Title: Soil pH, Acidity and Liming

Speaker: Bill Pan

online.wsu.edu

Soil pH, Acidity and Liming

Chapter 3 in Your Text

Thanks to R. Koenig for some slide material

Topics

Soil pH defined

Nature and extent of soil acidity

Sources of soil acidity

Importance of soil pH and acidity

Active and reserve acidity

Liming to increase soil pH

Soil pH Defined

In words: pH is equal to the negative log of the hydrogen ion concentration in moles per liter

Mathematically ◦ pH = -log [H+] ◦ [H+] = 10-pH

[H+] pH

1 x 10-5 5

1 x 10-6 6

1 x 10-7 7

3.44 x 10-6 5.46

pH is expressed using a log scale:

one unit change in pH equates to

10-fold change in [H+]

Higher [H+] = lower pH

pH Scale and Range in Soils

www.cropsoil.uga.edu/soilpHturf/

Figure 3.2 from your text

Worldwide, 25 to 30% of

agricultural soils are acidic

Acidic soils are associated

mainly with high rainfall areas;

alkaline (basic) soils are

associated mainly with arid

(low rainfall areas)

Why?

Nature and Extent of

Soil Acidity

One Effect of Precipitation: Leaching

Western U.S. Midwest U.S.

Leaching of

carbonates deep

into, or completely

out of, the soil

profile

Leaching of basic

cations from soil,

leaving acidic

cations

U.S. Soil pH Map forages.oregonstate.edu/maps

acidic

alkaline

Another Effect of Precipitation: Rainfall pH

jrscience.wcp.muohio.edu

Low rainfall

pH is

associated

with urban

and

industrial

areas (“acid

rain”)

Washington State

gocalifornia.about.com

High rainfall,

low soil pH

Low rainfall,

neutral to high

soil pH

Low to

intermediate

rainfall, low

soil pH

(recently

acidified

soils)

Sources of Soil Acidity

Rainfall and carbon dioxide ◦ H2O + CO2 ↔ H2CO3 ↔ H+ + HCO3

-

◦ In equilibrium with atmosphere, pH = 5.7

Other gasses in the atmosphere (“acid rain”) ◦ SO2 + ½O2 + H2O ↔ SO4

2- + 2H+

Soil organic matter ◦ “Soil atmosphere” has about 10-fold higher carbon dioxide concentration due to organic matter decomposition Effect on pH?

◦ Organic acids released during decomposition

Sources of Soil Acidity

Nutrient transformations and uptake (Table 3-2 in your text)

Process Reaction pH Effect

Nitrogen mole H+/mole N or S

Mineralization R-NH2 + H+ + H2O ↔ R-OH + NH4+ -1

Denitrification 2NO3- + 2H+ ↔ N2 + 2½O2 + H2O -1

Urea hydrolysis (NH2)2CO + 3H2O ↔ 2NH4+ + 2OH- + CO2 -1

NO3- uptake NO3

- + 8H+ + 8e- ↔ NH2 + 2H2O + OH- -1

SO4-2 uptake SO4

-2 + 8H+ + 8e- ↔ SH2 + 2H2O + 2OH- -2

Immobilization NH4+ + R-OH ↔ R-NH2 + H+ + H2O +1

Nitrification NH4+ + 2O2 ↔ NO3

- + H2O + 2H+ +2

Volatilization NH4+ + OH- ↔ NH3 + H2O +1

NH4+ uptake NH4

+ + R-OH ↔ R-NH2 + H+ + H2O +1

S Mineralization R-S + 1½O2 + H2O ↔ SO4-2 + 2H+ +2

Raises

pH

Lowers

pH

Consume H+

or release

OH-

Consume

OH- or

release H+

Aside: Basic and Acidic Cations

“Basic cations” ◦ Calcium (Ca2+), magnesium (Mg2+), potassium (K+), sodium (Na+) – no acid reaction

“Acidic cations” – acid or acid reaction ◦ H+

◦ Fe3+ and Al3+ via hydrolysis (water splitting) reactions:

Al3+ + H2O ↔ Al(OH)2+ + H+

Al(OH)2+ + H2O ↔ Al(OH)2+ + H+

Al(OH)2+ + H2O ↔ Al(OH)3(s) + H+

Importance of Soil pH and Acidity pH influences most inorganic chemical

and biological reactions – extremely important

Affects nutrient availability in soil Affects availability, and therefore

toxicity, of certain elements Affects microbial activity Affects many soil-borne pathogens Affects the number of cation exchange

sites in soil Overall, pH significantly affects plant

growth

pH Affects “Availability”

pH-induced deficiency

(Fe): nutrient is present

but unavailable due to

chemical form in soil:

Fe3+ + 3H2O ↔ Fe(OH)3(s) + 3H+

toxicity

deficiency

pH-induced toxicity:

element is present in

high concentration in a

plant-available form

Al3+ + 3H2O ↔ Al(OH)3(s) + 3H+

High pH soil

neutralizes

H+,

removing

from

reaction

Low pH soil

contributes

H+ to the

reaction

www.ca.uky.edu

One High pH Problem: Iron Deficiency

“Interveinal chlorosis”

on younger leaves

One Low pH Problem: Aluminum Toxicity

Aluminum toxicity on wheat

seedlings (Brown, 2006)

Soil pH optimums

vary by plant

species

Figure 3-13

from your text

Yield vs. Soil pH (N. Idaho)

Opportunity: pH and Ornamentals

Hydrangea flowers respond to Al availability ◦ Blue at pH <5.5

◦ Pink at pH >6.0

www.meltonrossnewbarnetby.co.uk

pH Affects Microbial Activity

Many microbial processes have optimum pH range ◦ In general, fungi tolerate low pH; bacteria high pH

◦ Mineralization (both fungal and bacterial process) occurs over a broad range of soil pHs

◦ Nitrification and denitrification (primarily bacterial processes) are optimum at near neutral pH

Soil-borne pathogens have pH optimum ◦ Fungal pathogens favored at low pH ◦ Bacterial pathogens favored at higher pH

Reduced Organic Matter Decomposition at Low Soil pH

Reduced nitrogen mineralization from soil organic matter and organic fertilizers

Thatch accumulation in turfgrass sods

Thatch Accumulation

Unusual Disease Epidemics

Dreschlera leaf spot of ryegrass overseeding in Feb.

Bermudagrass rust during spring green-up Fusarium and Ascochyta blights of ryegrass

and bermudagrass

ALL OCCURRED WITH SOIL pH around 4.0

http://virtual.clemson.edu/groups/

turfornamental/tmi/fertlime/disease.htm

and lowph.htm

Fairy Ring Symptoms on Bermudagrass

Ideal Environment for Fairy Ring Fungi

mycelium

Manganese deficient soybean from

over-liming an

Atlantic Coastal Plain soil

pH Affects Cation Exchange Sites

Sites on organic matter:

R – COOH ↔ R-COO- + H+ (H+ dissociates and is neutralized

at high pH)

Sites on the edges of Fe and Al oxides and clay minerals:

OH2+ OH O-

Al ↔ Al ↔ Al

OH2+ OH O-

Low pH ----- Neutral to high pH ----- Edge of

mineral

Potential vs. Active Acidity

Soil particles ↔ Soil solution

Potential acidity (quantity) ↔ Active acidity (intensity)

Majority of acidity in soil Minority of acidity in soil

Root are exposed to active portion

- -

- - -

-

-

- - - -

-

-

- -

-

-

-

- -

-

-

- - - -

- -

-

-

Al3+

Al3+

Al3+

Al3+

H+

H+

H+

H+

H+ Ca2+

Ca2+

Ca2+

Ca2+

Mg2+

K+

K+

Na+

↔

Al3+

H+

Ca2+

Mg2+

K+

Na+

Measuring Potential Acidity

Titrations with base or incubation with different amounts of lime

SMP buffer method

Both methods assess acidic

cations on CEC sites

SMP Buffer Interpretation Table

Soil+

buffer

pH

7.0 6.5 6.0 5.2

6.8 1.4 1.2 1.0 0.7

6.0 9.6 8.1 6.6 5.1

5.5 14.8 12.5 10.5 7.8

------ tons CaCO3/acre ------

Percent Base Saturation

- -

- - -

-

-

- - - -

-

-

- -

-

-

-

- -

-

-

- - - -

- -

-

-

Al3+

Al3+

Al3+

Al3+

H+

H+

H+

H+

H+ Ca2+

Ca2+

Ca2+

Ca2+

Mg2+

K+

K+

Na+

% base saturation =

% acid saturation = ?

cmol(+) exchangeable bases x 100

CEC (cmol[+] per kilogram)

43% in this example

Generally, as base saturation

increases pH also increases

Base saturation can be used

to develop a lime requirement

Measuring Active Acidity

pH electrode ◦ Saturated soil paste

◦ 1:1, 1:2 or 1:10 soil:water ratios

◦ Others solutions may also be used

Stratification of acidity under direct seeding

6 inches

Broadcast lime

Untreated

Within Field Surface Soil pH Variability near Pullman, WA

Liming to Increase Soil pH

Neutralize toxic elements: Al, Mn, H

Improve overall nutrient availability (recall the graphic)

Increase microbial activity

Increase effective CEC

Improve soil structure with Ca

Improve Ca and Mg availability

Overall, improve plant growth

Some Liming Reactions

CaCO3

◦ CaCO3 + 2H+ ↔ Ca2+ + CO2 + H2O

Ca(OH)2

◦ Ca(OH)2 + 2H+ ↔ Ca2+ + 2H2O

CaO ◦ CaO + 2H+ ↔ Ca2+ + H2O

Neutralizing Acidity

-

-

- -

- -

-

-

-

- Al3+

H+

H+

Ca2+

3 CaCO3

= 6 H+ can be

neutralized; 3

Ca2+ released

↔ Al3+

2H+

H+

H+

H+

} = 5H+

Neutralizing Acidity-Step 2

Review Equivalent Weight (pg 18-27) For an ion, the atomic weight divided by

the charge ◦ Ca2+: 40 g/mol ÷2 = 20 g/equivalent= 20 mg/milliequivalent (meq)

For a molecule of lime, the molecular weight divided by the number of moles of H+ neutralized ◦ CaCO3: 100 g/mol ÷2 = 50 g/equivalent= 50 mg/meq

Importance of meq?

It’s just a number

CEC is expressed in units of meq/100 grams of soil (or cmol(-)/kg soil …it’s the same number)

Exchangeable acidity is expressed in meq H+/100 grams of soil

Lime Effectiveness

Chemical composition and purity (Calcium carbonate equivalent)

Fineness (finer particles react faster to neutralize acidity; big particles stay unreactive longer)

Application and incorporation method…incorporation speeds the soil reaction…topdressing without incorporation slows the reaction.

Lime Neutralizing Value (Calcium Carbonate Equiv.)

MATERIAL MW Calcium

Carbonate

Equivalent

calcite 100 100%

dolomite

CaMg(CO3)2

184 2(100)/184=

109%

burned lime

CaO

56 100/56=

179%

hydrated lime

Ca(OH)2

74 100/74=136%

Lime Reacted in 1 to 3 years

0

20

40

60

80

100

4 to 8 8 to 20 20 to 50 50 to 100

# Mesh Holes per area of sieve

fine coarse

% L

ime R

eacte

d

Depth of Lime Incorporation: Lime Moves Slowly, Should Be Tilled-In to Be Effective