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)

http://www.traylorchemical.com/images/faqs/phchart.jpg

Most nutrients are highest and most toxins are lower at pH 5.5-7

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

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Methods for measuring soil pH

Pools of Acidity: Reserve

Exchangeable or KCl-extractable acidity [(Al+3 + H+) / CEC]

Mostly Al+3 on clay mineral sitesOrganic acid groups:

RCOOH = RCOO- + H+

Residual or non-exchangeable acidity (H+ and Al+3 not displaced by KCl or salt solution):

SOM-Al complexesSolid 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 pHOxidation 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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http://web.missouri.edu/~umcsnrsoilwww/290_2003/images/gillpic1.gif

Acid sulfate soilsDredging waterways, draining swamps, spoil piles, mine tailings

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

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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 waterCO2 + 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

• 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

Liming to increase soil pH

http://wwwlb.aub.edu.lb/~webeco/SIM215acidsoilsandlimimg_files/image002.gif

To increase pH from 6 to 7 requires more lime than from 4 to 5