Soil Acidity: It’s not just pH

Jim Harsh

Crop & Soil Sciences

harsh@wsu.edu

For the chemist: pH = -log [H+

(aq)]

[H+

(aq)] is the concentration in the soil solution.

What is “soil pH”?

0

0.0002

0.0004

0.0006

0.0008

0.001

2 3 4 5 6 7

[H]

(mo

l/L

)

pH

Problems with Decreasing Soil pH

oRoot growth issues (acute)

oAluminum toxicity

oNutrient deficiencies (P, Mo)

oLimits nitrogen fixation by legumes

oDisease interactions

oHerbicide interactions

oReduced nutrient use efficiency

oReduced biological activity

oReduced cation exchange capacity (CEC)

…and the list goes on…

pH affects:

•Plant

nutrient

availability

•Soil

organism

activity

•Occurrence

of toxic

elements

pH and Al

0

20

40

60

80

100

120

140

160

4.00 4.20 4.40 4.60 4.80 5.00 5.20 5.40

KC

l A

lum

inu

m (

mg

/kg

)

1:1 pH

Al Toxicity to Plants

Sensitivity of dryland (Palouse) crops to soil acidification

oLegumes

oSpring wheat

oWinter wheat

oTriticale

Sensitive

Tolerant

Al

Tolerant

(Alikat)

Al

Intolerant

(Scarlet)

Genetic solutions to aluminum

toxicity in low pH soils

Mike Pumphrey

WSU Spring Wheat Breeding and Genetics

Alpowa Louise

OSU wheat breeding nursery near Enid, OK; Dr. Brett Carver

Aluminum tolerance screen – PNW spring wheat 2011

Tolerance rating 0-5 (0 = tolerant, 5 = sensitive)

JD 0 LOUISE 3

ALPOWA 0 WA008124 3

BABE 0 KELSE 3

WHIT 0 WESTBRED 926 3

NICK 0 WB-FUZION 3

PENAWAWA 1 BR7030 3

HANK 1 WAKANZ 4

OTIS 1 EDEN 4

TARA 2002 1 SCARLET 4

MACON 1 Glee 4

Diva 2 JEFFERSON 4

BUCK PRONTO 2 WA008113 5

Dayn 2 LASSIK 5

ZAK 3 BULLSEYE 5

IDO599 3 UI WINCHESTER 5

Legion 1 5J050348-1 3

Madsen 1 5J050390-1 3

SSD060040 1 Finch 3

ARS-Amber 2 Legacy 3

Badger 2 SSD060298 3

Brundage 96 2 Tubbs 3

Chukar 2 WA008114 3

Masami 2 WA008115 3

Paladin 2 WA008135 3

WA008134 2 WA008137 3

WA008136 2 WA008138 3

WB-528 2 Eltan 4

Whetstone 2 5J050509-2 5

Xerpha 2 Skiles 5

5J040150-1 333 Stephens 5

Aluminum tolerance screen – PNW winter wheat 2011

Tolerance rating 0-5 (0 = tolerant, 5 = sensitive)

Why are some varieties of wheat acid

tolerant?

They have a gene called ALMT1, that

causes malate to be secreted from roots,

which binds the Al3+

Not all Al has the same

toxicity

100

80

60

40

20

0

Dis

trib

ution (

%)

4 5 6 7 8 9

Al3+

AlOH2+

Al(OH)+2

Al(OH)-4

Al(OH)°3

pH

What is Soil Acidity?

oSoil pH is only one part of soil acidity and…

o It is the most important factor in toxicity but…

o It is only a small part.

It does not tell you how much lime to add to get

the pH you want!

Types of Soil Acidity

oActive acidity = pH

o [H+] in solution, soil water solution.

o Measure directly with pH meter, e.g. 1:1 soil:water

suspension

o pH controls chemical properties

o pH affects both biological and physical properties

oExchangeable acidity. Buffer pH

o amount of aluminum [Al+++

], Hydrogen [H+] and

(some) Iron [Fe+++

] that occupies exchange sites

on clays, soil aggregates, and organic matter

oResidual acidity.

o not readily available

o bound aluminum and hydrogen in clay minerals

and soil aggregates

H+

H+

CATIONS (+) adsorbed to exchange sites

(CEC) of Clay, Soil Aggregates and

Organic Matter

CATIONS (+) in Solution

H+

Al+++Ca++

Al+++

Al+++

Al+++

Al+++

H+

Al+++

K+

Al+++

EXAMPLE

SCENARIO acid

cations dominating

both exchange

sites and cations in

solution

Exchangeable ACIDITY Active ACIDITY

Note: Overly simplified to minimize mental exhaustion

-

-

--

----------------

Fe+++

Fe++

Mg++

CATIONS (+) adsorbed to exchange sites (CEC) of Clay, Soil Aggregates and Organic Matter

CATIONS (+) in Solution

H+

Apply Ag LIME• CaCO3 ± MgCO3

• Replace acid cations on exchange sites with base cations

Exchangeable ACIDITY Active ACIDITY

Note: Overly simplified to minimize mental exhaustion

K+

Na+

K+

Mg++

Ca++Na+ Ca++

-

-

--

----------------

Al+++

Al+++

Al+++

K+

K+

Al+++

Fe+++

Fe++

Mg++

Mg++

Ca++

Ca++

Ca++

Ca++

H+

Bas

e S

atu

rati

on

= 7

0%

Bas

e S

atu

rati

on

= 1

0%

Buffering of pH in Soils

Soil factors influencing acidity

opH

oOrganic Matter

oMineralogy

oAge and Climate

oTexture

Processes that Cause Soil Acidity

oCarbonic (CO2!) and Organic Acids

oDecomposition of Organic Matter

oOxidation of Nitrogen (Nitrification)

oOxidation of Sulfur

oPlant Uptake/Leaching of Cations

oAcidic Phosphate Fertilizers

Sources of Soil Acidity. CO2

o Conversion of carbon dioxide to carbonic acid

o Respiration = uptake up O2 and subsequent release of CO2

o Plant roots, microbial activity, soil fauna, etc. are

primary source of CO2 in soil

CO2(g) + H2O(l) H+(aq) + HCO3-(aq)

Sources of Soil Acidity. N Fertilizers

NH3 + 2O2 = H+ + NO3– + H2O

(nitrification of ammonia)

NH4–N Source lb CaCO

3/lb N

oAnhydrous Ammonia 1.8

oUrea 1.8

oAmmonium Nitrate 1.8

oAmmonium Sulfate 5.4??

oMonoammonium Phosphate 5.4??

oDiammonium Phosphate 3.6

Sources of Soil Acidity. Humus

oSoil organic matter contains compounds that behave like weak acidso Hydrogen ions are released as crop residues decompose

oOrganic acids contribute to soil acidityo humic and fulvic acids

oSOM serves as pH buffer. o Acts as both [H+] donor and [H+] acceptor

o Buffering capacity of soil increases with increasing soil organic matter content

oHumus has higher CEC than most clay minerals

oSOM binds Al3+

oCation removal by crop uptake will either

increase or decrease soil acidification

o Legumes tend to acidify soils more than cereals

because the removal rate of base cations by

legumes is significantly higher

oRemoval of base cations when baling straw

o Each 1,000-Lb bale of straw contains plant nutrients

with a replacement value of about $10 per bale

o Burning retains cations

o N, C, and S are oxidized – and float away in the breeze

o Fertilizer Cost to balance nutrient losses = $ 12 to $55

per Acre

(source: report by Huggins 2013)

Sources of Soil Acidity. Straw Removal

Acidification from Wheat Production (hypothetical, simplification)

1. 150 lb/A N applied as anhydrous ammonia

2. Nitrification results in acidification equal to

270 lb/A CaCO3 (150 lb N/a x 1.8 lb CaCO3/lb N)

3. Wheat uptake of 75 lb/A of N neutralizes acidity

equivalent to 135 lb/A CaCO3 (75 x 1.8)

4. Removal of cations results in additional acidity

equal to 13 lb/A CaCO3 (50/100 x 25)

5. CaCO3 required to neutralize soil acidification

is 148 lb/A

6. 1 ton/A of CaCO3 needed to maintain soil pH

for every 13.5 years of wheat production

Changing Soil pHAcid soils: Add “lime”

o Lime = carbonates, oxides or hydroxides of Ca

(or Ca and Mg), many types!o Ca2+ (or Mg2+) replaces H+ on exch. sites

o Carbonate (CO32-) or OH- reacts with H+ to form H2O

and CO2

The Reaction of Lime with Acids

CaCO3(s) + 2H

+ Ca

2++ H

2O + CO

2

RAISING SOIL pH BY LIMING

How soil texture affects lime requirement

Determining Lime Requirement

oField determination with lime

oTitration in the laboratory

oIncubation in the laboratory

o“Quick Test”: We need one for the Palouse!

All require calibration

Managing Acid Soils. Amendments

o Add organic matter

o Chelates aluminum, iron and

manganese

o Amend with calcium carbonate

o Calibrated application rates raise

soil pH

Lime Requirement Depends on:

oLiming goal:opH goal or tolerable level of exchangeable Al

oSoil buffering capacity

oNeutralization value of

liming material

oFineness of liming material

Lime Reactivity with Soil

o100-mesh sieve or finer, reacts with soil in

6 months or less

o60-mesh or finer, reacts in 1st year

o20-mesh but not 60 mesh, about 50% in 1st

year

oNot passing 20-mesh sieve--little liming

value

oIdaho fertilizer law requires that the

percentages of lime passing 10-, 60-,

and 100-mesh sieves be specified.

Lime TypeSource of

LimeCa2+ Dry

Matter

CaCO3

Equivalent

(100% DM

Basis)

Fineness

Factor

Lime

Score

% %

Moses Lake Sugar

Lime

Cascade

Agronomics18 91.7 83.8 85 to 98 65 to 75

Limestone (Ground)Pioneer

Enterprises37.5 99.2 95 80 75 to 89

Microna Ag H2O -

Calcium (solution

grade)

Columbia

River

Carbonates

38 99 98 100 97

Microna Agricultural

Lime

Columbia

River

Carbonates

38 99 98 100 97

Lime Sources

Soil pH Stratification

Koenig and coworkers

Soil pH profile, Rockford, WA 2011

Koenig and coworkers

oImplement soil sampling plan to identify

acidified soil strata. Consider depth of

fertilizer placement.

oBudget CaCO3

applications to counter fertilizer

effects

e.g., 1.8 lb CaCO3

per lb NH4-N applied.

oAmend acid soils four to six months prior to

planting your sensitive crop. Speed of Ag

Lime reaction most rapid with very fine

products.

oAg Lime transportation costs are a significant

limiting factor when selecting product.

Consider long-term effects of particle size

among Ag Lime sources.

oConsider organic matter and micronutrient

content of Ag Lime

Practical Considerations

Acknowledgments

All those I stole from:

oWayne Thompson

oDave Huggins

oRich Koenig

oBrady and Weil

Questions?