RS 478: Lecture 11 Chemical Limitations to Soil Restoration
• Soil pH: soil pH influences metal availability
• When soil is acidic, toxicity is usually associated with Al and Mn, unless the soil
has been contaminated with heavy metals
• In soils that have been contaminated, the metals that are most likely of concern
are Cd, Cu, Pb, Zn, and the metaloid As
• Soil pH influences bacteria and fungi: In general, bacteria have narrower pH
range than fungi and fungi are generally more adapted to acidic soils. Below pH
5.5 fungi tend to be more active in decomposition; pH ≥ 6 bacteria and
actinomycetes are more abundant.
• Non-symbiotic N fixers (Azotobacter and Clostridium) can function over a wide
range of pH (3 to 9) but optimum is near neutral
• Symbiotic N fixers (Rhizobium or Frankia) will function in whatever pH range the
host plant is adapted. However, N-fixing bacteria tend to decrease their activity
as pH decreases.
• Soil pH also influences plants: general phytotoxicity thresholds in soils for most
common metal problems (plant available)
o Zinc = about 400 mg/kg, Copper = about 125 mg/kg, Cadmium = about 10
mg/kg, Lead = about 400 mg/kg, Arsenic = about 100 mg/kg, Boron =
about 50 mg/kg.
• Soil pH generally changes with successional development but plant and soil
communities exert some control. In restoration, we need to be mindful of these
• Later successional (forest) communities tend to have lower pH with more fungi
(adapted to low pH) than bacteria.
• When a disturbed site has sulfur minerals present, oxidation of those sulfides,
like pyrite, can lead to soil acidity that can cause serious problems for successful
• There are two issues: 1) how do we determine the acid generating potential? 2)
how do we treat the problem?
o To determine extent of acidity, we must measure pH, quantify acid
forming sulfur minerals, and measure neutralization potential (so acid
potential is balanced against total neutralization potential—carbonates
and exchangeable bases)
• Must determine current and potential acidity and neutralize both • Raising soil pH on Acidic Soils
o pH can be raised by adding a base or a compound made of limestone –
widely used on mined lands.
o The amount to add (for current acidity) is determined by the Buffer Index
test, which measures the responsiveness of the soil to lime applications.
(Remember that potential (future) acidity must also be addressed when
sulfur minerals are present).
o The soil test lab will usually give recommendations on lime application
rates. Or, soil supplements such as fertilizers, lime, manure, gypsum,
mulch, etc. can ameliorate acidic soils
• Soil amelioration: the process of modifying soils to provide what the native or
existing soils do not naturally provide.
• Lowering pH on Alkaline soils:
o Use of amendments: OM, Elemental Sulfur, Ammonium sulfate and
sulfur-coated urea (N fertilizer), Lime-sulfur solution, Sulfuric acid, Iron
sulfate, Aluminum sulfate
• Measuring soil p