadcasting.
In
this
experi-
ment,
the
soil
surface
water
distribution
was
within
an
approximate
15
cm
radius
of
the
emit-
ter.
There
were
740
trees
per
ha
and
2
emitters
per
tree.
Therefore,
as
fertigation
concentrated
the
fertiliser
to
within
the
wetted
area,
the
50
kg
N
ha
-1
yr
-1
treatment
was
applied
at
an
application
rate
equivalent
to
4779
kg
N
ha
-1
yr
-1
onto
the
wetted
soil
areas.
In
addition
to
the
restricted
surface
distribution,
the
water
and
therefore
urea
spatial
distribution
within
the
soil
profile
was
also
laterally
restricted
due
to
the
soil
physical
properties.
Also,
coarse
textured
soils
have
an
inherently
low
buffering
capacity.
Tak-
ing
all
factors
into
consideration,
soil
acidifica-
tion
could
be
expected
to
be
severe.
Significant
(p
0.0001)
soil
acidification
was
recorded
at
time
2
to
5
directly
below
the
emitter.
Time
1
received
2
hours
of
post
fertiga-
tion
irrigation,
which
compared
to
5
minute
irrigation
after
other
treatment
times,
may
have
resulted
in
considerable
leaching,
and
thus
nul-
lifying
the
treatment.
At
soil
sampling
times
2
and
3,
acidification
associated
with
urea
applica-
tion
was
more
severe
at
the
lower
soil
depths.
This
could
reflect
the
declining
buffering
capacity
related
to
declining
organic
matter
levels
with
increasing
soil
profile
depths.
At
times
4
and
5,
the
interaction
between
urea
application
and
depth
was
not
significant,
indicating
that
the
severity
of
soil
acidification
probably
had
over-
come
the
buffering
capacity
effect
down
the
profile
(Fig.
1).
As
expected,
acidification
was
intensified
with
consecutive
fertigations
due
to
a
cumulative
effect.
The
largest
soil
acidification
was
recorded
directly
below
the
emitter
at
time
5
under
50
kg
N
ha
-1
yr
-1
where
an estimated
pH
drop
of
1.6
units
was
recorded
at
all
profile
depths
(Fig.
1).
In
this
trial,
the
lowest
estimated
pH
value
of
4.3
occurred
at
a
profile
depth
of
27
cm.
This
subsoil
acidity
is
difficult
to
ameliorate
at
sur-
K
(meq
k5
1
)
10.0
1.5
1
5.0
2.5
0.0
15
25
Depth
(cm)
245
face-applied
liming
materials
usually
raise
only
the
pH
of
the
surface
soil
[10].
An
estimated
pH
drop
of
1.5
was
recorded
for
all
soil
depths
at
time
4.
This
indicates
that
acidification,
and
therefore
nitrification,
was
still
occurring
4
weeks
after
the
final
fertigation.
In
comparison,
complete
nitrification
of
fertigated
urea
within
2-3
weeks
of
application
has
been
reported
[13].
Resultant
pH
values
were
considerably
lower
than
those
recommended
(pH
5.8
to
6.8)
for
pip
fruit
production
in
these
soils
[15].
Edwards
et
al.
[7]
also
recorded
a
decrease
in
soil
pH
from
6.2
to
4.5
in
6
months
and
to
as
low
as
3.7
after
2
seasons
when
132
kg
N
ha
-1
yr
-1
of
NI-1
4
1\10
3
was
fertigated
onto
a
fine
sandy
orchard
soil.
Lateral
water
movement
within
the
soil
profile
appeared
to
be
restricted,
as
indicated
by
the
lack
of
significant
soil
chemical
changes
detected
in
the
core
samples
taken
20
cm
away
from
the
emitter.
At
this
location,
the
only
significant
soil
change
was
pH
at
time
5
under
50
kg
N
ha
-1
yr
-1
(Table
2).
Although
high
levels
of
exchangeable
Al
were
detected
20
cm
from
the
emitter
at
time
5
under
50
kg
N
ha
-1
yr
-1
,
there
was
consider-
able
variability,
indicating
that
lateral
water
and
urea
movement
to
20
cm
from
the
emitter
was
irregular
(Table
2).
This
irregular,
non-cylindri-
cal
waster
distribution,
probably
reflected
an
uneven
soil
surface
as
well
as
demonstrating
the
limited
lateral
movement
down
the
profile
due
to
the
coarse
soil
texture
and
the
rate
of
emitter
discharge.
Haynes
[11]
reported
that
increasing
the
discharge
rate
from
2
to
41h
1
reduced
the
downward
movement
of
urea
and
therefore
contained
acidification
from
40
to
20
cm
depth,
by
enhancing
lateral
flow
in
the
upper
soil
depth
of
a
silt
loam.
Thus,
wen
applying
a
constant
volume
of
water,
a
higher
emitter
discharge
rate
would
reduce
subsoil
acidification.
The
inverse
relationship
between
soil
pH
and
Al
solubility
which
was
obtained
in
this
study
(Fig.
1
and
Fig.
2)
is
well
recognised
[3].
Soil
acidification
below
pH
5.6
resulted
in
a
rapid
increase
in
exchangeable
Al.
Generally,
Al
levels
above
10
meq
kg
-1
oven
dried
soil
are
considered
toxic
to
most
plants
[16].
In
the
present
study
these
levels
occurred
at
pH
<
4.5
at
all
soil
depths.
Edwards
et
al.
[7]
also
reported
exchangeable
Al
levels
of
7.3
and
14.5
meq
kg
-1
under
NH
4
NO
3
fertigation
when
soil
pH
decreased
to
4.5
in
6
months
and
3.2
in
2
years
respectively.
These
authors
attributed
the
reduced
peach
root
growth
and
fruit
yield
to
Al
toxicity.
Assuming
that
apple
roots
behave
in
a
similar
physiological
manner
to
peach
tree
roots,
the
present
results
indicate
that
within
2
seasons
of
applying
half
the
usual
N
application
rate,
fruit
tree
vigour
and
yield
may
be
severely
affected.
Extractable
Mn
levels
increased
significantly
(p
0.001)
in
the
upper
soil
profile
with
increas-
ing
rates
of
urea
fertigated
at
time
5
only
(Fig.
3).
In
the
lower
soil
profile
depth,
extractable
Mn
levels
were
negligible
at
all
urea
application
rates
indicating
low
inherent
Mn
levels
and
minimal
Mn
leaching
from
upper
soil
profiles.
With
subsequent
rain
and/or
irrigation,
leaching
of
Mn
may
become
apparent
as
reported
also
by
other
workers
[12].
Toxic
Mn
levels
can
occur
at
pH
<
5.5,
but
are
dependent
on
total
soil
Mn
levels.
In
this
study,
level
of
extractable
Mn
reached
78
Ag
g
-
'
which
could
be
potentially
toxic
to
most
plants
[1].
The
displacement
of
base
cations
by
A1
3
"
,
H'
and
NH4
occurs
with
soil
acidification
[12].
In
general,
Ca
is
the
most
prevalent
exchange
cation
and
is
therefore
displaced
in
the
largest
quantities
[12,
18].
A
significant
(p
<0.005)
linear
decrease
in
exchangeable
Ca
was
detected
with
increasing
urea
fertigation
rates
directly
below
the
emitter,
at
all
profile
depths
at
time
5
only.
Large
losses
of
Ca
recorded
from
all
profile
depths
under
50
kg
urea-N
ha
-1
yr
-1
indicates
that
displaced
Ca
was
leached
beyond
the
mea-
sured
soil
profile
depth.
Decreased
soil
Ca
to
profile
depths
of
60
cm
with
NH
4
NO
3
fertigation
have
also
been
reported
[7].
As
well
as
affecting
the
soil
nutrient
balance
losses
of
Ca,
it
can
have
detrimental
effects
on
soil
structure
[5].
Further-
more,
a
reduction
in
soil
permeability
under
NH4
and
urea
fertigation
has
been
recorded
[2].
Magnesium
displacement
was
more
severe
in
the
upper
profile,
particularly
at
time
5,
where
the
loss
of
63%
of
exchangeable
Mg
in
the
upper
soil
profile
occurred.
(Fig.
5).
At
both
times
4
and
5,
the
linear
decrease
in
exchangeable
Mg
was
more
apparent
at
low
rates
of
urea
fertiga-
246
tion,
suggesting
that
at
higher
fertigation
rates,
Mg
from
the
upper
soil
profile
accumulated
in
the
lower
profile
depths.
Soil
acidification
with
urea
fertigation
(50
kg
N
ha
-1
yr
-1
)
reduced
upper
soil
profile
exchangeable
K
levels
by
27%,
while
increasing
levels
of
K
at
lower
soil
depths
(Fig.
6).
Clearly,
acidification
resulted
in
the
displacement
of
K
in
the
upper
profile
which
has
been
leached
to
the
lower
profile.
Subsequent
rain
and/or
irrigation
would
probably
result
in
K
loss
from
all
soil
profile
depths.
Likewise,
the
losses
of
Ca
and
Mg
would
result
in
levels
below
those
recommended
for
pip
fruit
production
in
these
soils
[15].
Reduced
Ca
:
K,
Mg
:K
and
Ca
:
Mg
+
K
ratios
with
the
acidification
of
a
coarse-textured
or-
chard
soil
have
been
recorded
[18].
Low
Ca
:K
and/or
Mg
ratios
can
result
in
ionic
antagonism
of
Ca
uptake
[16].
However
during
this
experi-
ment,
the
percentage
losses
of
Ca,
Mg
and
K
in
the
upper
soil
profile
under
the
50
kg
N
ha
-1
yr
-1
rate
were
23%,
63%
and
27%
respectively
and
thus
showing
increases
in
the
Ca
:
Mg,
Ca
:
K,
K
:
Mg
and
Ca
:
Mg
+
K
ratios.
Although
the
wetted
zone
under
trickle
irriga-
tion
may
only
represent
a
small
proportion
of
the
soil
utilised
by
the
root
system
of
a
mature
apple
tree,
the
root
activity
tends
to
proliferate
in
the
wetter
zone
[9].
This
may
explain
why
trees
have
responded
to
mineral
imbalances
within
this
relatively
small
root
zone.
A
higher
proportion
of
the
root
system
of
first
season
tress,
as
in
this
study,
would
occupy
the
wetted
zone
compared
to
mature
trees.
Therefore,
establishing
trees
may
be
even
more
susceptible
to
adverse
soil
conditions
under
the
emitters.
At
all
stages
of
tree
maturity
if
the
wetted
zone
becomes
adverse
to
root
growth
and
functions,
any
beneficial
effects
of
both
irrigation
and
N
fertigation
would
then
be
reduced.
Avoiding
or
rapid
correction
of
subsoil
acidifi-
cation
would
be
essential
to
justify
further
urea
fertigation
emitters
on
this
or
similar
soil
types.
Application
of
fertiliser
to
counteract
deficien-
cies
is
insufficient
unless
acidification
is
corrected
as
Al
and
Mn
toxicities
would
still
prevail.
Incorporation
of
liming
materials
in
the
subsoil
at
pre-planting
would
reduce
initial
acidification
[21].
However,
the
severity
of
acidification
in
this
trial
after
one
season
indicates
that
regular
correction
of
subsoil
pH
would
be
required
following
continuous
urea
fertigation.
Some
suc-
cess
in
correcting
subsoil
pH
was
achieved
by
fertigation
with
alkaline
solutions
such
as
potas-
sium
hydroxide
and
sodium
hydroxide
[10].
However,
avoiding
subsoil
acidification
so
that
surface
applied
liming
materials
can
be
effective
appears
to
be
the
most
viable
management
option.
This
could
be
achieved
in
two
ways.
Firstly,
urea
fertigation
at
lower
application
rates,
and
only
to
productive
trees
at
high
N
demand
phases,
would
reduce
the
severity
of
acidification.
Secondly,
increasing
lateral
spread
over
a
wider
soil
surface
area
at
application
would
increase
the
area
of
potential
soil
buffer-
ing
capacity
and
reduce
the
depth
of
the
wetted
volume.
Fertigation
through
minisprinklers
re-
sults
in
a
wider
distribution
area
and
therefore
acidification
was
confined
to
shallower
soil
depths
than
trickle
applied
[10].
Increasing
the
application
rate
of
trickle
fertigation
also
in-
creased
the
lateral
distribution
in
a
silt
loam
[11]
but
soil
surface
erosion
may
then
become
an
issue,
expecially
in
sandy
soils.
Conclusion
Fertigation
of
urea
by
trickle
emitters
at
50
kg
N
ha
-1
yr
-1
resulted
in
severe
acidification
and
mineral
element
imbalance
in
a
sandy
loam
soil
in
a
New
Zealand
apple
orchard
after
only
one
season.
These
soil
changes
were
potentially
detrimental
to
plant
growth
and
fruit
production.
The
viability
of
urea
fertigation
of
coarse tex-
tured
soils
is
limited
unless
subsoil
acidification
can
be
prevented
or
rapidly
corrected
to
prevent
further
mineral
imbalances.
Avoiding
subsoil
acidification
by
using
wide
distribution
mini-
sprinklers
for
urea
fertigation
warrants
further
investigation
on
this
or
similar
soil
types.
Acknowledgment
Dr
R.J.
Haynes,
MAF
Technology,
Canterbury
Agricultural
and
Science
Centre,
Lincoln,
for
the
use
of
his
orchard
trial
plots
for
the
field
experi-
ment.
247
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