Soil Acidity and pH
Causes, remediation, and
implications
http://www.msichicago.org/ed/learninglabs/imgs/waters_ph_chart.jpg
Fig 9.1 The relationship between pH, pOH, and the concentrations of
hydrogen and hydroxyl ions in water solution
pH is a ‘master’ variable
• affects chemical, physical, and biological
properties of soils
• Nutrient availability (optimum pH for most
crops is 5.5 - 7)
• Metal toxicity and solubility e.g., Al toxicity
at pH <5.5 (also Mn solubility and toxicity)
• Microbial activity (especially important in
the N cycle)
Most nutrients
are highest
and most
toxins are
lower at
pH 5.5-7
http://www.traylorchemical.com/images/faqs/phchart.jpg
Figure 9.11 Relationships
existing in mineral soils
between pH and the
availability of plant nutrients.
A pH range of about 5.5 to 7.0
seems to be best to promote
the availability of plant
nutrients. In short, if the soil
pH is suitably adjusted for
phosphorus, the other plant
nutrients, if present in
adequate amounts, will be
satisfactorily available in most
cases.
Pools of Acidity: Active
acidity that is in solution
(H+) that is measured with a pH ‘specific ion’
electrode (best), color indicators, dyes, litmus
papers.
Includes Al+3 in solution that hydrolyzes to form
H+ and Al(OH)x species
Relatively speaking, active acidity is only a
small amount compared to reserve acidity
Methods for measuring
soil pH
http://img.alibaba.com/photo/51008043/Soil_pH_Meter.jpg
http://www.biconet.com/testing/GIFs/st-t2.jpg
Pools of Acidity: Reserve
Exchangeable or KCl-extractable acidity [(Al+3
+ H+) / CEC]
Mostly Al+3 on clay mineral sites
Organic acid groups:
RCOOH = RCOO- + H+
Residual or non-exchangeable acidity (H+ and
Al+3 not displaced by KCl or salt solution):
SOM-Al complexes
Solid phase Al+3 + H+ in soil minerals
http://hubcap.clemson.edu/~blpprt/acid_photos/Buffering2.jpg
http://www.landfood.ubc.ca/soil200/images/15_3acidity.jpg
http://www.landfood.ubc.ca/soil200/interaction/acidity.htm
Soil pH alteration (naturally and
manmade)
• Management and land use
– Fertilizers, organic matter, and other amendments
– Submergence and subsequent uplift of land exposing
reduced sediments to oxidation processes
• Pollution
– Acid rain
– Mining
• Climate
– Weathering and leaching
– Rainfall leaching
– Plant growth: uptake of cations and release of protons
• Metal hydrolysis
Fertilizers can lower soil pH
• Oxidation of Ammonium, or ‘Nitrification’
NH4+ + 2O2  NO3- + H2O + 2H+
• Phosphate fertilizers:
Triple superphosphate  hydroxyapatite + H+
Ca(H2PO4)2  Ca5OH(PO4)3 + H+
Organic matter
Organic acid groups deprotonate:
RCOOH = RCOO- + H+
http://www.rsc.org/ej/gt/2000/b001869o/b001869o-f3.gif
Amendments that lower soil pH
Oxidation of elemental sulfur
produces sulfuric acid which
dissociates easily
S0 + 3/2O2 + H2O  H2SO4
Some growers even use sulfuric
acid – but it is very dangerous,
expensive, and doesn’t last long in
arid zone soils
Alum, KAl(SO4)2 is a commercial
product for lowering pH
Acid sulfate soils
Dredging waterways,
draining swamps, spoil
piles, mine tailings
http://www.latrobe.edu.au/envsci/assets/images/publicity/amd2-edit.jpg
http://web.missouri.edu/~umcsnrsoilwww/290_2003/images/gillpic1.gif
http://www.ozcoasts.org.au/indicators/Images/iron_staining_ASS.jpg
Iron staining is often a good indicator of disturbed acid sulfate
soils.
When acid sulfate soils are disturbed and undergo
oxidization, the sulfuric acid produced mobilizes iron,
aluminum and heavy metals present in the soil.
Toxic amounts of dissolved iron can then be washed into
waterways.
This iron can precipitate when in contact with less acid water,
such as rainwater or seawater.
This results in a rust-colored iron oxide scum or ‘floc' which
can smother vegetation and stain concrete and soil.
('QASSIT, Qld Department of Natural Resources and Mines').
Acid rain
• Oxidation of sulfur (SO2) in coal (power
plants) and NOx (car exhaust) to sulfuric
and nitric acid
• pH 4-5 (pure rainfall = pH 5.6)
• Extensive in heavily populated areas with
heavy rainfall (soils already slightly acid)
• Recent regulations have improved some
conditions.
• Lakes and forests impacted, low buffering
capacity
http://www.maine.gov/dep/air/acidrain/images/ARAIN1.jpg
http://www.newsroom.ucr.edu/releases/images/257_3.gif
Sulfide oxidation
FeS2 + H2O + O2  4H+ + 2SO4-2 + Fe(OH)3
Most metal ores are in sulfide form (ZnS,
PbS, CuS, etc) that oxidizes when
exposed to air in tailings piles once
exhumed from below ground.
Same concept as exposed submerged soil
in coastal zones (acid sulfate soils)
http://www.agf.gov.bc.ca/clad/britannia/images/acid_mine_fig1.jpg
Acid mine drainage
Climatic effects
• Excessive rainfall: Leaching of cations through
the soil profile by rain, weathering of the soil
– Carbonation; hydrolysis; hydration…
• Excessive irrigation: unlikely cause of acidity
since most irrigation occurs in arid or semi-arid
regions with accumulated salts, carbonates, etc
(buffer pH)
– Most irrigated regions are neutral to alkaline (they are
irrigated because there isn’t enough rain to support
crops, therefore the salts and cations don’t leach out
of soils)
Carbonic acid formation
•
forms in rainwater or soil water
CO2 + H2O  H2CO3
H2CO3  H+ + HCO3---------------------------CO2 + H2O  H+ + HCO3-
•
[CO2] is higher in soils than aboveground
•
Most unpolluted rainfall is slightly acidic
As CO2 concentration
increases, proton (H+)
production increases
and pH decreases
Soda pop or
carbonated beverages
have pH 3 - 4
Metal hydrolysis
• Polyvalent metals go through several
hydrolysis steps releasing protons
• Alum (KAl(SO4)2) is a commercial product
for lowering pH
http://www.wou.edu/las/physci/ch412/hydrxn2.jpg
Hydrolysis of Al+3
H2O  OH- + H+
Al+3 + H2O  Al(OH)+2 + H+
Al(OH)2+ + H2O  Al(OH)2+1 + H+
Al(OH)2+1 + H2O  Al(OH)3 + H+
Al(OH)3 + H2O  Al(OH)4-1 + H+
Increasing soil pH
• Burning plant residues or adding ashes
– Wood ashes are a source of K, Ca, Mg CO3’s
• Liming materials (pure calcium carbonate or
dolomitic lime) will increase soil pH.
– Lime is a certified organic product
– Slow-release product. Do not add every year.
– 15-25 lbs lime per 1000 sq ft is recommended
• Gypsum is calcium sulfate.
– It is not a substitute for lime, and has very little effect on
soil pH. Gypsum only improves structure in soils that
have extremely high sodium contents
Lime material
•
•
•
•
•
CaCO3 calcic limestone
CaMg(CO3)2 Dolomite
CaO: Quick lime
Byproducts: ground shells, cement
factory
Gypsum is NOT a liming material, as it
has very slight effect on pH, but can
provide Ca as a nutrient or exchange
with Na
Liming to increase soil pH
• Lime characteristics
– cost
– purity
– speed of effect (fine ground vs coarse)
– ease of handling
• Lime requirement
– depends on pH, CEC and buffer capacity of
the soil
• Lime Application: small amounts split and
incorporated into the soil
To increase pH from 6 to 7 requires more
lime than from 4 to 5
http://wwwlb.aub.edu.lb/~webeco/SIM215acidsoilsandlimimg_files/image002.gif