Practice exam #3 - QUESTION 2

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a)These soils contain iron sulfides, which when dried react with oxygen and form sulfuric acid. Release of this sulfuric acid from the soil can in turn weather other constituents (like clay minerals containing Fe/Al) in the soil and release iron and aluminum (amongst other things). Via reactions with these compounds many excess H+ ions are released, lowering soil acidity.

b)Lime the soil, or do something to contribute to it’s buffering capacity?

I am particularly bad at explaining this stuff…..but I get it! I think….

b) Could you also add organic matter or a clay with high SSA like montmorillonite that would increase the CEC, creating a bigger buffering capacity. Also not sure how you would do it but could you increase the base forming cations (such as Ca, maybe that's what lime does)?

Bennett, increasing the buffering capacity would have minimal impact on the extreme acidity produced by the oxidation of Fe-sulfides (e.g., acid mine drainage). See Blaise's suggestion below.

a) On soils containing iron sulphide, the iron sulphide will oxidize to produce iron oxides and sulphuric acid. Sulphuric acid is one of the most acidic acids in existence, and easily releases its H+ cations into the soil solution, which will drastically lower the soil pH.

b) To reduce H+ release from these soils, you could keep it at saturation for part of the year. This would create anaerobic conditions that cause reductive conditions to dominate that would prevent FeS2 from oxidizing and releasing H+ ions. Alternatively, you could add large amounts of basic compounds like lime to soils to absorb the release H+ ions and offset increases in acidity.

Blaise has a good option, keeping these soils saturated.... preventing oxidation and the release of H⁺ ions in the first place.
As indicated here as well, lime will help nuetralize acidic conditions, but large amounts required.

2.) a.) FeS2 disassociates in solution forming Fe(OH)3 and SO4- releasing H+ ions. b.) The Addition of Na and Mg ions can help slow the release of H+ by forming salts.

a) The reason for that kind of parent material end up with having very low pH is iron sulfide would have the chemical reaction (oxidation). When the soil is unsaturated or drained it reacts with oxygen and releases hydrogen to lower the pH.

b) First: keep the soil saturated Second: add basic ion (C, Mg, K, Na)

Oxidation of pyrite (or iron sulfide) produces soil pH of 3 or 3.5. Hence, to bring such low pH to something that most plants can tolerate (around at least pH 5) would require large quantities of lime (or any other material that would contain Ca and/or Mg) and frequent applications of that material would be needed. Pls note that we do not want to add material containing sodium because of sodium's negative effect of aggregate formation. Sodium is a bad news for soils ;-)

a) Low pH indicates a high concentration of hydrogen ions. The reaction of pyrite with water and oxygen releases just such a preponderance of hydrogen: 2FeS2 + 9O2 + 4H2O -> 8H^+ + 4SO4^2- + 2Fe(OH)3

b) The acidity is a product of the dissolution of pyrite in water. Therefore, programs to limit the amount of water in the soil may reduce the acidity. This could be done by improving drainage, potentially by adding sand or larger size-fraction particles to the soil. Alternately, alkaline compounds such as lime could be added to the soil to raise the pH.

Spencer, please see Maja's explanation about S oxidation above, and refer to lecture #24.So when answering part b) limiting the amount of water would not reduce the oxidation of the sulfides (and thus would not reduce the acidity). In fact, drainage would make the problem worse (not better) - see comments above e.g. from Blaise.