KEMASAMAN TANAH
&
pH
Soil Acidity and pH
Causes, remediation, and
implications
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http://www.msichicago.org/ed/learninglabs/imgs/waters_ph_chart.jpg
The relationship between pH, pOH, and the concentrations of hydrogen
and hydroxyl ions in water solution
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pH is a ‘master’ variable
1. affects chemical, physical, and biological
properties of soils
2. Nutrient availability (optimum pH for most
crops is 5.5 - 7)
3. Metal toxicity and solubility e.g., Al toxicity
at pH <5.5 (also Mn solubility and toxicity)
4. Microbial activity (especially important in
the N cycle)
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Most nutrients are highest and most toxins are lower at pH 5.5-7
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http://www.traylorchemical.com/images/faqs/phchart.jpg
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.
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Kemasaman Aktif
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
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Methods for measuring
soil pH
http://img.alibaba.com/photo/51008043/Soil_pH_Meter.jpg
http://www.biconet.com/testing/GIFs/st-t2.jpg
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Kemasaman Cadangan
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
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http://hubcap.clemson.edu/~blpprt/acid_photos/Buffering2.jpg
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http://www.landfood.ubc.ca/soil200/images/15_3acidity.jpg
http://www.landfood.ubc.ca/soil200/interaction/acidity.htm
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Perubahan pH tanah (Alamiah dan Buatan)
• Management dan land use
– Fertilizers, organic matter, and other amendments
– Submergence and subsequent uplift of land exposing
reduced sediments to oxidation processes
• Pencemaran
– Acid rain
– Mining
• Iklim
– Weathering and leaching
– Rainfall leaching
– Plant growth: uptake of cations and release of protons
• Hidrolisis Logam
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Pupuk dapat menurunkan pH tanah
• Oxidation of Ammonium, or ‘Nitrification’
NH4+ + 2O2  NO3- + H2O + 2H+
• Phosphate fertilizers:
Triple superphosphate  hydroxyapatite + H+
Ca(H2PO4)2  Ca5OH(PO4)3 + H+
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Bahan Organik
Organic acid groups deprotonate:
RCOOH = RCOO- + H+
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http://www.rsc.org/ej/gt/2000/b001869o/b001869o-f3.gif
Bahan Amendment yg menurunkan pH tanah
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
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Acid sulfate soils
Dredging waterways, draining swamps, spoil piles, mine tailings
http://www.latrobe.edu.au/envsci/assets/images/publicity/amd2-edit.jpg
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http://web.missouri.edu/~umcsnrsoilwww/290_2003/images/gillpic1.gif
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.
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Hujan Asam
1. Oxidation of sulfur (SO2) in coal (power plants)
and NOx (car exhaust) to sulfuric and nitric acid
2. pH 4-5 (pure rainfall = pH 5.6)
3. Extensive in heavily populated areas with
heavy rainfall (soils already slightly acid)
4. Recent regulations have improved some
conditions.
5. Lakes and forests impacted, low buffering
capacity
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http://www.maine.gov/dep/air/acidrain/images/ARAIN1.jpg
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http://www.newsroom.ucr.edu/releases/images/257_3.gif
Oksidasi Sulfida
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)
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http://www.agf.gov.bc.ca/clad/britannia/images/acid_mine_fig1.jpg
Acid mine drainage
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Efek Iklim
• 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)
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Pembentukan Asam Karbonat
•
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
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As CO2
concentration
increases, proton
(H+) production
increases and pH
decreases
Soda pop or
carbonated
beverages have pH
3-4
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Hidrolisis Logam
1. Polyvalent metals go through several hydrolysis
steps releasing protons
2. Alum (KAl(SO4)2) is a commercial product for
lowering pH
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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+
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Peningkatan pH Tanah
• 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
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Bahan Kapur
1.
2.
3.
4.
CaCO3 calcic limestone
CaMg(CO3)2 Dolomite
CaO: Quick lime
Byproducts: ground shells, cement
factory
5. 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
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Pengapuran untuk menaikkan pH Tanah
• Karakteristik Kapur
–
–
–
–
cost
purity
speed of effect (fine ground vs coarse)
ease of handling
• Kebutuhan Kapur
– depends on pH, CEC and buffer capacity of the
soil
• Aplikasi apur: small amounts split and
incorporated into the soil
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Untuk menaikkan pH dari 6 menjadi 7 memerlukan lebih banyak kapur,
dibandingkan dengan kenaikkan pH dari 4 menjadi 5
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http://wwwlb.aub.edu.lb/~webeco/SIM215acidsoilsandlimimg_files/image002.gif