Soil Acidity and Liming
Aqueous systems (like the soil) exhibit the property of being acid or basic
depending on the relative amounts of H+ and OH- ions
in areas with high rainfall are usually acid. Basic cations are leached
more readily than Al. the presence of exchangable
= acid peat soils
= pH range common for humid regions mineral soils
= common pH range for arid regions soils
and 11 = alkali mineral
B. Active and Potential Acidity
1. an acid ionizes into hydrogen ions and the accompanying
(Potential Acidity) =>H+ +A- (Active Acidity)
Acidity = Active + Potential Acidity
soils, active acidity is H+ in soil solution. Potential acidity
is exchangeable Al+3. Most of a soils acidity is potential.
+ 3H2O ==> Al(OH)3 + 3H+
active and potential acidity must be measured to determine how much lime is
C. Measuring Acidity
1. pH measures active acidity or the H+
concentration of the soil solution.
pH = log
1/[H+] where H+ is the concentration in moles per
unit change in pH means a 10-fold change in H+ concentration.
D. Causes of Soil Acidity
Material - Rocks from which soil was formed may have been basic or acidic
Rainfall - The higher the average annual rainfall the more leaching. Basic
cations are removed more readily than H+ and Al+3.
Native vegetation - Soils under forest are more acid than those developed
of O.M. forms acid
Fertilizer containing NH4+
of NH4+ => NO3- produces H+
Hydrolysis of Al
Al + H2O
===> AlOH3 + H+
come from clay structures
E. Reasons to Add Lime
neutralize toxic elements
Reduces root growth by inhibiting cell
Reduces Ca uptake
b. Mn 2+ -- toxicity is a problem on red,
clayey acid soils
at pH 4 or less H+ can damage the root membrane
Increases molybdenum availability. Mo is the only
micronutrient that is more available at higher pH's.
Supply Ca and Mg - ( two of the secondary
Increases microorganism activity for N fixation amd
efficiency of P fertilization. P is fixed and not available to plants at
Improves soil physical properties. (structures)
F. Determining lime requirements of a soil
Concept of buffer capacity
a. buffering - a
resistance to change in pH. Removal of H+
ions from the soil solution results in their replacement by H+
ions (Al+3) from the exchange complex.
higher CEC of a soil the greater will be its buffer capacity because more
reserve (potential) amount must be neutralized to change the pH. The percent OM must be taken into account as well
as the pH when estimating the amount of lime needed to raise the pH.
soils have a high buffer capacity.
soils have a high buffer capacity.
soils have a low buffer capacity.
of the effect of CEC on lime requirements and buffer capacity
to change from pH 5 to 6. We look at the curve and see that this is a
change from 25 to 75% base saturation or a 50% change.
meq of H+ must be neutralized if the
CEC of the soil is 2?
testing labs use an indirect method of measuring exchangable
cm3 soil + 10 ml water + 10 ml buffer at pH 6.6
been determined that each .1 decrease in pH of the solution equals 0.4 meq ac/100 cm3 of soil
rapid - large numbers
of soil samples can be processed.
Calculating lime rates (NCDA soil testing):
Lime rates for field crops are expressed in tons/acre (T) or lb/1000 ft² (M) for small areas such as lawns and
gardens. Rate calculation involves soil pH, exchangeable acidity (Ac),
target pH, and residual lime credit (RC).
The formula is:
tons lime/acre = Ac × [(target pH -
current pH) ÷ (6.6 - current pH)] - RC
To convert tons/acre to lb/1000 ft², multiply
tons/acre by 46. Example: 0.8 tons/acre × 46 = 36.8 lb/1000
RC is the amount of lime applied in the
last 12 months that has not been used in the neutralization of soil
acidity. Residual credit for lime applied varies with soil type over time.
RC is reduced by 8% per month for MIN soils and 16% per month for M-O and
ORG soils. The RC is reduced at a greater rate for the latter two soils
because they contain higher levels of acidity that increase the reaction
rate of lime. The equation for determining RC is as follows:
RC = months × rate × reduction percentage
· months is the number of months between lime application and the current
· rate is tons of lime applied per acre,
· reduction percentage is 0.08 for MIN soils and 0.16 for M-O and ORG
G. How lime neutralizes acidity.
H+ + CaCO3 ==> Ca2+ + H2O +
+ H2O ==> Ca2+ + HCO3- +OH -
OH-can react with H+ ==> HOH (water) or to precipitate Al as
reduces the concentration of H+ ions and increases the
concentration of OH - ions, and adds non acid
forming cations. the material must contain an
anion that combines with and neutralizes H+ ions and Al ions.
H. Factors influencing the quality of liming materials
1. purity - any impurities in the lime will reduce its
ability to neutralize acidity. ( sand, rocks,
clay, etc. )
2. fineness - large particles react more slowly and less
completely than fine particles.
Efficiency Rate (%)
C. regulation for lime particle size
100 percent must pass through 10 mesh screen; 40 percent must pass
through 100 mesh screen; and there shall be an investigational allowance of
must pass through 20 mesh
25% through 100 mesh
must pass through 20 mesh
35% through 100 mesh
3. neutralizing value - the ability to neutralize acids. expressed in terms of calcium carbonate equivalent.
Calcium carbonate is the standard by which other materials are measured ( 100%)
weight of CaCO3 is 100
molecule of each will neutralize the same amount of acid but on a weight
basis it only takes 84g of MgCO3 to dothe
job of 100g of CaCO3.
value (CCE) calcium carbonate equivalent of the pure forms of some commonly
used liming materials
I. Liming materials
considered a liming material an anion must produce OH - ions to
react with H+ and Al3+ ions. Oxides, hydroxides,
carbonates, and silicates
Calcium oxide (CaO)
names - burned lime, quicklime, unslaked lime
===> CaO + CO2
is immediate reaction with the soil.
- caustic, difficult to handle and apply
may occur. Through mixing is necessary
Calcium hydroxide (Ca(OH)2)
names -- slaked, hydrated, builders lime
CaO + H20
- quick reaction with the soil
- difficult and unpleasant to handle
3. Calcitic limestone (CaCO3)
from deposits. Quality depends on amount of impurities such as clay. Good
handling properties. Reaction time several months.
deposits of CaCO3. Usually contaminated with clay. Low in Mg.
Slags (CaSiO3) by-product of furnaces used for
Basic slag is
limestone or dolomite that has absorbed phosphate from the iron ore during
the steelmaking process.
Because of the slow release phosphate content, as well as for its liming
effect it is valued as fertilizer in
steelmaking areas. Basic slag containing less than 12% P2O5 must be labeled
National Slag Association
Fly ash or wood ash
J. Placement of lime
mixing of lime throughout zone of root growth is ideal.
Particles of lime do not move in the soil.
Application - Spread half of lime and plow down. Spread other half and
established sods lime must be topdressed.
Reaction is slower and less complete. * Add smaller amounts more often.
K. factors Determining the selection of a liming program.
requirements of crop to be grown. Plants differ in pH requirements.
blueberries, cranberies, azaleas and camellias
neutral alkaline - Sweet
clover, alfalfa, sugar beets.
crops in N.C. 5.8 - 6.2 is best.
6.5 except centipede 5.5
Texture and O.M. content affects amount of lime required to change pH and
frequency of application. Overliming coarse
textured soils is a possibility.
and frequency of liming - use soil test. Depends on texture and organic
matter content. Nitrogen fertilization and crop removal. Take soil samples
at least every 3-5 years.
Liming material to be used.
amount of material to
apply assuming 100% purity and proper size particles.
distribution pattern, no dust
finely divided - reacts quickly with soil
500-1000 pounds per acre applied at one time. Reg. annual applications.
4. 2 to
4 times more expensive
soil ph in turf
questions about liming
western regions of the country
- overlimed soils
- further acidification of soils for growth of plants such
as potatoes, azaleas, rhododendrons or camellias.
elemental sulfur, sulfuric
acid, aluminum sulfate, iron sulfate and ammonium sulfate.
Elemental sulfur - pound for pound is most effective. Converted to sulfuric
acid in warm moist soils by bacteria.
Sulfuric acid H2SO4
Aluminum sulfate - commonly used by Horticulturists for acidulating soil
for azaleas, camellias, rhododendrons, etc.
sulfate ( FeSO4) - reacts similar to
Alternative Liming Materials