Transcript 11Soilacidity09

Understanding pH
pH = - log (H+ ion concentration)
?
pH
=7
neutral
As pH decreases…
increases…
Brady and Weil, 2002
Optimum pH ranges have been identified for many crops
Native species also have pH preferences
http://asecular.com/forests/graphics.jpg
Understanding
Soil pH
Brady and Weil (2002)
Neutral
Do plants actually care about soil pH?
The acid infertility complex
Problems related to nutrient availability
and metal toxicity in acid soils
For most soils and crops,
Nutrient
total nutrient availability is
availability
optimized between
varies with pH
pH 5.5 and 7.
Molybdenum is more available at high pHs.
most
http://www.farmtested.com/research_pp.html
Understanding aluminum toxicity
Toxic forms
Aluminum
of Al
are
toxicity is
bioavailable
at lowminimal
pHs
above
pH 5.5
http://www2.ctahr.hawaii.edu/tpss/research_extension/rxsoil/alroot.gif
Crop varieties differ in their sensitivity to Al toxicity
Brady and Weil, 2002
Reserve vs. active acidity
Soil pH is
primarily a
measure of
active acidity
Reserve acidity
Active
acidity
Brady and Weil, 2002
Understanding reserve acidity
Very little
Reserve
lime isacidity
neededresupplies
to neutralize
the the
active
active
acidity
acidity in soils
BIG
pH
change
Small pH change
Reserve
acidity
High CEC soil
Active
acidity
Reserve
acidity
Active
acidity
Low CEC soil
Each charge depicted on this diagram represents 1
centimol of charge per kg of soil
K+
Ca+2
Mg+2
- --Humus
-
What is the CEC of
this soil ?
H+
H20
Exchangeable
exchangeable
acidity
ions
H20
H20
soil
solution
H20
Clay
SO4
-2
++
--
-Al
- K
+
Ca+2
+3
H20
H20
+ 3H2O  Al(OH)3 + 3H+
What is the “base”
saturation ?
Each charge depicted on this diagram represents 1
centimol of charge per kg of soil
K+
Many people refer to
Ca+2
non-acid cations as
Mg+2
base cations
H+
- --Humus
-
H20
Exchangeable
exchangeable
acidity
cations
H20
H20
Clay
SO4
-2
++
-Al
- K
+3
--
H20
soil
solution
H20
H20
+ 3H2O ↔ Al(OH)3 + 3H+
+
Ca+2
What is the “base”
saturation of this soil?
It isIs
probably
more accurate
say that
pH is related to?acid saturation
pH related
to tobase
saturation
100
80
60
40
Acid Saturation, %
20
0
Soil acidity increases when H+ producing
processes exceed H+ consuming
processes.
Many processes add H+ ions to soils
1) Carbonic acid forms when carbon dioxide dissolves in water.
H+ ions are released when carbonic acid dissociates:
H2CO3 -> H+ + HCO32) Organic acids form during the decomposition of organic matter.
H+ ions are released when these organic acids dissociate.
3) Sulfuric and nitric acids form during the oxidation of reduced forms
of N and S (e.g., NH4+ from fertilizer, elemental S).
4) Sulfuric and nitric acids form when sulfur oxides and nitric oxides
(released into the atmosphere by automobile emissions, industry
smoke stacks, volcanoes, forest fires) dissolve in precipitation.
H2SO4 and HNO3 are strong acids and fully dissociate in water.
5) Roots release H+ to balance internal charge when cation uptake
exceeds anion uptake.
Many processes consume H+ ions in soils
1) Weathering of most minerals (e.g., silicates, carbonates…)
2) Decomposition of organic anions
3) Reduction of oxidized forms of N, S and Fe.
4) Roots release OH- or HCO3- to balance internal charge when anion
uptake exceeds cation uptake.
5) Inner sphere adsorption of anions (especially sulfate)
Sources of pH
buffering
Carbonates
Young soil
Lime
(CaCO3)
↓
fountain
of soil
youth?
Old soil
Chadwick and Chorover ( 2001)
Acid inputs promote leaching of non-acid cations
Brady and Weil, 2002
S and N oxides cause acid precipitation
Brady and Weil, 2002
plant
uptake
mineralization
Alfalfa field with
dead strip where
lime was not
applied
How much lime
should be
applied ?
Lime
requirements
should be guided
by soil testing
Pocket pH meters can be very useful
but require regular calibration !!!
Salt pH vs. water pH
Why do labs
in arid
regions use
a salt
solution?
Brady and Weil, 2002
Sources of variation in soil pH measurements
1. The nature and type of inorganic and organic
constituents that contribute to soil acidity.
2. The soil to solution ratio used in measuring pH.
3. The salt content of the diluting solution used to
achieve the desired soil to solution ratio.
4. The carbon dioxide content of the soil and
solution.
5. Errors associated with standardization of the
equipment used to measure pH.
The amount of lime needed to
bring about a 1 unit change in
pH varies widely between soils
“Illinois method” of determining lime requirement
How do
you know
which line
to use ?
http://iah.aces.uiuc.edu/pdf/Agronomy_HB/11chapter.pdf
Choosing the right line
Line A: Dark colored silty clays and silty clay loams (CEC > 24)
Line B: Light and medium colored silty clays and silty clay loams,
dark colored silts and clay loams (CEC 15-24)
Line C: Light and medium colored silt and clay loams, dark and
medium colored loams, dark colored sandy loams (CEC 8-15)
Line D: Light colored loams, light and medium colored sandy
loams and all sands (CEC < 8)
Line E: Mucks and peat (organic soils).
Light colored (< 2.5% OM)
Medium colored (2.5-4.5% OM)
Dark colored (4.5% OM)
Lime requirements determined using the “Illinois
method” assume the following:
A. A 9-inch tillage depth. If tillage is less than 9 inches, reduce the
amount of limestone; if more than 9 inches, increase the lime rate
proportionately. In no-till systems, use a 3-inch depth for calculations
(one-third the amount suggested for soil moldboard-plowed 9 inches
deep).
Rates of lime should be
adjusted if any of these
B. Typical fineness of limestone. Ten percent of the particles are
greater thanassumptions
8-mesh; 30 percent pass an are
8-mesh not
and are held on 30mesh; 30 percent pass a 30-mesh and are held on 60-mesh; and 30
percent pass a 60-mesh.accurate
C. A calcium carbonate equivalent (total neutralizing power) of 90
percent. The rate of application may be adjusted according to the
deviation from 90.
It takes time for lime to react in soil
Soil pH and lime
requirement can
vary widely
within fields
Both past management and inherent
soil properties affect soil pH and lime requirement
Insufficient lime is applied in IL to
neutralize the acidity from N fertilizers
http://iah.aces.uiuc.edu/pdf/Agronomy_HB/11chapter.pdf
Have you
ever seen a
stream look
like this ?