soil acidity: it’s not just ph
TRANSCRIPT
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?