Topic 6


Soil Acidity and Liming

Soil Acidity and Liming (NCSU Extension)

Lime terminology

A. Introduction

1. Aqueous systems (like the soil) exhibit the property of being acid or basic depending on the relative amounts of H+ and OH- ions present.

2. Soils in areas with high rainfall are usually acid. Basic cations are leached more readily than Al. the presence of exchangable Al+3

3 = acid peat soils

4, 5, 6 = pH range common for humid regions mineral soils

7, 8, 9 = common pH range for arid regions soils

10 and 11 = alkali mineral

B. Active and Potential Acidity

1. an acid ionizes into hydrogen ions and the accompanying anion.

HA (Potential Acidity) =>H+ +A- (Active Acidity)

Total Acidity = Active + Potential Acidity

2. In soils, active acidity is H+ in soil solution. Potential acidity is exchangeable Al+3. Most of a soils acidity is potential.

Al+3 + 3H2O ==> Al(OH)3 + 3H+

Both active and potential acidity must be measured to determine how much lime is needed.

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 liter













A one unit change in pH means a 10-fold change in H+ concentration.

D. Causes of Soil Acidity

1. Parent Material - Rocks from which soil was formed may have been basic or acidic

2. Rainfall - The higher the average annual rainfall the more leaching. Basic cations are removed more readily than H+ and Al+3.

3. Native vegetation - Soils under forest are more acid than those developed under grassland.

Decomposition of O.M. forms acid

CO2 forms H2CO3

4. Fertilizer containing NH4+

Conversion of NH4+ => NO3- produces H+ ions

5. Hydrolysis of Al

Al + H2O ===> AlOH3 + H+

Al can come from clay structures

E. Reasons to Add Lime

1. to neutralize toxic elements

a. Al+3

1. Reduces root growth by inhibiting cell

2. Reduces Ca uptake

3. Fixes soil Phosphorus

b. Mn 2+ -- toxicity is a problem on red, clayey acid soils

c. H+ at pH 4 or less H+ can damage the root membrane

2. Increases molybdenum availability. Mo is the only micronutrient that is more available at higher pH's.

3. Supply Ca and Mg - ( two of the secondary nutrients).

4. Increases microorganism activity for N fixation amd nitrification

5. Increases efficiency of P fertilization. P is fixed and not available to plants at low pH's.

6. Improves soil physical properties. (structures)

F. Determining lime requirements of a soil

1. 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.

b. The 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.

Clay soils have a high buffer capacity.

Organic soils have a high buffer capacity.

Sandy soils have a low buffer capacity.

Example of the effect of CEC on lime requirements and buffer capacity

We want 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.

How many meq of H+ must be neutralized if the CEC of the soil is 2?

2. Soil testing labs use an indirect method of measuring exchangable acidity.

N.C. method

add 10 cm3 soil + 10 ml water + 10 ml buffer at pH 6.6

measure the pH

It has 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 ft².

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 soil test,
· rate is tons of lime applied per acre,
· reduction percentage is 0.08 for MIN soils and 0.16 for M-O and ORG soils.

G. How lime neutralizes acidity.

H+ H+ + CaCO3 ==> Ca2+ + H2O + CO2

in solution

CaCO3 + H2O ==> Ca2+ + HCO3- +OH -

This OH-can react with H+ ==> HOH (water) or to precipitate Al as Al(OH)3

Lime 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.

CO3 ............. does


oxides .......................CaO



silicates .....................SiO3-

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.

Particle Size



Efficiency Rate (%)


larger than

4 mesh




4-8 mesh




8-20 mesh




20-60 mesh




60-80 mesh




100 mesh



N. 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 5 percent;




90% must pass through 20 mesh

and 25% through 100 mesh




90% must pass through 20 mesh

and 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%)

Molecular weight of CaCO3 is 100

MgCO3 is 84

1 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.

Neutralizing value (CCE) calcium carbonate equivalent of the pure forms of some commonly used liming materials


Neutralizing Value












I. Liming materials

To be considered a liming material an anion must produce OH - ions to react with H+ and Al3+ ions. Oxides, hydroxides, carbonates, and silicates

1. Calcium oxide (CaO)

Common names - burned lime, quicklime, unslaked lime

CaCO3 ===> CaO + CO2

Advantage is immediate reaction with the soil.

Disadvantage - caustic, difficult to handle and apply

Caking may occur. Through mixing is necessary


2. Calcium hydroxide (Ca(OH)2)

common names -- slaked, hydrated, builders lime

CaO + H20 ===> Ca(OH)2

Advantage - quick reaction with the soil

Disadvantage - difficult and unpleasant to handle

3. Calcitic limestone (CaCO3)

Dolomitic limestone (CaMg(CO3)2)

Mined from deposits. Quality depends on amount of impurities such as clay. Good handling properties. Reaction time several months.

4. Marl (CaCO3)

Unconsolidated deposits of CaCO3. Usually contaminated with clay. Low in Mg.

5. Slags (CaSiO3) by-product of furnaces used for making steel.  

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 low phosphate.

blast furnace

 National Slag Association

6. Fly ash or wood ash

J. Placement of lime

Through mixing of lime throughout zone of root growth is ideal.

1. Particles of lime do not move in the soil.

2. Application - Spread half of lime and plow down. Spread other half and disk.

On 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.

1. Lime requirements of crop to be grown. Plants differ in pH requirements.

acid - blueberries, cranberies, azaleas and camellias

neutral alkaline - Sweet clover, alfalfa, sugar beets.

Field crops in N.C. 5.8 - 6.2 is best.

Turf - 6.5 except centipede 5.5

2. Texture and O.M. content affects amount of lime required to change pH and frequency of application. Overliming coarse textured soils is a possibility.

3. Time 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.

4. Liming material to be used.



calcium carbonate equivalent

amount of material to apply assuming 100% purity and proper size particles.

Fluid Lime

1. good distribution pattern, no dust

2. finely divided - reacts quickly with soil

3. 500-1000 pounds per acre applied at one time. Reg. annual applications.

4. 2 to 4 times more expensive

Managing soil ph in turf

Top ten questions about liming

L. Acidulating the soil

- arid 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.

1. Elemental sulfur - pound for pound is most effective. Converted to sulfuric acid in warm moist soils by bacteria.

2. Sulfuric acid H2SO4

3. Aluminum sulfate - commonly used by Horticulturists for acidulating soil for azaleas, camellias, rhododendrons, etc.

4. Iron sulfate ( FeSO4) - reacts similar to aluminum sulfate.

Alternative Liming Materials





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