Soil acidity

Understanding

Natural pH impacts optimal pH
Soil Acidity

In humid regions, most soils
are naturally acidic but the
following factors contribute to Neutral
greater acidity:

- parent material w/ low ANC
- forest vegetation
- ↑duration and intensity of
chemical weathering

ANC = acid neutralizing capacity

Brady and Weil (2002)

Do plant roots really care about the H+ concentration crops
Optimum pH ranges have been proposed for many in soil?

Collective term for the challenges
faced by crops growing in acid soils

The acid infertility complex

Nutrient
availability
varies with pH

For most soils, nutrient
availability is optimized
between pH 5.5 and 7.

Molybdenum becomes more available as pH goes up !

most
^

http://www.farmtested.com/research_pp.html

Understanding aluminum toxicity
Fe and Mn toxicities also
occur at lower pHs

Toxic forms
of Al are
bioavailable
at lower pHs
Aluminum toxicity
is minimal above
a water pH of 5.5

http://www2.ctahr.hawaii.edu/tpss/research_extension/rxsoil/alroot.gif

Many biological processes
are sensitive to aluminum toxicity

Crop varieties differ in their sensitivity to Al toxicity

Brady and Weil, 2002

Multiple forms of soil acidity

H+
H+
H+
H+
Soil pH is
primarily a
Al+3
measure of
active acidity
H+
H+

Active
Reserve acidity acidity
Sometimes called residual acidity

Understanding pH
pH = -log(H+)


Understanding pH
pH = -log(10-x)
X -log(H+)

neutral


So what is the H+ ion concentration when the pH = 6?

Understanding pH
pH = -log(10-x)
X -log(H+)

neutral


So what is the pH if the H+ ion concentration is 10 x higher?

Understanding reserve acidity
Very little lime is needed to neutralize the active acidity in soils

Both soils
initially have
the same pH
(i.e., the same
amount of
activity
acidity)

Reserve Active Reserve Active
acidity acidity acidity acidity

High CEC soil Low CEC soil

If only enough lime is added to deplete the active acidity,
reserve acidity will quickly begin resupplying the active acidity



More lime is needed to bring about persistent pH change
in soil with more reserve acidity

Effect of ΔpH
adding the
same rate ΔpH
of lime to
soils with
different
amounts of
reserve
acidity


Each charge depicted on this diagram represents 1
centimol of charge per kg of soil
K+ Sum of non-acid cations

--
_____________________

--
%BS =
Ca+2 * 100
Sum of all cations

-
Mg+2
Humus - H+
H20
H20
Exchangeable
exchangeable soil H20
acidity
cations solution
H20
H20
-
H20

- Al
Clay

-- K
+3 + H2O ↔ Al(OH)3 + 3H+
+
SO4 -2 + -- +
What is the “base”
Ca+2
saturation of this soil ?

Is pH related to base saturation ?

100 80 60 40 20 0
Acid Saturation, %

Isis probably more accuratesaturation ?
It pH related to base to say that active
acidity is related to acid saturation

100 80 60 40 20 0
Acid Saturation, %

pH dependent charge

In contrast, the
charge on humus is
higher at higher pHs

The dominant clay minerals in
IL have mostly permanent
charge created by isomorphic
substitution

The charge on humic
substances (and low
activity clays) is very pH
dependent
H+

H+ H+

H+

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).
Nitrification NH4+ + 2O2 → NO3- + 2H+ + H2O

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.

K+
H+

- The pH of a plant’s
NO3 rhizosphere changes
as the plant regulates
OH- its internal charge
balance.

Which plant received nitrate (NO3-)?
Which plant received ammonium (NH4+)?

http://departments.agri.huji.ac.il/plantscience/topics_irrigation/uzifert/4thmeet.htm

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

Elements that
Oxides of have traditionally
Alkalinity Ca, Mg and K been called
“bases”

C, N and S oxides cause acid precipitation


Forest damaged by acid rain

Looks great but
may be devoid
of life if acid rain
has created Al
toxicity

Forest damaged by acid rain

Looks great but
may be devoid
of life Monument getting
if acid rain
has created Al by acid rain
dissolved
toxicity

Sliding down the acidity slope

The effect of
added acidity on
Carbonates soil pH
depends on the
soil’s buffer
capacity

Chadwick and Chorover ( 2001)

Acid inputs promote leaching of non-acid cations
Nitric acid = HNO3 → NO3- + H+

Why does
leaching of
these anions
cause soil
acidification ?


released into
the soil 1H+
consumed
Nitrification is an acidifying process, right??
1H+
NH3 consumed

Complete N cycle (no net acidification)

released into
the soil 1H+
consumed

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

Very important in places where lime is expensive!

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.
Lime required
Nitrogen source Composition
(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
Monoammonium
10-52-0 5.4
phosphate
Diammonium
18-46-0 3.6
phosphate

Harvest of crop biomass removes alkalinity
from agricultural fields
Lime required to
replace alkalinity
Cation : N ratio
Crop removed in harvest
in plant biomass
(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

http://www.ianrpubs.unl.edu/epublic/pages/publicationD.jsp?publicationId=111

Scenario
Corn/soybean rotation
200 bu corn, 50 bu soybeans
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 ~ 190 lbs of lime
supply P removed in harvest
1.8 x 150 lbs of N in AA ~ 270 lbs of lime

Acidity from grain harvest
25 x 180 lbs of N harvested/100 ~ 45 lbs of lime

25 x 200 lbs of N harvested/100 ~ 50 lbs of lime

Projected lime requirement ~ 0.3 tons/rotation

In many parts of the world, notably the US Midwest and Europe, soils
are often limed to a near neutral pH 6.5–7.0. Because plants do not
directly respond to H+ concentration, it is pertinent to inquire why this
approach to liming has enjoyed such widespread popularity.

The original near-neutral pH of many of the soils was no doubt a
consideration as was the use of acid-sensitive forage legumes to
supply N in rotations during the era when the original lime
experiments were conducted.

The introduction of the pH meter at about the same time as N
fertilizers found widespread popularity (replacing forage legumes in
rotations) facilitated measurement of soil acidity and removed the
focus from the real problems of soil acidity, namely, toxic levels of Al
and Mn and deficiencies of nutrients such as Ca, Mg, N, S, P and Mo.

Even after forage legumes disappeared from most rotations, high
target pH values were retained.

Liming experiments throughout the world reveal that, with very few
exceptions, all grain crops including legumes cease to respond to lime
above pH 5:5–5:8; provided that the nutrients (Ca, Mg, Mo, B, P, etc)
negatively impacted by soil acidity are optimized.

On highly weathered soils (e.g., NC and Brazil), liming to near
neutrality can have disastrous effects on yields of many crops.

Many examples are presented in the article of the few benefits of
liming to neutrality and the many benefits of farming with levels of
acidity somewhat more intense than has normally been the case.

Among the latter benefits are increased profitability from higher
nutrient efficiencies, reduced diseases and pests, slower
nitrification with less water pollution, improved soil tilth,
improved availability of many metals and P.

Do you remember this graph?
Impact of pH and an inhibitor on % nitrification
% Nitrification

w/ N serve

Soil pH
http://soil.scijournals.org/cgi/content/full/68/2/545/FIG4

According to Sumner:

Ever since pH became an easily measured soil parameter
(invention of the pH meter), we have focused on an indicator of
soil acidity (pH) rather than on the actual plant limiting factors
associated with acidity (toxicities, deficiencies and
imbalances).

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.

Water pH > Salt pH

Salt solutions
are normally
used when
measuring the
pH of soils in
arid regions
(i.e. places
where salinity
is often an
issue)


Soil pH depends on the 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+ Ca+2
soil colloid + CaCO3 soil colloid + H2O + CO2
H+

“Illinois method” of determining lime requirement

How do
you know
which line
to use ?
The lines represent
different levels of
reserve acidity

Steeper line = more reserve acidity

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)

“Buffer pH” is a measure of reserve acidity

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).

B. Typical fineness of limestone. Ten percent of the particles are
greater than 8-mesh; 30 percent pass an 8-mesh and are held on 30-
mesh; 30 percent pass a 30-mesh and are held on 60-mesh; and 30
percent pass a 60-mesh.

C. A calcium carbonate equivalent (total neutralizing power) of 90
percent. The rate of application may be adjusted according to the
deviation from 90.

Lime rates should be adjusted if any of these
assumptions are not accurate

It takes time for lime to react in soil

pH measurements on the fly

Soil pH often
varies widely
within fields

Don’t forget that some measure of OM, CEC or clay
content is also needed to make a variable rate lime map.

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?

Both past management and inherent
soil properties affect soil pH and lime requirement

Over-liming and under-
Why is variable rate lime
liming likely tonegative
more have pay than
variable rate N, P or K?
effects on yield

Insufficient lime is applied to neutralize
total acid inputs to IL soils

South eastern IL
has fewer 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 long-
term 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

Soil acidity

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