1. Causes, remediation, and implications
Soil Acidity and pH
Causes, remediation, and implications

3. Fig 9.1 The relationship between pH, pOH, and the concentrations of hydrogen and hydroxyl ions in water solution

4. pH is a ‘master’ variable
affects chemical, physical, and biological properties of soils
Nutrient availability (optimum pH for most crops is )
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)

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

6. 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.

7. 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

8. Methods for measuring soil pH

9. 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

13. 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

14. 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+

15. Organic matter Organic acid groups deprotonate: RCOOH = RCOO- + H+

16. 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

17. Acid sulfate soils
Dredging waterways, draining swamps, spoil piles, mine tailings

19. 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').

21. 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

24. 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)

26. Acid mine drainage

27. 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)

28. 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

29. As CO2 concentration increases, proton (H+) production increases and pH decreases
Soda pop or carbonated beverages have pH 3 - 4

30. Metal hydrolysis
Polyvalent metals go through several hydrolysis steps releasing protons
Alum (KAl(SO4)2) is a commercial product for lowering pH

31. 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+

32. 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

33. 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

34. 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

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