Soils Exam 2 - Lectures 17 and 18

What are the three different pools of soil acidity, and where are they found?

1. Active acidity – hydrogen (H+) and aluminum (Al+3) in the soil solution.

2. Salt replaceable (exchangeable) acidity – hydrogen and aluminum easily exchangeable in a simple salt solution (on the CEC).

3. Residual acidity – that H and Al that has the potential to come into the soil solution.

three different terms for residual acidity

• Residual acidity

• Reserve acidity

• Potential acidity

Are the soil pH strips (paper ones) that you buy in stores a reliable measure of soil pH?

no.

reactions for how free aluminum in the soil solution creates hydrogen ions

Al+3 + H2O ← → AlOH+2 + H+

(this occurs in acid soils, and the result is the H+ released into the soil solution)

What happens to free aluminum as soil pH is increased?

Al+3 <__> AlOH+2 <__> Al(OH)2+ <__> Al(OH)3

Soil pH is increasing (adding OH’s) →

reasons soils become acidic over time

1) removal of bases

2) application of ammonium containing fertilizers

3) acid parent material

4) organic acids from root respiration and OM decomp

5) acid rain

What is the relationship between pyrite, air, water and acid mine drainage?

acid mine drainage forms when pyrite (an iron sulfide mineral) is exposed to air (oxygen) and water, triggering a chemical oxidation process that produces sulfuric acid and dissolved heavy metals

At each soil pH, what is the major mechanism that controls soil pH?

2-4 — Oxidation of S minerals, pyrite (FeS2)

4-5.5 — Exchangeable hydrogen and aluminum

5.5-6.8 — Exchangeable hydrogen

6.8-7.2 — Dissolved carbon dioxide

7.2-8.5 — Carbonate dissolution/CaCO3

8.5-10.5 — Exchangeable sodium/dissolution of sodium carbonate

What kind of scale does pH have? Logarithmic or arithmetic?

logarithmic; a pH of 5.0 is ten times more acid that one with a pH of 6.0 (multiply by 10 each time)

how to answer: If the concentration of hydroxyl ions is 10^-8 , what is the soil pH?

1. find the pOH — pOH = -log(10^#)

2. find the pH — the sum of pOH and pH ALWAYS equals 14 → pH = 14 - pOH

What is the total range of soil pH?

4.5—7 (0—14)

What two things (groups of things) MUST a liming material contain?

• Ca or Mg

• sufficient carbonates or oxides to raise soil pH

Dolomite/dolomitic limestone characteristics

mineral that is 40-45% magnesium carbonate and 50-55% calcium carbonate.

Dolomitic limestone - dolomite with impurities

Common in AL!!!

Calcite characteristics

CaCO3 – the mineral form

• Calcite is found in many materials that have a liming effect, including eggshells, marble, limestone and chalk.

• 100% CaCO3 is the standard against which we compare all liming materials.

Marl characteristics

Natural loosely consolidated mix of shell, soil and calcium carbonate.

Calcium oxide (CaO) characteristics

• Quicklime, Burnt Lime

• Portland cement

• Formed after rapidly after heating limestone to drive off the CO2.

• If calcium oxide is mixed with water, it becomes slaked lime, a viscous, slurry material that is highly suitable for mortar applications.

• Very caustic, difficult to handle.

• Fast acting.

• Because of the caustic nature long used for drying, dessication.

• Disposal of cadavers.

• Outhouses.

Calcium hydroxide characteristics

• Hydrated lime – Ca(OH)2

• May also see this called ‘slaked’ lime

• Also caustic

• Also called ‘builder’s lime’

• Fast acting

Wood ash characteristics

• Burnt wood contains oxides of Ca and Mg

• Fly ash – result from the industrial process

Slag characteristics

• Byproduct of the manufacture of pig iron from iron ore.

• Has a liming value

• Also contains Si, Mg and Ca

• Often called Calcium Silicate Slag

What is nixtamalization? How did it effect health (pellagra) in the south, several hundred years ago?

• Cooking/soaking of grain (usually corn) in lime water.

• Dried field corn + Ca(OH)2 in water.

European settlers did not always adopt the nixtamalization process, except in the case of hominy grits, though maize became a staple among the poor of the southern states. This led to endemic pellagra in poor populations throughout the southern US in the early 20th century.

the steps for how lime works to increase soil pH

1. Add lime to the soil

2. Lime reacts to form calcium bicarbonate

3. Ca in the Ca bicarbonate replaces H on exchange sites

4. A Ca (+2 charge) replaces two H

5. Displaced H react with bicarbonate to form carbonic acid

6. Carbonic acid breaks down into water and carbon dioxide

What is pellagra? When and where in the US was it a problem, and who was affected?

a disease caused by low levels of niacin, also known as vitamin B-3. It’s marked by dementia, diarrhea, and dermatitis, also known as “the three Ds”

European settlers did not always adopt the nixtamalization process, except in the case of hominy grits, though maize became a staple among the poor of the southern states. This led to endemic pellagra in poor populations throughout the southern US in the early 20th century.

Can you really apply a lot less liquid lime, and get the same changes in soil pH as applying ground ag lime?

no.