Humus - soil
organics
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Unformatted text preview: soil
organics
 Spodosol

 Myakka.
The
Official
State
Soil
of
Florida
 A
 E
 Bh
 Carbon
(cycle
later)
 •  dominantly
adding
C
to
 the
soil
 •  so
we’re
talking
more
 about
the
upper
 horizon
A
of
the
soil
 dominantly.
 SOM
 soil
organic
maFer
 •  Organic
maFer
is
 essenGal
to
producGve
 soils

 •  improves
physical
and
 chemical
properGes
 •  inflluences
soil
 aggregaGon
and
thus
 Glth
 SOM
–
aggregates
‐
structure
 •  It
bonds
clays
together
 in
groups

 –  more
resistant
to
being
 broken
apart,

 –  protects
against
further
 decomposiGon

 –  
important
for
ability
of
 peds
to
resist
crushing,
 breakdown,
raindrop
 erosion.
 “crumb”
 •  an
IDEAL
STRUCTURE
‐
 called
TILTH
of
the
soil

 –  easy
to
Gll,

 –  good
seed
bed,

 –  seedling
emergence,
 –  root
penetraGon
etc
 •  Permeability
‐
increases
 •  Coats
mineral
fracGon
 stopping
weathering
 •  Soil
aeraGon
increases
 •  Erosion
decreases
 •  Increases
water
holding
 capacity
 •  increases
CEC
‐
higher
 than
inorganic
colloids
 SOM
 •  ALSO
‐
contain
essenGal
 plant
nutrients
 •  primary
source
of
N
 •  important
source
of
S,
P
 +
others
 •  we
think
of
organic
 maFer
making
the
soil
 ferGle

 •  BUT
a
soil
of
100%
 organic
material
is
 naturally
inferGle.
 •  Organic
material
in
a
 forest
or
grassland
can
be
 considered
in
3
areas
 •  1)
the
plants
‐
38%
in
a
 forest
 •  2)
the
floor
‐
9%
in
a
 forest
 •  3)
the
soil
+
roots
‐
this
is
 about
53%
in
a
forest
‐
up
 to
95%
in
a
grassland
 area.
 •  The
source
of
the
 organic
material

(SOM‐ soil
organic
material)
 are
plants
 •  then
consumed
and
 decomposed
by
animals
 and
microbes‐
LAST
 WEEK
 PLANTS

 •  take
carbon
from
CO2
 •  during
photosynthesis,
 energy
in
the
form
of
 light
is
used
to
convert
 water
and
CO2
and
 minerals
into
oxygen
 and
energy‐rich
organic
 compounds
mainly
 carbohydrates
 •  the
light
is
absorbed
by
 CHLOROPHYLL
in
green
 plants
 •  acGve
healthy
plants
 can
produce
several
 liters
of
O2
/
hour
for
 each
gram
of
 chlorophyll
‐
about
1
kg
 of
plant
Gssue.
 •  rates
of
photosynthesis

 ‐
dependent
on
light
 supply,
CO2
supply
and
 temperature.
 •  We
get
complex
organic
 materials
such
as
 –  Glucose
 –  Cellulose
 –  Starch
 –  Fructose
 –  sucrose
 Glucose
 •  also
other
products
‐
 amino
acids,


 •  proGens

 •  (animo
acids
+
C
+
H
+
N
 +
O
+
S),
 •  
lignin,


 •  lipids
(fats),

 •  pigments
 Protein
‐
alanine
 •  minerals
supply
the
 other
material
NOT
 produced
by
the
plant
 •  N,
P,
S,
 •  +


K,
Na

etc.
 K
–
Na
‐
tartrate
 •  CHANGES
WITH
TIME
‐
 when
we/
nature
adds
a
 mass
of
SOM
–
 •  Hand
out
too
 •  1)
proteins
and
solubles
 ‐
bacteria
eat
this
that
 were
already
in
the
soil
 but
dormant
‐
produces
 much
CO2
 •  2)
cellulose
and
 hemicellulose
 •  3)
lignin
 •  compare
to
microbial
 acGvity
‐
consumed
and
 decomposed
by
soil
 organisms
 •  they
use
thecarbon
‐
in
 its
many
forms
 •  use
the
easiest
first
‐
 when
all
gone
will
 decrease
in
#
of
 decomposers

‐
most
 die
some
inacGve
 •  just
a
very
few
can
 survive
on
the
very
 resistant
material

 •  Die
–
and
the

bodies
 and
products
of
 organisms
add
to
the
 soil
 HUMUS
 •  the
more
or
less
stable
 organic
fracGon
that
is
 lei
is
called
‐
HUMUS
 HUMUS

 •  A
collecGon
of
complex
 organic
compounds
of
 microbial
or
higher
 plant
origin
that
are
 resistant
to
decay
and
/
 or
compounds
 protected
from
decay
 by
adsorpGon
onto
 minerals
surfaces.
 •  material
is
NOW
 –  50‐60
%
C
and

 –  
N
+
P
+
S
+
O

 •  Colloidal
 •  highly
charged
 •  some
parts
are
very
 long
Gme
residents
‐
 some
1000’
years
 •  we
can
envision
the
 humus
content
to
be
in
 equilibrium
with
the
 soil
in
a
steady
state
 •  new
humus
produced
=
 humus
lost
 •  how
much
humic
 substances?
 •  depends
on
‐
OUR
SOIL
 FORMING
FACTORS
 Climate
 •  type
and
amount
of
 vegetaGon
+
rate
of
 decomposiGon
 •  general
o.m.
inc
with
 inc
ppt
‐
also
temp
 consideraGon
 VegetaGon
 •  affects
type,
amount
 •  annuals
‐
also
root
 and
placement
of
 decay
‐
higher
basic
 organics
 caGon
content
 •  trees
/
shrubs
‐
leaf
fall
‐
 •  Grasslands?
 O
and
thin
A
 •  evergreen
needles
have
 low
basic
caGons
‐
Ca
 and
K
‐
are
acidic
‐
more
 Al
etc
‐
more
soil
 weathering
 Topography
 •  affects
runoff
and
 •  organic
soils
‐
 erosion
‐
steep
slopes
 HISTOSOLS
 less
water
for
plants+
 oien
O
and
A
are
 removed
 •  gentle
slopes
‐
thicker
A
 +
addiGons
 •  BUT
‐
oien
poor
 drainage
‐
much
 organics
e.g.
swamps
 parent
material
 •  sand
‐
less
humus

 •  clays
‐
more
humus
+
 also
wet
‐
see
above
 age
(Gme)
 •  Gme
adds
more
 organics
 •  NOT
when
cropped….
 age
(Gme)
 •  •  •  •  •  irrigaGon,

 drainage,

 ferGlizers

 
MAN

etc

 will
all
effect
the
 balance

 –  Adding
OM
as
adding
a
 drain
system
 WHERE
DOES
HUMUS
ORIGINATE
 •  1)
LIGNIN
‐
the
main
 chemical
consGtuents
 of
wood
are
cellulose,
 hemicellulose
and
lignin
 •  Cellulose
are
long
 •  like
‐
REBAR
AND
 molecules
that
form
 CEMENT
 microfibrils
‐
give
wood
 it’s
strength
 •  Hemicellulose
and
lignin
 glue
these
together
‐
 the
structural
integrity
 •  lignin
is
responsible
for
 the
resistance
to
 degradaGon
–
 •  
BUT
is
poorly
 understood
 (incompletely
 characterized)

‐
know
it
 is
a
6‐carbon
ring
 structure
 •  LIGNIN
‐
decomposes
 very
very
slowly
0.001%
 day
or
less
 •  (fastest
SOM
at
8%
/
 day)
 •  Much
of
the
humus
 bonds
with
the
 minerals
parCcles
such
 as
the
clays.
 •  even
unstable
proteins
 can
therefore
survive
 longer
in
the
soil
when
 adsorbed
 •  HUMUS
is
composed
of
 two
groups
of
 compounds
 •  1)
nonhumic
‐
20‐30
%
‐
 amino
acids,
lipids
and
 carbohydrates
 •  2)
humic
substances
‐
 humic
acid;
fulvic
acid
 and
humin
 •  HUMIC
ACID
‐dark
 colored
amorphous
 organic
material
 •  soluble
in
alkalis
BUT
 insoluble
in
dilute
acid
 •  
molecular
weight
‐
 20,000
‐
1360,000
 •  FULVIC
ACID
‐
colored
 organic
material
 •  soluble
in
acid
and
alkali
 •  molecular
weight
 275‐2100
 •  HUMIN
‐
insoluble
in
 both
‐
most
resistant
 CATION
EXCHANGE
CAPACITY
OF
 HUMUS
 •  Humic
and
Fulvic
acids
 are
hydrophyllic
(high
 affinity
for
water)
 •  
colloids
 •  usually
with
a
negaGve
 charge
‐‐‐
 •  caGons
are
aFracted
to
 this
charge
‐
again
CEC
 •  BUT
also
part
of
AEC
in
 soils
 •  pH
buffers,

 •  chelates
etc
 •  pH
buffers
 •  H+
caGon
 •  High
pH
number
–
less
 H+

 •  Humus
micelles
have
an
 even
greater
CEC
than
 montmorillonite
or
 vermiculite,

 •  and
can
be
equally
Gny.
 •  Colloid


Avg.
CEC
(meq/ 100g)
 •  Humus200
 •  Vermiculite150
 •  Montmorillonite100
 •  Illite
and
chlorite
30
 •  Kaolinite

8
 •  Humus
soils
therefore
 can
hold
lots
of
 nutrients,
and
shrink
 and
swell
(think
of
a
 plant
poFed
in
Pro‐mix
 allowed
to
dry).
 •  humus
can
release
or
 take
up
H+
depending
 on
the
pH
of
the
soil
 •  1)

can
buffer
pH
 –  release
H+
as
soil
becomes
 more
alkali
 –  takes
up
H+
as
soil
 becomes
more
acid
 –  When
a
soil
is
limed
and
 the
acidity
decreases,
 there
is
a
greater
tendency
 for
the
H+
to
be
removed
 from
humic
acids
and
to
 react
with
hydroxyl
(OH‐)
 to
form
water.
 •  2)
has
a
pH
dependant
 surface
charge
‐
a
pH
 dependent
CEC
 •  if
the
soil
is
more
alkali
H+
 is
released
and
the
humus
 becomes
more
–ve
 •  CEC
increases
 •  more
acidic
CEC
 decreases
 •  3)
ability
to
chelate
 certain
caGons
‐
e.g.
Cu,
 Zn,
Co,
Ni
 •  these
metals
are
now
 immobile
as
humus
is
 immobile
 •  can
cause
deficiencies
 or
reduce
toxiciGes
 •  chelates
etc
 ...
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