Selection of topdressing material for rehabilitation of disturbed areas in the Hunter Valley


Elliott, G.L.; Veness, R.A.

Journal of the Soil Conservation Service of NSW 37(1): 37-40

1981


A procedure for the recognition in the field of soil material suitable for topdressing disturbed sites is described. The criteria used to distinguish suitable and unsuitable material are soil structure, coherence, mottling, macrostructure, ped strength, texture, gravel and sand content, pH, salt content, colour and cutan distribution.

Selection
of
Topdressing
Material
for
Rehab
itation
of
Disturbed
Areas
in
the
Hunte
Valley
G.
L.
Elliott
and
R.
A.
Veness
Soil
material
available
for
topdressing
of
disturbed
areas
is
not
always
suitable
for
agricultural
use.
Physically,
such
material
may
be
too
weakly
structured,
too
poorly
drained,
or
too
sandy
or
gravelly
to
support
vegetation.
This
paper
presents
a
method
for
the
recognition,
in
the
field,
of
material
suitable
for
topdressing.
The
physical
soil
properties
used
to
separate
suitable
and
unsuitable
material
are
soil
structure,
macrostructure,
coherence,
texture
and
the
force
needed
to
disrupt
peds.
The
relevance
of
these
properties
is
discussed.
G.
L.
Elliott
is
a
Research
Officer
at
Scone
Research
Centre
R.
A.
Veness
is
a
Soil
Conservationist
at
Scone.
S
preading
topsoil
to
assist
in
the
revegetation
of
bare
eroding
sites
is
an
accepted
technique.
The
estab-
lishment
of
vegetation
on
industrial
sites
(Junor
1968),
gravel
pits
(Longworth
1962),
road
batters
(Olsen
1976),
coal
ash
dams
(Junor
1978),
dispersible
soil
(Reynolds
and
Lang
1979)
and
mine
overburdens
is
often
improved
by
the
use
of
suitable
topsoil.
In
New
South
Wales
the
topdressing
of
coal
mine
overburden
is
a
condition
attached
to
the
Right
to
Mine
(Department
of
Mineral
Resources
1979).
The
descriptive
term
"topdressing
material"
rather
than
"topsoil"
is
used
in
this
paper
to
avoid
confusion.
Where
there
is
insufficient
topsoil
or
A
horizon
material
available,
or
where
large
quantities
of
material
are
required,
it
may
be
necessary
to
use
B
horizon
soil
material
in
topdressing
operations.
It
is
often
assumed
that
a
mixture
of
A
and
B
horizon
material
recovered
from
a
site
is
better,
from
an
agri-
cultural
point
of
view,
than
any
subsurface
material
remaining
after
site
disturbance.
However,
this
may
not
always
be
so.
This
has
been
observed
on
several
sites
where
large
scale
topdressing
operations
have
been
conducted.
It
is
acknowledged
that
sometimes
material
from
below
the
soil
C
horizon
is
equal,
or
superior,
to
recognisable
soil
material
in
supporting
plant
growth
(Schafer
1979).
There
is
thus
a
need
for
a
relatively
simple
scheme
to
aid
in
the
identification
of
suitable
topdressing
material,
as
the
use
of
unsuitable
material
may
prevent
successful
rehabilitation.
This
paper
presents
criteria
which
have
been
found
to
be
useful,
in
the
Hunter
Valley,
for
the
recognition
of
such
topdressing
materials.
Suitable
techniques
for
the
application
of
material
are
assumed.
(e.g.
Quilty
et
al.
1978).
The
scheme
presented
here,
while
found
to
work
satisfactorily,
is
not
intended
to
be
definitive
and
is
subject
to
improvement.
The
scheme
should
not
be
interpreted
as
a
recommendation
to
use
B
horizon
material
exclusively
in
rehabilitation.
It
is
a
tool
for
the
selection
of
suitable
material
in
situations
where
insufficient
topsoil
exists.
37
PROCEDURE
FOR
THE
RECOGNITION
OF
SUITABLE
TOPDRESSING
MATERIAL
Experience
in
assessing
the
behaviour
of
soil
material
involved
in
the
rehabilitation
of
coal
mine
overburden
in
the
Hunter
Valley
has
been
the
basis
for
identifying
certain
properties
critical
to
the
selection
of
topdressing
material
(table
1).
Also
considered
were
such
factors
as
erodibility
and
natural
regeneration
of
sites
exposed
by
erosion
in
many
parts
of
the
Hunter
Valley.
The
table
is
arranged
with
the
most
significant
features
at
the
start
of
the
scheme.
All
features
should
be
con-
sidered.
The
notes
which
follow
explain
the
importance
of
each
feature.
Some
additional
features
are
also
discussed.
Structure
Water
entry
must
be
considered
in
the
selection
of
topdressing
material
because
water
is
essential
for
the
germination
of
plants
and
available
water
is
needed
for
plant
establishment.
Infiltration
generally
varies
with
structure
grade
(Charman
1978),
and
is
known
to
depend
on
the
proportion
of
coarse
peds
in
the
soil
surface.
Russell
(1974)
stated
that
the
rate
of
water
entry
into
soil
is
controlled
by
every
factor
which
affects
the
number
and
stability
of
large
pores.
Regardless
of
these
factors,
the
visible
evidence
of
the
pores
is
in
the
structure,
coherence
and
colour
of
the
material.
Structure
has
been
used
as
the
primary
determinant
of
the
suitability
of
material.
Coherence
Russell
also
considered
that
surface
soil
should
be
crumbly
and
that
the
crumbs
should
be
large
enough
not
to
blow
away
but
be
small
enough
to
allow
good
germination
of
seed
and
be
sufficiently
non-sticky
when
moist
to
maintain
their
individuality
when
dis-
turbed.
Soil
material
with
the
higher
grades
of
structure
(Butler
1955),
or
the
more
pedal
soils,
are
considered
suitable
for
use
in
topdressing.
Coherence,
in
the
less
pedal
soils,
is
used
as
a
measure
of
the
ability
of
soil
to
maintain
its
structure
grade.
Soils
which
are
structure-
less,
or
in
which
structure
is
likely
to
be
destroyed
by
mechanical
work
associated
with
the
extraction,
trans-
portation
and
spreading
of
topdressing
material
are
not
considered
suitable
for
revegetation.
Surface
sealing
and
reduced
infiltration
of
water
will
restrict
germination
and
establishment
in
these
soils.
Mottling
Mottling
is
an
indicator
of
poor
drainage
conditions
(Bouma
1977).
Poor
moisture
transmission
rates
will
unfavourably
affect
infiltration,
available
moisture
and
air
porosity.
Mottles,
especially
value-chroma
class
two
mottles
(Northcote
1974),
indicate
material
which
is
not
suitable
for
use
in
revegetation.
Macrostructure
Macrostructure
refers
to
the
form
and
orderliness
of
the
arrangement
of
peds
in
the
soil,
and
is
equivalent
to
the
term
primary
ped
used
by
Corbett
(1969).
The
size
of
macrostructure
units
is
an
indication
of
the
tendency
of
a
soil
horizon
to
form
a
massive
structure
when
it
is
wet.
The
void
space
is
reduced
in
such
situations
and
thus
the
tendency
is
undesirable
in
topdressing
materials.
Peds
in
better
drained
soils
have
been
noticed
to
be
smaller
than
those
in
poorly
drained
soils
(van
de
Graff
1978).
This
is
another
reason
to
consider
ped
size
in
the
selection
of
topdressing
materials.
Ped
Strength
The
force
to
disrupt
peds
,
assessed
on
soil
at
about
the
moderately
moist
state,
is
taken
as
an
indicator
of
the
method
of
ped
formation.
Deflocculated
soil
will
dry
to
a
hard
ped
and
re-wet
to
a
dispersed
state.
Flocculated
soil
dries
to
crumbly
peds
and
retains
these
peds
on
wetting
(Russell
1974).
The
"raw"
soils
discussed
by
Russell
and
the
"harsh"
soils
referred
to
by
Paton
(1978)
are
not
suitable
for
revegetation
and
would
normally
be
identified
by
the
strong
force
required
to
break
aggregates.
When
peds
are
difficult
to
disrupt,
the
soil
material
is
unsuited
to
the
objectives
of
top-
d
ressi
ng.
Texture
Junor
(1968,
1978)
indicated
that
topdressing
material
of
any
texture
could
be
used
in
revegetation.
However,
Charman
(1978)
stated
that
sands
are
extremely
erodible
and
Thomasson
(1978)
considered
that
sandy
soils
with
low
available
water
are
poorly
suited
to
plant
growth.
Schafer
(1979)
considered
that
materials
with
textures
coarser
than
sandy
loam
were
less
desirable
for
use
in
strip
mine
reclamation
than
medium
textured
soils.
Favourable
topographic
situation
or
reliable
climate
may
obviate
the
adverse
effects
of
using
sandy
material,
and
good
moisture
relations
in
the
underlying
layers
may
permit
the
use
of
sandy
topdressing
materials.
However,
they
are
generally
not
suited
for
use
in
the
climate
of
the
Hunter
Valley.
It
is
possible
that
topdressing
material
which
retains
a
large
amount
of
available
water,
especially
in
relation
to
underlaying
strata,
may
not
be
suitable
for
re-
vegetation
in
the
long
term.
This
is
because
roots
of
established
plants
will
concentrate
in
this
layer
with
the
following
effects:
inhibition
of
natural
regeneration
by
competition
with
germinating
seedlings
and
by
dessication
of
the
zone
in
which
germination
is
occurring;
poor
drought
tolerance,
because
of
poor
root
distribution;
establishment
of
isolated
plants
with
little
ground
cover
between
plants;
leading
to
increased
erosion
and
weed
invasion
in
periods
favourable
for
growth.
This
problem
requires
further
study
of
the
long
term
effects.
The
heavier
textured
soils
which
have
relatively
high
available
water
capacities
have
given
satisfactory
short
term
establishment.
Gravel
and
Sand
Content
If
the
combined
amount
of
gravel
and
sand
exceeds
60
per
cent
of
the
soil,
plant
growth
may
be
retarded.
Too
much
coarse
grained
material
can
result
in
weakened
soil
structure
and
in
reduced
moisture
holding
capacity.
pH
and
Salt
Content
Before
the
final
assessment
of
the
suitability
of
material
is
made
both
pH
and
salt
content
should
be
measured.
Adverse
effects
of
high
salt
content
and
both
high
and
low
pH
values
are
well
known.
Values
of
pH
should
he
between
the
limits
4.5
and
8.4,
or
preferably
between
5.5
and
7.5.
Values
of
electrical
conductivity,
considered
as
an
index
of
soluble
salt
levels,
should
he
less
than
1.5
x
10
-3
S.
cm
'
measured
in
a
1:5
soil:water
sus-
pension
(Blake
1965).
38
TABLE
1
Procedure
for
the
selection
of
material
for
use
in
topdressing
of
disturbed
areas.
STRUCTURE
GRADE
OF
MATERIAL*
I
II
II
III
<
30%
peds
30-50%
peds
50-80%
peds
>
80%
peds
Not
suitable
COHERENCE
*
Coherent
dry,
not
coherent
wet
Coherent,
Not
coherent,
wet
or
dry.
wet
a
d
dry
Not
suitable
MOTTLE**
Present
Absent
Not
suitable
MACROSTRUCTURE
In
situ
macrostructure
dimension
in
X—Y
plane
<10
cm
FORCE
TO
DISRUPT
PEDS*
/
\
1-3
4-5
Not
suitable
TEXTURE
As
fine
or
finer
than
FSL
As
coarse
or
coarser
than
SL
'GRAVEL
AND
SAND
CONTENT
>
60%
/
<
60%
Not
suitable
>
10
cm
Not
suitable
KEY
*
Butler
(1955
)
*"
Northcote
(1974
)
+
Gravel
2
—60
mm
Not
suitable
pH
<4.5
or>8.4
4.5
to
8.4
Not
suitable
/
SALT
CONTENT'
Measured
as
electrical
conductivity
(S.
cm
—I
)
/
—'
„4
>
1.5x10
-3
<1.5x10
/
\
SUITABLE
Not
suitable
39
Soil
Colour
Colour
of
soils
has
been
found
to
assist
in
the
deter-
mination
of
susceptibility
to
dispersion
and
to
erosion
in
the
Hunter
Valley
(Loughran
et
al.
1980).
Charman
(1978)
stated
that
where
red
and
yellow
soils
occur
in
toposequence,
as
a.
general
rule
the
red
soils
will
be
the
more
stable
with
respect
to
soil
conservation
structures,
cultivation,
etc.
Stapledon
and
Casinader
(1976)
found
that
45
per
cent
of
yellow-brown
(B
/C
horizons
were
dispersive,
compared
to
less
than
25
per
cent
of
red-
brown
B
horizons.
Most
of
the
dispersion
in
red-brown
soils
was
actually
slaking
along
fracture
planes.
Murphy
(1980)
presented
data
on
aggregate
dispersion
in
duplex
red
and
yellow
soils
showing
stronger
aggregation
in
the
red
soils.
Soils
as
red
as,
or
redder
than,
7.5
YR,
with
value-chroma
classes
one
or
five,
or
with
value-chroma
class
four
and
colour
value
less
than
or
equal
to
six,
are
generally
suitable
with
respect
to
aggregate
stability
and
to
erodibility.
Soils
as
yellow
as,
or
yellower
than,
7.5
YR
and
with
value-chroma
classes
two
or
three,
or
with
value-chroma
class
four
and
colour
value
greater
than
or
equal
to
seven,
are
generally
not
suitable
with
respect
to
stability
and
erodibility.
Cutans
The
presence
of
cutans
in
the
soil
has
been
found
to
be
a
useful
indicator
of
permeability,
especially
when
the
distribution
of
cutans
is
considered.
A
uniform
distri-
bution
of
cutans
indicates
uniform
and
deep
wetting.
A
discontinuous
distribution
indicates
restrictions
to
permeability
or
a
limitation
to
the
expansion
of
a
wetting
front.
Following
Brewer's
(1964)
terminology,
grain
cutans,
tertiary
ped
cutans,
channel
and
plane
cutans
are
useful
indicators
of
satisfactory
permeability.
Void
cutans
and
cutans
restricted
to
primary
peds
may
indicate
restricted
permeability.
TOPDRESSING
OPERATION
Having
decided
the
suitability
of
each
layer
in
the
soil
profile
it
is
possible
that
some
materials
that
are
suitable
for
use
occur
alternately
with
materials
that
are
not.
Unsuitable
materials
may
be
included
in
the
topdressing
mixture
of
soil
horizons
if
they
are
less
than
20
cm
thick
and
comprise
less
than
30
per
cent
of
the
total
material
to
be
used.
If
the
layer
of
unsuit
a
bl
e
material
is
more
than
20
cm
thick
it
is
probably
practi
ca
l
to
selectively
remove
and
discard
it.
CONCLUSION
Use
of
the
scheme
suggested,
for
the
selection
of
topdressing
material,
will
result
in
the
choice
of
materials
stable
to
erosion
and
suitable
for
plant
germination
and
establishment.
For
the
revegetation
of
disturbed
areas,
the
soil
properties
considered
to
be
critical
are
soil
structure
grade,
coherence,
mottling,
macrostructure,
ped
strength,
texture,
gravel
and
sand
content,
pH
and
salt
content.
These
were
found
to
be
the
important
soil
properties.
The
other
properties
mentioned
(colour,
presence
of
cutans)
should
also
be
used
but
have
not
been
included
in
the
scheme
as
they
are
less
definitive
factors,
the
limits
for
which
cannot
be
evaluated.
Some
possible
problems
associated
with
the
use of
material
which
retains
a
relatively
large
amount
of
available
water
have
been
identified.
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C.
D.
(ed.)
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of
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J.
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study
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some
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