ACID, SALINE, AND SODIC SOILS
Chapter 3
Why study acid, saline, and sodic
soils?
• Acid, saline, and sodic soils have unique chemical and physical properties that
•
•
•
•
•
influence how plants grow.
Since availability of nutrient ions is determined by their chemistry, it is
important to understand how nutrient availability will be influenced by the
special chemical properties of these soils.
What are acid soils?
Acid soils, technically defined, are soils that have a pH less than 7.0, since by
convention pH of 7.0 is neutral, above 7.0 is basic (or alkaline) and below 7.0 is
acidic. From the standpoint of plant growth, soil management is usually not
affected until the pH is less than about 6.2 for legumes and 5.5 for nonlegumes.
Understanding the concept of pH is fundamental to understanding and
managing acid soils. Since pH is defined as the –log H+ activity, a pH change of
one unit (e.g. from pH of 6.0 to pH of 5.0) represents a 10-fold increase in
acidity.
.
What is an acid soil?
• basic = high base saturation
• acid = low base saturation
Ca++ H+
Mg++
K+
H+ H+
K+
Ca++
H+
80% base saturation
H+
Ca++
K+
Mg++
H+
H+
50% base saturation
Why study acid soils?
46% of the Oklahoma tested samples
had a pH of <6.0 (PPI 2005).
7.5
7.7
2007 Wheat Fields:
Canadian
Garfield
Grant
Kay
5.4
5.4
5.4
5.7
7.5
6.6
7.2
7.0
7.1
7.1
6.4
5.8
6.1
5.7
7.0
7.2
5.4
5.3
5.6
5.6
5.7
5.9
6.1
5.8
5.7
5.9
5.7
6.7
6.0
5.7 5.5 5.6
5.8
5.8
5.7
5.6
5.5 5.5
5.8
5.7
5.6
5.7
5.4
5.5
5.7
5.6
6.3 5.9 5.8
5.3
5.5
6.4 6.7
6.3
6.1
5.6 5.2 5.3
7.4
5.8
5.8
6.5
7.2
5.6
6.1
6.8
5.4
6.6 6.0
5.7
5.7
5.9
5.6
5.5
5.5
5.7 6.1 5.7
Median Soil pH Values of OK Counties (all Ag. soils)
Acid Soils
What causes soil acidity?
• Acid soils are a natural phenomenon related to soil
parent material and rainfall conditions under which the
soil developed.
• Soils developed from limestone parent material, for
example will often be neutral or alkaline in their pH
(e.g. pH > 7).
• Granitic parent material, on the other hand, will favor
development of an acid soil
Acid Soils
• Under high rainfall conditions (> 30 inches/year) parent material
that is permeable, such as sandstone, will likely become acidic
because there is sufficient leaching over geological time (tens and
hundreds of thousands of years) to remove even basic materials like
limestone.
• Rainfall, by nature is slightly acidic because water and carbon
dioxide form carbonic acid in the atmosphere (i.e. “acid rain” is
normal). Thus, as basic materials are leached out of the parent
material, H+ may remain to cause the soil to be acidic.
• CO2 + H2O = H+ + HCO3atmosphere
carbonic acid
• Two other factors, that contribute to soil acidity, are the removal of
basic cations and use of N fertilizers associated with intensive crop
production.
Removal of Base Cations
• As base cations decrease and soil pH drops, Al+3 saturation increases
• Nutrient removal from fields (hay or stubble)
Aluminum in solution goes through stages of
hydrolysis and produces acid H+
 Al3+ H20 = +Al(OH)2+ + H+
H
Ca2
2
- Ca
+
2+ + H 0
+Al(OH)
+ = Al(OH) + + H+
 Mg
2
K
2
2
- +
+
H0
+
+
- KAl(OH)2 + H2H0+ = Al(OH)3 + H+

Ca2+
Mg2+
K+
Ca2+
Ca2+
Mg2+
K+
-
Ca2
+
Ca2
+ 2
Mg
++
K
Ca2
+
Ca2
+
3 3 3
3 Al3 AlAl
Al
Al(OH)
Al
x+ + +
+
+
insoluble
Mg2
+
H+
The production of acidity during
nitrification
Nitrification
NH4+
NO3- + 2H+
Nitrification of ammonia or
ammonium forming fertilizers
is a source of acidity in
agricultural soils
“Basic” and “Acidic” Cations
• The term “basic cations” is used to designate cations that, when combined with
hydroxide (OH-) form a compound that would dissolve in water and create an
alkaline solution
• The cations Na+, K+, Ca 2+ , and Mg 2+ are good examples. In contrast, the
hydroxides of Al 3+ and Fe 3+ are so insoluble the ions would not be present in
solution unless the solution were acidified to dissolve them.
• Al 3+ and Fe 3+ , are usually referred to as acidic ions for this reason. Plants
generally absorb nutrient cations in excess of nutrient anions. In this process,
electrical neutrality or ion-charge balance may be maintained by simultaneous
absorption of OH- or the exudation of H+ by the plant root.
• In either case the result is a contribution of acidity to the soil.
• Plant uptake of basic cations in excess of anions in a natural, non-agricultural
environment contribute little to soil acidity because plants die and recycle the
cations in-place.
• Intensive agriculture accelerates the acidification because the bases are generally
removed from the field with harvest and are not recycled.
Root
0-2 inches
Urea
NH3 + H2O = NH4 + OH-
NH4+O2 = NO3+ 2H
NH4
H+
NO3
NO3
Net H+ addition
2-6 inches
OH-
NO3
Net OH- addition
OH-
Assumption:
For each NH4 or NO3 take up by the plant 1 H+ or OH-1 will be exchanged.
This is not the case.
Plant Uptake and Exchange
NO3OHNH4+
H+
Intensive agriculture relies heavily on the use of ammoniacal
sources of N. These fertilizer materials undergo biological oxidation
to NO3- according to the overall general reaction
NH4+ + 2O2  NO3- + 2H+ + H2O
which produces two protons for every mole of N oxidized
• __________________________________________
atomic mass units
charge
(amu)
__________________________________________
proton
1.007594
+
electron
0.000549
neutron
1.008986
none
__________________________________________
m E
Z
1 H
1
4 He
2
E- element
m – mass
z - atomic number (# of protons in the nucleus)
All hydrogen atoms have one proton
__________________________________________
1 H
2 H
3 H
1
1
1
__________________________________________
stable
stable
radioactive
deuterium
tritium
mass = 1
mass=2
mass=3
no neutron
1 neutron
2 neutrons
1 proton
1 proton
1 proton
1 electron
1 electron
1 electron
__________________________________________
12 C
13 C
14 C
6
6
6
__________________________________________
stable
stable
radioactive
mass=12
mass=13
mass=14
6 neutrons
7 neutrons
8 neutrons
6 protons
6 protons
6 protons
6 electrons
6 electrons
6 electrons
__________________________________________
What is the nature of soil acidity and
soil buffer capacity?
Soils behave as a system made up of the salt from a weak acid and strong
base.
Clay and soil organic matter, provide surfaces for adsorption of cations
Clays have a net negative charge resulting from isomorphic substitution of
divalent for trivalent ions (Mg 2+ for Al 3+ ) and trivalent for tetravalent
ions (Al 3+ for Si 4+ ) within the mineral structure.
Soil organic matter contributes to the net negative charge of soils because
of dissociated H+ from exposed carboxyl and phenol groups.
The cation exchange capacity (CEC) of organic matter is pH dependent,
whereas most of the CEC from clays is not.
A small contribution to soil CEC is from unsatisfied charges at broken
edges of clays.
The strength with which cations are adsorbed to cation exchange sites is
directly proportional to the product of the charges involved and inversely
proportional to the square of the distance between charges (Coulomb’s
law). Consequently, the lyotropic series describing the adsorption of
cations on clay particles in soils is generally considered being
Al
3+
 H+ > Ca
2+
 Mg
2+
> K+  NH4+ > Na+.
• The similarity in strength of adsorption for Al+++ and
H+ is because H+, although only 1/3 the charge
strength of Al+++, is much smaller in diameter,
allowing it to get closer to the internal negative
charge of clays than is possible for the larger Al+++.
• The electrostatic adsorption of cations on clay and
organic matter surfaces creates a reservoir of these
ions for the soil solution. The adsorbed ions are in
equilibrium with like ions in the soil solution
(-)
(-)
(-)
clay
particle
K+
Ca2+
H+
==
==
==
K+
Ca2+
H+
soil
solution
Soil pH
• Relative amounts of each ion adsorbed and in solution varies depending
upon their relative concentrations in the soil solution and how strongly
the ion is adsorbed (lyotropic series).
• Amount of H+ in the soil solution is 1/100th the amount adsorbed on
cation exchange sites
• We might expect the amount of Ca 2+ and K+ to be present in the soil
solution at about 1/50th and 1/10th their amount adsorbed on cation
exchange sites
• When soil pH is determined, only the H+ in the soil solution is measured.
• Soil pH referred to as “active” acidity, whereas the H+ adsorbed on
exchange sites is called “potential” or “reserve” acidity.
• The buffer capacity of soils, that is, their ability to resist change in pH
when a small amount of acid or base is added, is a function of their
exchangeable acidic and basic cations.
• Soils with low CEC (e.g. sandy, low organic matter) have weak buffer
capacity, while soils with high CEC (e.g. clayey, high organic matter) have
strong buffer capacity.
Effect of soil acidity on plants
• Plant species vary in their response to acidic soil conditions.
Those which have evolved and are cultivated in humid regions
(e.g., fescue, blueberries, and azalea) tolerate acidic soils better
than other species (e.g., bermudagrass and wheat) grown in arid
and semiarid climates.
• The chemical environment that plants must tolerate, or can
benefit from, may be inferred from the relationship of percent
base saturation and pH
7
Soil
6
Soil
pHpH
5
4
0
20
40
60
80
Percent Base Saturation
100
pH and pOH
• pH = -log [H+]
• pOH = - log [OH-]
• pH + pOH = -log
Kw = 14
Kw = ion-product constant for water
Kw = [H+][OH-] = 1 x 10-14
Ka = acid-dissociation constant
Ka = [HA] + H2O
[H3O+][A-]
(A- conjugate base of the acid)
Kb = base-dissociation constant
Kb = [A-] + H2O
[OH-][A+]
(A+ conjugate acid of the base)
Ka * Kb = Kw
Ksp = solubility-product constant
-degree to which a solid is soluble in water
-equilibrium constant for the equilibrium between an ionic solid and
its saturated solution
Solubility
• Solubility of a substance:
quantity that dissolves to form a saturated solution (g of
solute/L)
• Solubility product:
Equilibrium constant for the equilibrium between an ionic
solid and its saturated solution
AgCl
 Ag+ + Cl-
Ksp = [Ag+][Cl-]
At equilibrium, conc of Ag+ = 1.34 x 10-5
conc of Cl- = 1.34 x 10-5
Ksp = (1.34 x 10-5)(1.34 x 10-5)
= 1.80 x 10-10
• The percentage base saturation identifies the proportion of the CEC that is
occupied by cations like Na+, K+, NH4+, Ca 2+ , and Mg 2+ compared to the
acidic cations of H+ and Al 3+ .
• This relationship is responsible for the fact that deficiencies of Ca, Mg and
K are rare in soils with a pH near or above neutral.
• Aluminum oxides (Al(OH)3, also expressed as (Al2O3  3H2O) are of such low
solubility that Al 3+ usually is not present in the soil solution or on cation
exchange sites until the soil pH is less than about 5.5.
• The “apparent solubility” product constant (Ksp) for Al(OH)3 in soils is
about 10-30. From this, the concentration of Al+++ in the soil solution and its
change with change in pH can be calculated.
Al(OH)3 =  Al3+ + 3 OH-
(Al3+ ) (OH-)3 = 10 -30
Ksp = 10 -30
Aluminum
Al(OH)3 =  Al3+ + 3 OH-
Ksp = 10 -30
(Al3+ ) (OH-)3 = 10 -30
(Al3+ ) = 10 -30 /(OH-)3 .
Solving the above at pH of 5, OH- would be equal to 10-9
(Al3+ ) = 10 -30 /(10 -9 )3
(Al3+ ) = 10 -30 /10 -27
(Al3+ ) = 10 -3 .
The concentration of Al+++ (10-3) is moles/liter.
Since the atomic weight of Al is about 27, a mole/liter would be
27 grams/liter (g/L) and the concentration of 10-3 is equal to
0.027 g/L, or 27 ppm.
27 ppm at a pH of 5
Solubility
Critical to the management and growth of plants in acid soils is the
knowledge that Al+++ in the soil solution increases dramatically
with decrease in pH below about 5.5. When solved for a soil pH of
4.0 (OH- is equal to 10-10), we have
(Al3+ ) = 10 -30 /(10 -10 )3
(Al3+ ) = 10 -30 /10 -30
(Al3+ ) = 1
A concentration of 1.0 mole/L is equal to 27 g/L or 27,000 ppm.
While there may not be a 1000-fold increase in soil solution Al 3+
concentration when pH changes from 5.0 to 4.0, these calculations
should make it clear why Al 3+ concentrations may be significant at
pH 4.5, for example, and immeasurable at 5.5.
Al toxicity
• Soluble Al is toxic to winter wheat at concentrations of about
25 ppm.
• Adverse effect of soil acidity on non-legume plants is usually a
result of Al and Mn toxicity.
• In winter wheat, Al toxicity inhibits or “prunes” the root
system and often causes stunted growth and a purple
discoloration of the lower leaves.
• These symptoms are characteristic of P deficiency, and are
likely a result of the plants reduced ability to extract soil P.
• Al toxicity versus P deficiency?
pH preferences of common crops
• “pH” is not an essential plant nutrient, and plants obtain
their large H requirement from H2O and not H+.
• Thus, it is the chemical environment, for which pH is an
index, that crops are responsive to rather than the pH
itself.
• Non-legumes require a soil pH above 5.5 because more
acidic soils tend to have toxic levels of Mn and Al
present.
• Crops which grow well in soils more acidic than this can
tolerate these metal ions and perhaps are ineffective in
obtaining Fe from less acidic soils.
• Legumes usually grow best at soil pH above 6.0 because
the rhizobium involved in fixing atmospheric N2 seem to
thrive in an environment rich in basic cations.
Soil pH Impacts
• It is more than Aluminum toxicity.
• Nutrient Availability is greatly influenced by pH
• Some herbicides are pH “sensitive”
• Physiological impact.
Soil pH 4.1
Soil pH 5.1
Soil pH 4.0
Soil pH 5.5
Soil pH 4.7
Soil pH 6.7
Nutrient Availability
ALS inhibitors
Group 2
• Imidazolinones
–Pursuit
–Raptor/Beyond
• Sulfonanilides
–PowerFlex
–FirstRate
–Python
• Sulfonylureas
• Maverick
• Osprey
• Classic
• Sulfonylaminocarbonyl-
triazolinones
–Olympus
–Everest
Herbicide concentration
SUs are more persistent
at higher soil pH
Glean (chlorsulfuron)
Soil pH 7.5
Frederickson and Shea, Weed Sci. 34:328-332
Half-life ≈ 10 weeks
Soil ph 5.6
Half-life ≈ 2 weeks
IMIs are more persistent
at lower soil pH
Pursuit (imazethapyr)
Soil ph 4.6
Soil ph 5.6
Soil ph 6.5
Loux and Reese, Weed Tech. 7:452-458
Effect of soil pH on
herbicides
•PSII inhibitors—atrazine,
Sencor
• More persistent at high soil pH
Atrazine is more persistent
at higher soil pH
Weed growth
No weed control
Complete weed control
Hiltbold and Buchanan, Weed Sci. 25:515-520
10% control after
2 months
90% control after 2
months
How is soil acidity neutralized
Most effective way to neutralize soil acidity is by incorporation
of aglime.
(-)
(-)
(-)
acid clay
particle
2+
Ca
H+
H+
+ CaCO3 ===
(-)
(-)
(-)
Ca2+
Ca2+
+ H2O + CO2
neutral clay
particle
Neutralization of acid soil using aglime (CaCO3)
resulting in increasing exchangeable Ca and formation
of water and carbon dioxide.
Lime
• Aglime is effective because it is the salt of a relatively strong
base (calcium hydroxide) and a weak acid (carbonic acid), and
is therefore basic
• Ca(OH)2 + H2CO3 === CaCO3 + H2O
carbonic acid
Lime needed to neutralize soil
acidity
• Exchangeable acidity must be neutralized in order to
change soil pH because it represents most (99 %) of the
soil acidity. Since the amount of exchangeable acidity in
the soil, at a given pH, depends on the soil CEC, the
amount of lime required is a function of clay content,
organic matter content, and soil pH.
•
Lime requirements can be determined directly in a
laboratory by quantitatively adding small amounts of a
solution of known strength base (e.g. 0.1 normal NaOH),
to a known amount of the acid soil mixed with water.
pH and Lime
• By measuring pH as the base is added, the amount of base required
to obtain any pH can be estimated
Buffer index of 6.2
pH scale of 14? Why?
Lime
• Direct determination of lime requirement is very time consuming
and is not usually done in the routine determination of lime
requirement by soil testing laboratories.
• Direct determination identifies the amount of base, such as CaCO3,
that must be applied if all the acidity is able to react with the base
that is added
• In practice, this is virtually impossible because of size differences
between clay and organic matter colloids (very small) and the finely
ground (relatively large) lime particles.
• Field studies (calibration) can be conducted to develop the
relationship between amounts of aglime identified by direct
laboratory titration and crop response.
Lime Requirements
• Most soil testing laboratories use an indirect method of
determining aglime requirement.
• Involves adding a known quantity of a lime-like chemical
solution (i.e., buffer solution of pH 7.2) to an acid soil and
water mixture.
• After equilibrium has been obtained (about two hours)
the pH is measured.
• This pH is often called the “buffer pH” or “buffer index”.
The buffer index, by itself, does not identify how much
lime must be added to neutralize an acid soil.
• Field studies relating lime additions to soil pH are
required to calibrate the buffer index, just as they would
be in a direct titration approach.
Lime Requirements
Table 3.1. Calibration of SMP buffer index and lime requirement for acid soils.
Buffer
Lime Requirement* for pH 6.8
Lime Requirement* for pH 6.4
2
Index
ton/acre
lb/1000 ft.
ton/acre
lb/1000 ft.2
>7.1
0
0
0
0
7.1
0.5
23
0
0
7.0
0.7
32
0
0
6.9
1
46
0
0
6.8
1.2
55
0.7
32
6.7
1.4
64
1.2
55
6.6
1.9
87
1.7
78
6.5
2.5
115
2.2
101
6.4
3.1
142
2.7
124
6.3
3.7
170
3.2
147
6.2
4.2
193
3.7
170
*Lime requirement is in units of effective calcium carbonate equivalent (ECCE) lime.
Buffering Capacity
• Buffer capacity is a function of CEC (e.g. clay and soil organic
matter content).
• Amount of lime required to neutralize acidity in a sandy soil
(e.g. Meno fine sandy loam) and a fine textured soil (e.g. Pond
Creek silt loam) will be quite different even when they have
the same soil pH
Soil
pH7.06.5-
Buffer
Index
7.1-
Pond Creek
silt loam
6.93.7 ton
ECCE lime
per acre
6.05.5-
6.76.5-
5.0-
6.3-
4.5-
6.1-
4.0-
Buffer
Index
Soil
pH7.06.56.0-
Meno fine
sandy loam
1.4 ton
ECCE
lime
per
acre
7.17.06.9-
5.5-
6.8-
5.0-
6.7-
4.5-
6.6-
4.0-
Amount of
potential
acidity that
needs to be
neutralized
How often should lime be applied
The answer to this question will depend on how intensively the soil is
managed and how large is the soil buffer capacity. For example, the
amount of basic cations removed in a 30-bushel wheat crop in grain
and straw is shown to be about the same as that removed by a ton
of good quality alfalfa hay
Table 3.2. The amount of lime in equivalence removal of basic elements
yield of wheat.
Ca
K
Mg
Na
-------------------- Equivalent lbs ECCE lime -------------------Grain
2
10
10
2
Straw
11
45
14
9
Total
13
55
24
11
* A ton of alfalfa hay will remove slightly more than this amount.
in a 30 bushel
Total
24
79
103*
• Soil will become acidic faster, and require liming more
often, if both grain and straw are harvested.
• If two fields are yielding at the same level, it might be
expected that a sandy soil would need to be limed at
lower rates, but more frequently, than a fine textured
soil.
Common liming materials
•
Aglime. Any material that will react with, and neutralize, soil acidity
may be considered for use to “lime” an acid soil. The most common liming
material is “aglime”, a material that is primarily composed of calcium
carbonate, mined from geological deposits at or near the earth’s surface.
• Some deposits are high in magnesium carbonate and are called dolomitic
limestone. Dolomitic limestone is also a good source of Mg for deep, sandy,
acid soils where this nutrient may also be deficient. The mined limestone is
usually crushed and sieved to obtain material of a small enough particle size
to be effective for aglime.
• Quick lime. Mined limestone may be processed to improve its purity and
neutralizing strength. The term “lime” was initially used as a name for CaO,
which may also be called unslaked lime, burned lime, or quick lime. It may
be obtained by heating (burning) calcium carbonate to drive off carbon
dioxide.
CaCO3 + heat ==== CaO + CO2
Often used for stabilizing sewage sludge. When added to the
mixture of sewage solids and water, it quickly reacts to raise the pH
above 11
Liming Materials
• Hydrated lime. Hydrated lime, which may also be called
slaked lime or builders lime, is produced by reacting quick lime
with water.
CaO + H2O ==== Ca(OH)2
Special Formulations
• Liquid lime
• Formulated by mixing finely ground limestone with water and a small amount of clay.
• Clay is added to help keep the lime particles suspended in the water during application.
• Since the solubility of CaCO3 is low, most of the lime is present in solid form and will react
like an application of solid lime. The ECCE of the formulation will be much less (depends
on how much water was added) than that of the lime used in the mixture, even when the
dry lime had a high ECCE.
• Typically the dry lime has an ECCE of nearly 100 % and the liquid lime is about 50 %
because about ½ of it is water.
• Pelleted lime
• Pelleted lime is created by compressing, or otherwise forming pellets out of finely ground,
good quality CaCO3.
• Neutralizing effectiveness of liming materials depends upon being able to maximize their
surface contact with soil colloids.
• The advantage of liquid lime and pelleted lime compared to conventional aglime is to
minimize dust. The disadvantage is they are usually much more expensive, on a cost per
ton of ECCE, than conventional aglime.
Industrial by-products.
•
•
•
•
Kiln dust from cement manufacturing plants,
Fly-ash from coal burning power plants,
Residual lime from metropolitan water treatment plants.
Effectiveness of these materials will depend on particle size and
neutralizing strength of the material.
How are the neutralizing values of
liming materials compared
• Effective Calcium Carbonate Equivalent.
• Effectiveness of the aglime identified as effective calcium carbonate equivalent, or
ECCE.
• Expression of the “active ingredient” of the material for neutralizing soil acidity.
• ECCE of liming materials is expressed as a percentage of the material and takes into
account the particle size and neutralizing strength of the material
• Chemical Equivalence.
• Equivalence of compounds relative to their acid neutralizing strength provides insight
to their differences in neutralizing strength.
• Accomplished by calculating the equivalent weight of a liming material and comparing
it to the equivalent weight of CaCO3.
• Only possible if the materials are pure chemically. This consideration is of interest, for
example, when comparing the effectiveness of dolomitic lime (rich in MgCO3) to that
of normal aglime (primarily CaCO3). The equivalent weight of each material is
calculated, using the definition:
• An equivalent weight is the mass of a substance that will react with one gram of H+,
or one mole (6 x 1023) of charge.
Equivalent weights
• Equivalent weights are the chemists way of converting “apples and oranges”
(etc.), all to apples.
• Atomic (or molecular) weight of an ionic species, divided by its charge is
equal to its equivalent weight.
• For both CaCO3 and MgCO3 the charge of ions involved is two, and one mole
of the carbonate ion will neutralize two grams of H+, or two moles of charge.
• The molecular weight of CaCO3 is 100 and MgCO3 is 84.
• Equivalent weights are ½ their molecular weights, or
• CaCO3: 100/2 = 50
• MgCO3 84/2 = 42
• It only requires 42 g of MgCO3 to accomplish the same neutralizing as 50 g
of CaCO3, the MgCO3 is 50/42 or 1.19 times more effective than CaCO3.
• Applying the same comparison to CaO (eq. wt. 28) and Ca(OH)2 (eq. wt. 37)
it is clear that these materials would be required at much lower rates than
CaCO3 (eq. wt. 50)
Important considerations to improve success of
liming
•
•
•
•
•
•
•
•
•
•
Soil Testing.
Reliable soil test (representative)
soil pH may be variable in the area (year to year?) (within year?)
Amount of Lime. The buffer index from a soil test serves as a good guide for determining
how much lime should be added,
When non-legumes are grown successively in the same field, it is only necessary to apply
enough lime to eliminate current and future Al and Mn toxicities.
Lime recommendations for continuous wheat production in Oklahoma are to apply only
3/4 the amount required to raise the pH to 6.8.
This recommendation will raise the pH above 5.5 and keep it below 6.5 to minimize the
incidence of root-rot diseases.
Occasionally the buffer index for sandy, low organic matter soils will be so high that no lime
is recommended.
In these cases a minimum of 0.5 ton ECCE/acre for non-legumes and 1.0 ton ECCE for
legumes is recommended to assure the acidity will be corrected and the application is
economical.
When lime recommendations are extremely large the amount should be split into an initial
application of 5 ton/acre (230lb/1000 ft2) followed by the remainder applied a year later.
Considerations
• Incorporation and Timing.
• Lime must be physically mixed with the soil.
• Pastures, perennial plantings, or no-till productions, may require three to five years
before the lime causes a noticeable change in soil pH.
• Important to lime fields before they are planted to a perennial crop or managed as
no-till.
• Systems where alfalfa is rotated with a non-legume annuals like corn or wheat, the
field should be limed a year before the alfalfa is planted to take advantage of tillage
operations related to corn or wheat production and allow more time for lime to react
in the soil.
• When lime is incorporated well, and there is good soil moisture, it may still take a year
or more before noticeable change in soil pH occurs.
• Tillage Depth. Lime recommendations are usually made assuming a six-inch tillage
depth.
• Sandy soils are some times cultivated to eight or ten inches therefore a proportional
increase in the lime rate should be made.
• For crops with a shallow root system, such as some vegetables, it may be important to
reduce the lime rate to match a shallower depth of incorporation.
How Deep will lime impact soil pH in notill?
Depth Loc1 Loc1 Loc2
0
1Ton 0
ecce
0-1 5.8 6.7* 5.5
1-2 5.5 6.1* 5.1
2-3 5.7 6.0* 5.1
3-4 5.8 6.0 5.0
4-5 5.9 5.9 5.0
5-6 5.9 6.0 5.1
6-9 6.1 6.1 5.4
Data Present by Dr. Dave Mengel. KSU
Loc2
1Ton
ecce
6.5*
5.7*
5.2*
5.0
5.0
5.1
5.4
Loc3 Loc3
0
1Ton
ecce
5.6 6.2*
5.3 5.7*
5.4 5.6*
5.4 5.5
5.5 5.6
5.6 5.9
6.0 6.1
Production Induced Soil
Acidity
pH
depth (inches)
4
0
3
6
9
12
15
18
21
24
5
6
No-till
Normal tillage
7
How can acid soils be managed
without liming
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Liming Alternative.
Acid tolerant varieties or different plant species.
Karl and Custer are not acid tolerant whereas the variety 2163 is acid tolerant.
Rye more acid tolerant than wheat.
The Al and Mn toxicity that prevent normal seedling root development in wheat can
be alleviated by adding phosphate fertilizer in a band with the seed at planting.
Phosphate reacts strongly with Al to form insoluble aluminum phosphate, thus
removing Al+++ from solution and the exchange complex.
Rate of 60 lb P2O5/acre is required to obtain normal fall pasture but only 30 P2O5/acre
is needed if wheat is managed for grain only.
If P is not deficient, the cost of applying the P for two or three years will usually equal
the cost of an application of lime that would have lasted five to eight years.
These alternatives allow normal or near normal production but do not cause a change
in soil pH.
Eventually the soil must be limed for long-term production.
What are saline soils
• Classified as saline when they contain a high enough concentration of soluble salts to
interfere with normal growth and development of salt-sensitive plants.
• Soluble salts are compounds, like common table salt (NaCl), where ions that make up
the salt are weakly bound and have a strong attraction for water.
• These ions hold water quite tightly, salty water
• (higher boiling point)
• (lower freezing point)
• Salt is added to water used in food preparation to raise the boiling point and hasten
the process.
• Salt spread on icy sidewalks and roads to melt ice that would otherwise remain solid
at temperatures below freezing.
• Soluble salts in soils: soil water is held tightly enough by the ions that plants cannot
use it (apparent moisture stress)
• Saline soils characteristically remain moist longer than the rest of the field
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Occupy poorly drained areas of the landscape
White surface layer of salt after they become dry.
Occur in semi-arid, temperate regions
Saline soils are uncommon in the moisture extremes of deserts and tropical rain forests.
Saline Soils
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Saturating a soil sample with water (a paste condition) for about four hours,
Extracting the water (and dissolved salts)
Measuring its ability to conduct electricity.
Ions in water allow electricity to pass through it
More ions present the easier electricity is conducted
Conductivity is expressed in mhos/cm.
Conductivity of water is usually very low and expressed as mmhos/cm or micromhos/cm.
Soils are classified as saline when the extract of a saturated paste has an electrical
conductivity (EC) equal to or in excess of 4,000 micromhos/cm.
• Concentration of soluble salts, expressed as ppm, is roughly equal to 0.65 times the
conductivity expressed in micromhos/cm.
• Soil with an EC of 4,000 micromhos/cm will contain about 2600 ppm soluble salts in the
saturated soil solution.
• Saline soils Reclamation
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leaching soluble salts out of the soil.
create good surface and internal drainage.
incorporating large amounts of organic matter (create large pores in the surface soil)
Good quality irrigation water can be used to hasten the process.
Deep tillage should be avoided once the organic matter is incorporated
Salt tolerant species like bermudagrass or barley should be planted to provide a vegetative cover
** Practices to reduce surface evaporation and encourage water movement downward ???
What is a Sodic Soil
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Abnormally high levels of exchangeable sodium (Na+).
When enough Na+ is adsorbed, clay particles repel each other.
Occurs when the exchangeable Na+ percentage (ESP) is equal to or exceeds 15
Soil pH of sodic soils will often be above 8.
Dispersed colloids become oriented as water moves into soil and eventually they plug
soil pores.
Poor internal drainage resulting in dry subsoil and a moist or wet surface layer. Crops
fail because of excess surface water (“drown out”) or for lack of water (dry subsoil)
even though there may have been adequate rainfall or irrigation.
Reclaimed by improving surface and internal drainage and incorporating gypsum
(CaSO4) in the surface.
Gypsum dissolves to supply a high concentration of Ca++ in soil solution that replaces
exchangeable Na+, freeing it to be washed out of the soil
Ca++ helps bind colloids into aggregates and restore soil permeability. Reclamation of
sodic soils is similar to that of saline soils except that gypsum must be added to sodic
soils.
What are Saline-Sodic Soils
Contain salts in excess of 4,000 micromhos/cm and exchangeable Na+
in excess of 15 %
Have all the features of the saline soil, and if reclamation procedures
are used that do not include gypsum, they will become sodic soils
when the salts are leached out.
Many salt affected soils are saline-sodic because a primary soluble ion
is Na+.
Reclamation takes several (2 or more) years, dependent upon the time
required to get about two pore volumes of good quality water to
pass through the soil.
Most soils are about 50 % pore space and so a “pore volume-depth”
for a four foot profile would be about two feet and two pore
volumes about four feet.
Sandy soils in high rainfall regions may be reclaimed quite rapidly while
clayey soils in semi-arid regions may take many years if rainfall is the
only source of leaching water.
How Soluble is the Earth’s
Crust
• The extent to which the earth’s crust dissolves over time
depends upon solubility of rocks and mineral, abundance
of elements in the rocks and minerals, and rainfall.
• Naturally occurring compounds containing either Na or Cl
tend to be very soluble and, with time, end up in the
oceans and seas of the world.
Soluble salt content of sea water*.
Element
Cl
Na
Mg
S
Ca
K
Br
Total
Concentration (ppm)
18980
10561
1272
884
400
380
65
32,542