Soil pH and Liming

What is pH?• A measure of the relative acidity of a

substance, or• The negative logarithm of the hydrogen

ion concentration

0 7 14

pH Range< 7.0 Acid >7.0 Alkaline

Notes on pH• pH is a convenient notation

– (5.0 is easier to use than 0.00001 or 10-5)

• The H in pH stands for hydrogen ions

• A change in pH of one unit equals a 10 fold change in H+

concentration

• A change of pH 6.0 to 5.7 doubles the acidity

The Relationship Between pH and Hydrogen Ion Concentration

pH H+Concentration

5.0 0.00001 or 10-5

6.0 0.000001 or 10-6

7.0 0.0000001 or 10-7

8.0 0.00000001 or 10-8

Typical pH Range of Soils

Neutrality

Acidity AlkalinityVery

Strong Strong Moderate SlightSlight Moderate StrongVery

Strong

3 4 5 6 7 8 9 10 11

Extreme range in pH for most mineral soils

Acid peat soils

Alkali mineral

soils

Humid region mineral soils

Arid Region Mineral Soils

pH Scale

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

Neutral

Alkaline

Acid

Lye Solution

Hard Water

Milk of Magnesia

Coffee

Coca Cola

Stomach Acid

Battery Acid

Sulfuric Acid

Milk

Washing Soda

Normal pH Range

Typical pH Values

Importance of Soil pH• Low pH dissolves Al+++

• (1000x more soluble at pH 4.5 than at 5.5• Plant toxicity

• High pH and High pH: lower availability of some nutrients • P, Zn, Fe, etc.

• Low pH affects microbe activity which affects nutrient cycles, legume nodulation, residue decomposition, diseases, herbicide breakdown and carryover.

• Herbicide efficacy/carryover

Lime WheatRate Soil pH Extr. Al Yield

Lbs Ecc/A ppm Bu/A

0 4.6 102 153,000 5.1 26 396,000 5.9 0 3812,000 6.4 0 36

Initial Soil pH = 4.7, Extr. Al = 94 ppm & Lime Rec = 12,000 Lbs Ecc/A

Kansas - 4 year average

0-6 Inch Depth

Effect Of Lime Rate On Soil AcidityExtractable Al and Wheat Yield

Effect of Soil pH on Nutrient Availability?

Effect of Soil pH on Nutrient Availability?

Soil Reactions with Added P

Soil pH

CalciumCalciumphosphatesphosphates

Dis

trib

uti

on

(%

)

Fixation by hydrous Fixation by hydrous oxides of Al and Feoxides of Al and Fe

Adsor

bed

to c

lay

Adsor

bed

to c

lay

Fixation by Fixation by Fe, Al & MnFe, Al & Mn

Available phosphatesAvailable phosphates

Brady, 1990

One year old stand

One year old stand

Lime and Fertilizer P For Alfalfa In North Central KansasGordon and Whitney, 1991-93

Alfalfa established March 1991, Initial Soil pH = 5.1 and Bray P-1 Soil Test = 30 ppm

No Lime½ Lime

Full Lime

1991 1992 1993

5.1 5.1 56.0 6.16.5 6.5

Soil pH

1991 1992 1993 Total 1991-93

1.3 8.7 5.7 15.71.5 10.7 7.3 19.51.5 11.0 7.5 20.0

Alfalfa Yield (ton/acre)

No Lime½ Lime

Full Lime

Row Applied Grain SorghumLime Rate P2O5 Rate YieldLbs Ecc/A Lbs/A Bu/A

0 0 560 35 89

5,000 0 745,000 35 101

10,000 0 8810,000 35 101

Initial Soil pH = 4.6, Extr. Al = 24 ppm & Bray P1 = 44 ppm

Kansas, 3 year averageLime and Row Applied P For Grain Sorghum

Factors Affecting Soil pH• Parent Material• Annual Precipitation

Higher rainfall Lower pH• Native Vegetation• Crop Grown

– Legumes remove more Ca and Mg than nonlegumes• Microbial Nitrification

– 1-2 lbs of lime to neutralize 1 lb N

Nitrification Process Results In Residual Soil Acidity

When ammonium ion is converted to nitrate, H+

ions are released:

NH4+ + 2O2 NO3

- + 2H+ + H2ONitrifying Bacteria

Ammonium Oxygen Nitrate Hydrogen Water

Nitrogen SourceSoil pH Soil OM Bray P-1 NO3

--NNH4

+-N

SoilDensity

(%) - - - - - ppm - - - - (lb/cu ft)

Check (No N) 6.2 2.0 38 4 5 100Anhydrous Ammonia 5.2 1.8 27 27 9 99

Ammonium Nitrate 5.2 2.3 26 21 11 99Urea 5.1 2.3 24 31 12 99

UAN Solution 5.2 2.0 28 20 8 100

Effect Of 20 Years Of N Fertilizers On Soils (Kansas State University)

Residual Acidity of N Fertilizers

Pounds calcium carbonate per pound of N:

Amm. Nitrate, Ammonia, UAN, Urea 1.8Ammonium sulfate 5.4

AOAC, 1934

Nitrate Sources Are Not Residually Acidic(Calcium nitrate, potassium nitrate)

Correction of Soil Acidity

Reserve Acidity Active Acidity

H+

H+

H+H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+H+

H+

H+

H+

H+

H+

H+H+

H+

H+H+

Section of Soil Colloid

H+

H+

H+

Soil Solution H+Ions

(Active Acidity)

Reserve Acidity

Soil pH Vs. Buffer pH

Soil pH -- a measure of the acidity (H+) of the soil solution.Soil Buffer pH -- a measure of the soil’s potential acidity (H+) due to that

of the soil solution and the exchange sites of the soil colloidsthemselves.

Soils 1 and 2 have the same soil pH values; however, they have different Buffer pH values. It will take more lime to raise the pH of soil 2 than soil 1.

Soil 1

(Soil Air) (Soil Air)(Soil Water) (Soil Water)

Exchange sites of Clay particles

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

K+

K+

K+

K+Mg++

Mg++

Mg++

Mg++

Mg++

Ca++

Ca++

Ca++

Ca++

Ca++

Ca++

NH4+

NH4+

NH4+

K+

Soil 2

Soil Acidity• Active Acidity

– Affects soil chemical reactions and plant growth– Measured as soil or water pH– Neutralized by <1 pound calcium carbonate/acre

• Reserve Acidity– The total acidity– Affects the quantity of lime required– Measured by buffer pH (buffer index)

Soil Texture and Lime Requirement

Soil pH 5.8

50% H+

50% Ca++

Buffer pH 6.6

10% H+

90% Ca++

Soil pH 6.8

Clay Loam Soil

Requires 4.5 Ton/A

Lime

Finer textured soils require more Ag-lime than coarser textured soils even at the same pH values.

50% H+

50% Ca++

Soil pH 5.8

Buffer pH 7.0

10% H+

90% Ca++

Soil pH 6.8

Sandy Loam Soil

Requires 3.5 Ton/A

Lime

The lower surface area and CEC of the sandy soil holds less total hydrogen and thus requires less lime.

Soil Texture and Lime Requirement

How Lime Neutralizes Acidity

Calcium Carbonate - A Liming Agent

Calcium Sulfate (Gypsum) - A Non-liming Agent

Soil Colloid Soil Colloid+CaCO3H+

H+Ca + H2O +CO2

Soil Colloid H+

H++CaSO4 Soil Colloid Ca + 2H+ + SO4

=

Sulfuric Acid

Relative Neutralizing Value of Different Substances

Liming Material Composition Relative Neutralizing Value

Calcium Carbonate

Calcitic Lime

Dolomitic Lime

Quicklime (burned lime)

Hydrated lime (slaked lime)

Ground shells

Wood ashes

100

50-100

90-109

150-180

115-135

80-90

40-80

CaCO3

CaCO3 + Impurities

CaCO3 + MgCO3 +Impurities

CaO

Ca(OH)2

Neutralizing Value is Expressed As:

• ECC - Effective Calcium Carbonate – Example

• Soil Test Recommends 2500 lbs ECC/acre• ECCE of local Ag-lime = 70%• 2000 X 0.7= 1400 lb ECCE/ton• 2500 divided by 1400 = 1.79 tons of Ag-lime

What determines the quality of a What determines the quality of a liming material?liming material?

PurityCalcium carbonate equivalentDetermined in the laboratory

FinenessParticle sizeDry sieve analysis

Neutralizing Value of Lime• Particle Size

– Fine sized particles dissolve more quickly than coarse particles

• Purity– % liming agent vs % clay,

sand and other non-lime materials

• Often expressed as effective calcium carbonate equivalent (ECCE) also effective neutralizing value (ENV)

Kansas Ag Lime Law

% DissolvedSize Fraction

< 60 mesh

8-60 mesh

> 8 mesh

100

50

0

After 1 Year60 mesh

8-60 mesh

> 8 mesh

Lime Fineness Factors for KansasLime Fineness Factors for Kansas

Sieve Size Category

% of Particles Remaining In Size Category

Fineness Factor

% Available Based on Fineness

> 8 mesh 0 0.0 0

8-60 mesh 60 0.5 30

< 60 mesh 40 1.0 40

ECCE = 70

Example limestone material

Comparative Values of Limestone of Varying Particle Sizes Evaluated 1, 4, and 8 Years After Application

Time After Application, %Dissolved

Size Fraction

Through 60 mesh

30-60 mesh

8 to 30 mesh

Over 8 mesh

%

100

50

20

5

%

100

100

45

15

%

100

100

75

25

1 Year

4 Year

8 Year

Effect of Ag-lime on Soil pH in Years Following Application

Years After Application

Soil pH

2 4 6

6.0

5.5

6.5

7.0

X X

X

X 8000 lb ECCE/A

2000 lb ECCE/A

Ag Lime vs Fluid Lime

Soil pH

March April May June July August

6.0

5.0

6.5

7.0

5.5

xx

xx

x x x

Check

x

x

x x xx

x x 500 ECCE

x

x

x

xx

x xx

x

xx

xx

x

xx x

x 5000 ECCE Fluid

5000 Ag lime

Soil Depth 0 1,250 2,500 5,000

Ag Lime

0-3" 5.1 6.0 6.6 6.83-6" 4.9 5.2 5.5 5.8

Fluid Lime0-3" 5.1 6.4 6.5 7.23-6" 4.9 5.2 5.5 5.8

Effect Of Lime Rate On Soil AcidityKansas, 8 months after application - one disc, one field cultivation

Lime Rate (Lbs Ecc/A)

- - - Soil pH - - -

Normally Lowest CostResidual BenefitsHighest Farmer Profitability

Difficulty Of Uniform ApplicationTough On EquipmentNo Margins For Vendors

Higher Cost Than Ag LimeHigh Rates Are Not PracticalApplied As Fluid Slurry

Uniform ApplicationQuick Effect On Soil pHProfit For Vendors

Often Prohibitively High Cost Adequate Rates Are Not PracticalMarketing Hype By Some

Uniform ApplicationQuick Effect On Soil pHProfit For Vendors

Pel-Lime

Fluid Lime

Ag Lime

Depth Of Incorporation

Time and Method of Lime Application

• Since lime is relatively insoluble– Apply 3-12 months before crop– Mix thoroughly with soil– Adjust rate for tillage depth

• Example– 6 inches = 2,000 lbs lime/acre– 9 inches = _____ lbs lime/acre

Variable Rate Lime Application

Does Liming pay?

University of Nebraska, Lincoln

T. R. Fisher, Missouri

Effect Of Soil pH on Soybean Yields

Lime WheatRate Soil pH Extr. Al Yield

Lbs Ecc/A ppm Bu/A

0 4.6 102 153,000 5.1 26 396,000 5.9 0 3812,000 6.4 0 36

Initial Soil pH = 4.7, Extr. Al = 94 ppm & Lime Rec = 12,000 Lbs Ecc/A

Kansas - 4 year average

0-6 Inch Depth

Effect Of Lime Rate On Soil AcidityExtractable Al and Wheat Yield

Lime Recommendations – Know Assumptions Of Recommendations

Lime Quality

Soil Depth To Be AmendedRotational Tillage DepthTillage SystemCrop

Target pHCropGeographic AreaFinancial ConsiderationsLandlord/Tennant Situation

‘High’ pH Soils

• Calcareous Soils– Contain free undissolved lime– pH about 7.3-8.4– Common in low rainfall or poorly drained areas– Micronutrient (Zn & Fe) availability sometimes low– Lowering pH is economically impractical and not necessary– Each 1% free calcium carbonate in the surface 6-7 inches of soil

requires about 10 tons of sulfuric acid or 3.25 tons of elemental sulfur to neutralize

– Many soils contain levels of 5% to 10% or more

Salt Affected Soils

• Saline Soils– Salty soil - Excess NaCl, NaSO, CaCl, etc.– Plants grow poorly - moisture stress

• Alkali or Sodic Soils– Contain excess sodium

• >15% of exchange capacity

– May appear whitish in color– Usually very high pH (8.5-10)– Poor soil structure

Salt Affected Soils• Saline and alkali (or sodic) are terms describing salt

affected soils

• Saline soils contain excessive salts but relatively low levels of exchangeable sodium

• Alkali soils have relatively low soluble salt levels but are high in exchangeable sodium

• Saline/Alkali soils contain both soluble salts and exchangeable sodium

Saline Soils

• EC > 4.0 (reported as mmhos/cm) and ESP < 15%

• pH will be less than about 8.4

• Soil will have a white crust (deposited salts)

• Good soil structure and water permeability

• Excess salts compete with the crop for available water

Reclaiming Saline Soils• Identify cause of problem

• Pass excessive water through the soil profile to leach the saltsbelow the root zone

• Establish good internal drainage

• 6-inches of water to remove 50% of salts, 12-inches to remove 80% and 24-inches to remove 90%

• Under irrigation, reclamation may be relatively quick

• Dryland fields may be difficult or nearly impossible

• Crops vary in tolerance of high quantities of salt

Alkali (Sodic) Soils

• Alkali soils– ESP > 15% while EC < 4.0– pH is typically between 8.5 and 10.0 although there are a few

acidic alkali soils which have a lower pH– In general, when more than 15% of the CEC is associated with

sodium (ESP) soil physical properties deteriorate and lose soil structure, other situations can occur

– Impervious to water and air movement– Poor physical properties that make soils nonproductive rather

than sodium toxicity or high soil pH

Reclaiming Alkali Soil

SoilColloid

Na+

Na+

SoilColloid

SoilColloid Ca++

Ca++ + Na2SO4 leach with water

+ CaSO4

Reclamation of alkali and saline/alkali soils

• Not easy or fast• Identify and correct cause of sodium/salts accumulation• Establish internal soil drainage• Excess sodium needs to be replaced

• Add gypsum (calcium sultate)– Slowly dissolves and calcium replaces sodium of the CEC complex and

sodium is leached below the root zone

– Gypsum is limited to only sodic soils and are not effective on saline

• Sulfuric acid and elemental sulfur can only be used to reclaim alkali soils if they contain free calcium carbonate (excess lime)

– Elemental sulfur is converted to sulfuric acid by soil microbes, which in turn reacts with calcium carbonate and forms calcium sulfate

How a Salt Affected Soil is Born

Water Evaporation

Salt problem - seep

Evaporation Evaporation

Alkali and Saline/Alkali Soils

• Contain both excess salts and exchangeable sodium

• ESP > 15%, EC > 4.0, a pH of less than 8.4

• Reclamation is the same as for alkali soils

Proper Identification of Salt-Affected Soils

• Tests to perform– Cation Exchange Capacity (CEC)– Exchangeable sodium percentage of CEC, (ESP)– Electrical conductivity (EC)– Soil pH and excess lime (free calcium carbonate)

• Sampling– 0-6, 6-12, and 12-24 inches sampling depths

• What caused the salt-affected soil to develop?

Saline, Alkali and Calcareous Soils

Saline

Alkali

Saline/Alkali

Calcareous

EC*

>4.0

<4.0

>4.0

<4.0

ESP

<15%

>15%

>15%

<15%

pH

<8.4

>8.4

<8.4

7.3-8.4

Physical Condition

Good

Poor

Good-Poor

Good

Amendment Required

No

Yes

Yes

No

Soil Test

* EC of 4.0 for saturated paste - If 1:1 soil:water dilution then 1.5

Reclaiming Saline and Alkali Soils

Identifying/Correcting Cause

Good Internal Soil Drainage

Leaching with Excessive Water

Gypsum or Elemental Sulfur

Time Required for Reclamation

Deep Ripping

Manure Applications

Saline

Yes

Yes

Yes

No

Short

No

No

Alkali and Saline/Alkali

Yes

Yes

Yes

Yes

Long

May Help

May Help

Soil pH and Liming

Review Exercises. Of the following, soil pH can affect all except:

a. Plant growth.

b. Herbicide performance.

c. Nutrient availability.

d. Soil texture.

e. Fertilizer effectiveness.

. A pH of 6.5 is:

a. Strongly acid. b. Neutral. c. Slightly acid. d. Slightly alkaline.

. A soil with a pH of 5.0 is ___________ as acidic as a soil of pH 6.0.

a. Twice. b. 5 times. C. 6 times. d. 10 times.

. One consequence of an excessively low soil pH is that Al dissolves in amounts that can be toxic to plants.

a. True. b. False.

. With an excessively high soil pH, the availability of __________ to plants is decreased.

a. Calcium.

b. Phosphorus.

c. Sodium.

d. Molybdenum.

. Of the following, which crop can tolerate the lowest soil pH:

0. The __________ pH or index serves as the best indicator for determining the amount of lime required on acid soils.

a. Soil. b. Buffer. c. Water. d. Plant

1. All but __________ are effective liming materials.

a. Calcium carbonate. b. Calcium oxide. c. Gypsum. d. Dolomite.

2. A common measure of the neutralizing value of agricultural limestone is the effective calcium carbonate equivalent (ECCE) which is a function of purity and particle size of the lime.

a. True.

b False

4. __________ soils contain excessive amounts of sodium on the CEC sites of the soil colloids and generally have a soil pH greater than 8.4.

a. Calcareous.

b. Saline.

c. Alkali or sodic.

d. Gypsum.

5. All except __________ are relatively salt tolerant crops which might be considered when managing saline soils:

a. Barley.

b. Cotton.

c. Red clover.

7. Soils become more acidic (lower pH) as cations such as calcium and magnesium are replaced on the soil exchange complex with:

a. Nitrogen and phosphorus

b. Potassium and phosphorus

c. Hydrogen and aluminum

d. Sulfates and chlorides

8. If soil pH and magnesium soil test levels are too low, which of the following can be applied to correct both problems:

a. Dolomitic limestone

b. Magnesium sulfate

c. Calcitic limestone