Understanding
Soil
Acidity
Brady and Weil (2002)
Neutral
pH = - log (H+ ion concentration)
pH
=7
neutral
increases…
As pH decreases…
Brady and Weil, 2002
Optimum pH ranges have been identified for many crops
Collective term for the challenges
faced by crops growing in acid soils
The acid infertility complex
For Nutrient
most soils, nutrient
availability
is optimized
availability
between
pHpH
5.5 and 7.
varies with
Molybdenum becomes more available as pH goes up !
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
Multiple forms of soil 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
ΔpH
ΔpH
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
H+
- --Humus
-
H20
Exchangeable
exchangeable
acidity
cations
H20
H20
soil
solution
H20
Clay
SO4
-2
++
--
-Al
- K
+3
H20
H20
+ H2O ↔ Al(OH)3 + 3H+
+
Ca+2
What is the “base”
saturation ?
It is probably
accurateto
to say
that pH
is related to acid
Is pHmore
related
base
saturation
? saturation
100
80
60
40
Acid Saturation, %
20
0
pH dependent charge
The dominant
clay minerals in
IL have mostly
permanent
charge
The charge on humic
substances (and low
activity clays) is very pH
dependent
H+ ions dissociate when the soil pH increases
and reassociate when the pH drops.
Brady and Weil (2002)
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 → HCO3- + H+
VERY IMPORTANT PART OF SOIL FORMATION
2) 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).
NH4+ + O2 → NO3- + 2H+
S0 + O2 → SO4-2 + 2H+
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) which displaces
hydroxyl (OH-) groups
Acidity
What is liberated and what is left behind
when plant biomass is burned ?
Oxides of
C, N and S
Alkalinity
Oxides of
Ca, Mg and K
Elements that
have traditionally
been called
“bases”
C, N and S oxides cause acid precipitation
Brady and Weil, 2002
Sources of pH
buffering
in soils
Carbonates
Chadwick and Chorover ( 2001)
K+
H+
NO3
?
-
The pH of a plant’s
rhizosphere changes
as the plant regulates
its internal charge
balance.
Which plant received nitrate ?
Which plant received ammonium ?
http://departments.agri.huji.ac.il/plantscience/topics_irrigation/uzifert/4thmeet.htm
Acid inputs promote leaching of non-acid cations
Why does
leaching of
these anions
cause soil
acidification ?
Brady and Weil, 2002
Complete N cycle (no net acidification)
released into
the soil
1H+
consumed
Nitrification is an acidifying process, right??
1H+
NH3
consumed
The 2 H+ produced during nitrification are balanced by 2 H+ consumed
during the formation of NH4+ and the uptake of NO3- by plants
Excellent but focused
on Australian soils
Standard values for the quantity of lime needed to
neutralize the acidity generated by specific N fertilizers
Assumes: 1) all ammonium-N is converted to nitrate-N and
2) half of the nitrate is leached.
Composition
Lime required
(lb CaCO3 / lb N)
Anhydrous ammonia
82-0-0
1.8
Urea
46-0-0
1.8
Ammonium nitrate
34-0-0
1.8
Ammonium sulfate
21-0-0-24
5.4
10-52-0
5.4
18-46-0
3.6
Nitrogen source
Monoammonium
phosphate
Diammonium
phosphate
Harvest of crop biomass removes alkalinity
from agricultural fields
Cation : N ratio
in plant biomass
Lime required to
replace alkalinity
removed in harvest
(lb CaCO3 /100 lb of
N harvested)
Corn grain
0.14
25
Corn stover
0.73
131
Soybean
0.14
25
Oats grain
0.14
25
Oats straw
0.94
169
Alfalfa
1.41
254
Crop
http://www.ianrpubs.unl.edu/epublic/pages/publicationD.jsp?publicationId=111
Scenario
Corn/soybean rotation
200 bushels, 50 bushels
All P supplied as DAP
N applied as DAP and AA
Acidity from N fertilizer
3.6 x 52 lbs of N in DAP required to
supply P removed in harvest
1.8 x 150 lbs of N in AA
Acidity from grain harvest
25 x 180 lbs of N harvested/100
25 x 200 lbs of N harvested/100
~ 190 lbs of lime
~ 270 lbs of lime
~ 45 lbs of lime
~ 50 lbs of lime
Projected lime requirement ~ 0.3 tons/rotation
Alfalfa field with
dead strip where
lime was not
applied
How should
lime rates be
determined?
Lime rates should
be guided by soil
testing
Pocket pH meters can be very useful
but require regular calibration !!!
Sources of variation in soil pH measurements
1. The soil to solution ratio used when measuring pH.
2. The salt content of the diluting solution used to
achieve the desired soil to solution ratio.
3. The carbon dioxide content of the soil and solution.
4. Errors associated with standardization of the
instrument used to measure pH.
Why measure soil pH
Water
pH >solution
Salt pH ?
using
a salt
Brady and Weil, 2002
Soil pH depends on method
used to measure it !!
As a result, the method of measurement
should be reported whenever soil pH
data is discussed.
The amount of lime needed to
bring about a 1 unit change in
pH varies widely between soils
When a soil is limed, Ca+2 from the lime
displaces exchangeable acidity from the
soil colloids. The active acidity that is
generated reacts with the carbonate ions
from the lime, producing water and
carbon dioxide.
H+
soil colloid + CaCO3
H+
Ca+2
soil colloid + H2O + CO2
“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)
Not all limestone is the same !
Pure calcium carbonate has a calcium carbonate
equivalency (CCE) of 100 and is the standard against
which all liming materials are compared. A ton of material
with a CCE of 90 % can neutralize 10% less acid than a ton
of pure calcium carbonate.
Liming materials that are finely ground, have more surface
area in contact with the soil solution than coarser ground
materials and thus will neutralize soil acidity more rapidly.
Fineness of grind is rated according to the percentage of
material that will pass through 8-, 30-, and 60-mesh
screens.
http://www.agr.state.il.us/news/pub/2007LimeBook.pdf
Page from the 2008 IL Lime book
Multiply by these factors
Adjusting for differences in lime particle size distribution
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
pH measurements on the fly
Soil pH and lime
requirement can
vary widely
within fields
Both past management and inherent
soil properties affect soil pH and lime requirement
Why is variable rate lime
more likely to pay than
variable rate N, P or K?
Insufficient lime is applied to neutralize
total acid inputs to IL soils
South eastern IL
has few quarries
and the greatest
lime deficit
http://iah.aces.uiuc.edu/pdf/Agronomy_HB/11chapter.pdf
Barak P, Jobe BO, Krueger AR, Peterson LA, Laird DA 1997. Effects of longterm soil acidification due to nitrogen fertilizer inputs in Wisconsin.
PLANT AND SOIL. 197(1): 61-69
Abstract:
Agroecosystems are domesticated ecosystems intermediate between natural
ecosystems and fabricated ecosystems, and occupy nearly one-third of the
land areas of the earth. Chemical perturbations as a result of human activity
are particularly likely in agroecosystems because of the intensity of that
activity, which include nutrient inputs intended to supplement native nutrient
pools and to support greater biomass production and removal. At a long-term
fertility trial in South-Central Wisconsin, USA, application of ammoniacal N
fertilizer resulted in significant increases in exchangeable acidity accompanied
by decreases in cation exchange capacity (CEC), base saturation, and
exchangeable Ca2+ and Mg2+ . Plant analysis shows that a considerable
portion of the alkalinity generated by assimilation of N (and to a lesser extent
by S) is sequestered in the above-ground plant parts as organic anions and is
not returned to the soil if harvested. Elemental analysis of soil clays
indicates a loss of 16% of the CEC. The reversibility of this change is
doubtful if the changes are due to weathering of soil minerals.
Summary of common soil fertility problems that
rarely occur in soils with pHs between 5.5 and 7
pH << 5.5
pH >> 7.0
Al toxicity to plant roots
Fe deficiency
Mn toxicity to plant roots
Mn deficiency
Ca and Mg deficiency
Zn deficiency
Mo deficiency in legumes
*Osmotic stress from salts
P tied up by Fe and Al
P tied up by Ca and Mg
Slow N transformations
Potato scab