Soil
Acidity
What
is Soil Acidity?
Active
and Potential Acidity
Lime Requirement
Factors
in Acid Soils Detrimental to Plant Growth
Liming
Materials
Factors
Affecting Lime Reaction
What
is Soil Acidity?
An acid
is a substance that gives off hydrogen ions ( H+ ).
A base is a substance that gives off hydroxyl ions. ( OH-
).
pH is
an expression of the concentration of hydrogen ions in solution
pH =
- log ( H+ activity)
pH scale
ranges from 1 - 14, pH = 7 is neutral, > 7 is basic, < 7 is
acidic
Soils
are acid when a considerable portion of the cation exchange capacity
is filled with H+ and A13+, instead of the basic
cations, Ca2+, Mg2+, K+, and Na+.
H+ and Al3+ replace the basic cations from the
exchange complex and they can be leached deep into the soil profile
or groundwater.
In soils,
H+ arises from several factors:
1. Carbon
dioxide from decomposing organic matter and root respiration.
C02
+ HOH ——> H2CO3 ——>
HC03- + H+
2. Ammonium
and organic N are oxidized to form nitrate and H+.
NH4+
+ 202 ——> N03- +
H20 + 2H+
3. Sulfur
oxidation.
2S
+ 302 + 2HOH ——> 2S042-
+ 4H+
4. Root
release of H+ to maintain cation anion balance within the
plant.
5. Acid
rain -- sulfuric acid and nitric acid. Generally, minimal effects
except near point source discharges.
6. Crop
removal of basic cations.
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Active
and Potential Acidity
Active:
Hydrogen ion in solution
Potential:
Aluminum (primarily) and H+ on the exchange complex. When
the soil is limed Al3+ is displaced from the exchange sites
by Ca2+. The Al3+ hydrolyzes (see equations
below) and generates more H+. Lime must be added to the
soil to neutralize the H+ generated from Al3+
hydrolysis, as well as that present in the soil solution prior to
liming.
Al3+
+ H2O ----> Al(OH)2+ + H+
Al(OH)2+ + H2O ----> Al(OH)2+
+ H+
Al(OH)2+ + H2O ----> Al(OH)3
+ H+
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Liming
Requirement
Soil
pH indicates if lime is needed, not how much is needed.
Lime
requirement is dependent on initial soil pH, desired soil pH, and
a measure of the potential acidity.
Potential
acidity is often referred to as the soil's buffer capacity or resistance
to change in pH. Usually, the higher the clay and organic matter content
the greater the soil's buffer capacity. More lime is needed to change
the pH of a soil with a high buffer capacity than a soil with a low
buffer capacity. More lime is needed to change the pH of a clayey
soil than a sandy soil.
The buffer
capacity of a soil is determined in the laboratory by mixing soil
with a buffer solution. The buffer solution alone has a high pH. The
decrease in pH of the buffer/soil mix is an indication of the buffer
capacity of the soil. If the buffer capacity of the soil is low, the
pH of the buffer/soil mix will be close to that of the original pH
of the buffer. If the buffer capacity of the soil is high, then the
pH of the buffer/soil mix will be much lower than the initial pH of
the buffer. For example:
Initial
pH of buffer = 8.00
|
Soil
texture
|
initial
soil pH
|
target
pH
|
pH
of soil/buffer mix
|
lime
rec.
|
|
Sand
|
5.5
|
6.5
|
7.80
|
700
lb/a
|
|
Sandy
loam
|
5.5
|
6.5
|
7.60
|
1400 lb/a
|
|
Clay
loam
|
5.5
|
6.5
|
7.20
|
2800
lb/a
|
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Factors
in Acid Soils Detrimental to Plant Growth
Acid Soil
Complex -- plant growth problems at low pH are often multi-faceted.
1. ALUMINUM
toxicity.
Affects
plant roots Sparsely branched large diameter roots at low pH.
Tissue
levels of Al relate poorly to toxicity.
2. Manganese
toxicity.
Affects
plant tops. Occurs below pH 5.5. Cotton, soybean, and tobacco are
sensitive.
3. Iron
toxicity minor problem
4. Calcium
deficiency. Peanut, tobacco, and tomato are sensitive.
5. Magnesium
deficiency.
Conditions
that are conducive to Mg deficiency are:
1.
low CEC
2. use of low magnesium liming material
3. high rate of NH4+ fertilizer
4. high rate of K fertilizer
5. excessive leaching of soil profile
6. crops with high requirement for Mg -- tobacco, cotton, and soybean.
6. Molybdenum
deficiency (soybeans and cauliflower).
In most
cases liming to pH 5.5 to 6.0 alleviates Mo deficiency. In
some cases liming > 6.0 needed to eliminate Mo deficiency.
7. Nitrogen,
phosphorous, and sulfur deficiency because of slow organic matter decomposition
minor problem.
8. Reduced
phosphorous availability due to precipitation with aluminum.
9. Poor
nodulation of legumes.
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|
|
| |
Chemical
Formula
|
Rel.
Neutr. Value
|
Origin
|
| Calcitic
limestone |
CaC03
|
90
- 100
|
Natural
deposits
|
| Dolomitic
limestone |
CaC03:MgCO3
|
95
- 108
|
Natural
deposits
|
| Hydrated
lime |
Ca(OH)2
|
120
- 135
|
Steam
burned
|
| Burned
lime |
Ca0
|
150
- 175
|
Kiln
burned
|
| Calcium
silicate |
CaSi03
|
30
- 60
|
Slag
|
| Wood
ashes |
variable
|
40
- 85
|
Steam
boilers
|
Factors
Affecting Lime Reaction
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Relative
Neutralizing Value
Relative
ability of a liming material to neutralize acidity compared with pure
CaC03
Chemical
measure which:
reflects
chemical composition oxide, hydroxide, carbonates (handout)
level of impurities
Does
not necessarily relate to rate and extent of reaction in soil.
Fineness
The smaller
the particle the faster the reaction. (overhead)
Effective
calcium carbonate or neutralizing index. An attempt to integrate calcium
carbonate equivalence and fineness so that various liming materials
can be compared. (p. 254, note 8-2).
Lime
law (handout/overhead)
Chemical
Composition
Dolomite
particles are less reactive than calcitic.
"When
lime is required and soil test for magnesium is medium plus or above,
any source of lime may be used. When lime is required and soil test
magnesium is medium or less, use dolomitic limestone. The use of dolomitic
limestone every time the soil is limed will not result in magnesium
toxicity or cause a ration imbalance situation in the soils in South
Carolina." (Circular 476, p. 13)
Incorporation and Mixing
Expose
as much lime surface area to as much soil surface area as possible.
Time
and Frequency of Liming
At anytime
in the cropping sequence that soil pH is below the acceptable range.
Frequency
of liming is dependent on several factors.
1.
Neutralization by acid forming nutrient sources; particularly those
containing organic N, ammonium N, and ammonia N.
2.
Organic matter decomposition and carbon dioxide release.
3.
Leaching of basic cations.
4.
Removal of basic cations by crop harvesting.
5.
Erosion of topsoil, releaving acid subsoil behind.
6.
Acid rain.
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Reference
Materials
Soils:
An Introduction to Soils and Plant Growth. 6th Edition. Donahue, Miller,
and Shickluna.
Soil
Fertility and Fertilizers, 3rd Edition. Tisdale and Nelson. 1975.
Macmillan Publishing Co., Inc. New York, NY.
Soil
Acidity and Liming, 2nd Edition. F. Adams. 1984. ASA, Madison, WI.
