Soil Acidity and pH

Causes, remediation, and measurement

http://www.msichicago.org/ed/learninglabs/imgs/waters_ph_chart.jpg

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

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 (consume H+ and provide Ca2+)

Gypsum (calcium sulfate) is not a liming agent and has very little effect on soil pH. Gypsum may improve structure in soils that have extremely high sodium contents

More ways to increase soil pH

• Denitrification (reduction under anaerobic conditions)

5CH2O + 4NO3- + 4H+ 5CO2 + 2N2 + 7H2O

• Sulfate reduction

SO4-2 + 2CH2O + H+ 2CO2 + HS- + H2O

Factors affecting soil pH measurement1. Soil:Water ratio

– Saturated paste, 1:1, 1:2, 1:5, 1:10• Adding more water dilutes the [H+] and usually

causes a slight rise in pH due to “Dilution effect”

– The amount of increase is a function of soil texture and porosity

– Not linear (More water = more dissociation of H+ from soil surfaces and more hydrolysis of Al+3)

• Important to report how pH was measured (both instrument and method)

– Some pH measurements shouldn’t be compared directly e.g., 1:1 ≠ 1:10 or pHwater ≠ pHKCl

pH

Soil:water ratio

1:1 1:5 1:10

7.7

8.3

8.6

Brazito Sandy Loam pH

CaCl2 7.4

KCl 7.1

http://soilslab.ifas.ufl.edu/general%20graphics/Instrumentation/pHprobe.jpg

pH determines the soil's "active" acidity.

The ESTL uses 1:2 (Soil:water) volume ratio for determining soil pH.

This is the acidity that affects the plant roots.

1. Salts in suspension to measure pH

– Dissolved salts from saline soils– Fertilizers

– CaCl2 or KCl added for pH measurement (pHCaCl2 or pHKCl)

• As salt concentration increases, pH decreases (but not necessarily linearly)

– Displace Al+3 or H+ from exchange sites – Hydrolysis

3. Time of sampling and season

• Before or after summer rains, irrigation, or leaching

• Before or after fertilization

• During warming or cooling trends

• Processing and storage of soil– The pH of a soil that has been dried out and

sitting around will be different from a fresh sample.

Accuracy of pH measurements

• A measured soil pH can not really be much more accurate than +/- 0.5 pH unit. This means worrying about the last decimal point in a soil pH of 6.35 is not worth it when the same soil might vary seasonally between 5.8 and 6.8

• It's not the pH being wrong, it's just the nature of soil.

4. CO2 concentration and pH

• As [CO2] increases, the pH decreases• Formation of carbonic acid• Stirring of sample may decrease pH• Soil [CO2] is MUCH higher than

atmospheric [CO2] • Affects in-situ measurement of soil pH in

calcareous soils.– Measure pH immediately or– Minimize headspace in sample containers

and put on ice

5. Buffering capacity

• Soils high in SOM and clay minerals are more resistant to change in pH

• Sandy soils and highly weathered soils are least buffered

• Base Saturation = exchangeable bases CEC

BS = (exch Ca + Mg + Na + K)

(exch Ca + Mg + Na + K + Al + H)

6. Lime Requirement

• Amount of CaCO3 needed to increase the pH of the soil to an optimum pH

• Depends on soil mineralogy, % clay fraction, % OM, cultivation practices (leaching, fertilization, etc)

• Variety of liming materials

• Only practical to raise pH to ~6 (KCl-extractable acidity is ~0)

Lime material

• CaCO3 calcic limestone

• CaMg(CO3)2 Dolomite

• CaO: Quick lime

• CaOH calcium hydroxide

• Byproducts: ground shells, cement factory waste

• 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