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pH: degree or intensity of acidity and alkalinity
Soil pH also known as “soil reaction”
pH is a "master variable"
Controls the chemistry and reactions of solutions
Solution chemistry is soil chemistry
Also affects chemistry of solid surfaces in contact with solutions
Ion activity is affected by and controlled by pH
pH is equivalent to thermostat that controls activity of furnace or air conditioner
Redox potential is the other master variable
pH affects all soil properties; chemical, physical, biological
Affects chemical forms of elements, nutrient availability, cation exchange capacity (CEC)
Affects mineral solubility, mineral weathering, mineral formation
Affects microbial activity, root growth and development, ion exchange between root tissue and soil solution
pH: negative logarithm of hydrogen ion (H+ ) activity
pOH: negative logarithm of hydroxide ion (OH- ) activity
Acid: substance that donates hydrogen ions (H+ or proton) to another substance Base: substance that donates electrons or hydroxides ( e- or OH- ); accepts hydrogen ions (H+ or protons)
Reacts with acid to form salts
Example: HCl + NaOH + H2 O → H3 O+ + Cl- + Na+ + OH- → NaCl + 2H2 O
When mixed with water, acid ionizes into H+ and accompanying ion
Examples:
Hydrochloric acid: HCl ↔ H+ + Cl-
Acetic acid (“vinegar”): CH3 COOH ↔ H+ + CH3COO-
Strong acid: any acid that ionizes completely in water solution
All hydrogen ions dissociate from accompanying anions
Provides greatest number of hydrogen ions
Example: hydrochloric acid (100% dissociation of H+ )
Weak acid: acid that dissociates incompletely; releases only some of its hydrogen ions into water solution
Significant amount of undisassociated acid remains in solution
Example: acetic acid (1% dissociation of H+ )
¶ Table 1. Relationship Between pH and Concentrations of H+ and OH-
----- H+ concentration ----- pH ----- OH- concentration -----
(hydrogen ions) ? (hydroxyl ions)
mol/L mg/L mg/L mol/L
1 x 100 10,000 0 0.0000000001 1 x 10-14
1 x 10-1 1,000 1 0.000000001 1 x 10-13
1 x 10-2 100 2 0.00000001 1 x 10-12
1 x 10-3 10 3 0.0000001 1 x 10-11
1 x 10-4 1.0 4 0.000001 1 x 10-10
1 x 10-5 0.1 5 0.00001 1 x 10-9
1 x 10-6 0.01 6 0.0001 1 x 10-8
1 x 10-7 0.001 7 0.001 1 x 10-7
1 x 10-8 0.0001 8 0.01 1 x 10-6
1 x 10-9 0.00001 9 0.1 1 x 10-5
1 x 10-10 0.000001 10 1.0 1 x 10-4
1 x 10-11 0.0000001 11 10 1 x 10-3
1 x 10-12 0.00000001 12 100 1 x 10-2
1 x 10-13 0.000000001 13 1,000 1 x 10-1
1 x 10-14 0.0000000001 14 10,000 1 x 100
Expresses relative concentration of H+ and OH- in solution
Degree or intensity of acidity or alkalinity
In soils, ionic activities are generally about the same as ionic concentrations
For simplicity, negative logarithm (pH) used as unit of measure for solution acidity (see Table 1 )
Concentrations of H+ and OH- are equal in pure water at equilibrium
H2O ↔ H+ + OH-
Can express this equilibrium concentration as:
228,000,000,000,000,000 hydrogen ions per gallon
60.22 quadrillion hydrogen ions per liter
hydrogen ion concentration of 0.0000001 gram equivalents per liter
hydrogen ion concentration of 0.0000001 moles per liter (mol/L)
Concentrations of H+ and OH- are both 10-7 mol/L on logarithmic scale
pH and pOH are “shorthand” method for expressing concentrations in pure water
pH 7 = H+ 10-7 mol/L
pOH 7 = OH- 10-7 mol/L
Solution with pH of 7 considered “neutral”
Solution with pH less than 7 considered acidic
Increasing H+ concentration
Decreasing OH- concentration
Solution with pH greater than 7 considered basic or alkaline
Decreasing H+ concentration
Increasing OH- concentration
Pure water is neutral, being neither acid nor base
Because it is logarithmic, one unit change in pH is ten-fold change in concentration
Solution at pH 6 is ten times more acidic than at pH 7
Solution at pH 5 is one hundred times more acidic than at pH 7 (10 * 10 = 100)
Solution at pH 4 is one thousand times more acidic than at pH 7 (10 * 10 *10 = 1,000)
Solution at pH 8 is ten times less acidic than at pH 7 and is ten times more alkaline than at pH 7
Solution at pH 9 is one hundred times more alkaline than at pH 7 (10 * 10 = 100) or one hundred times less acidic than at pH 7 (0.1 * 0.1 = 0.01)
Accepted range of pH scale normally from 0 to 14
pH value can actually be less than 0 for very strong acids; greater than 14 for very strong bases
pH scale is traceable to set of standard solutions
pH of standard solutions established by international agreement
pH represents H+ ion concentration in solution, not potential acidity (or undisassociated H+ )
Hydrochloric acid (HCl) is strong acid, completely dissociates; pH is 1.0
Acetic acid (CH3 COOH) is weak acid, partially dissociates; pH is 3.0
Acetic acid (ph = 3.0) is one hundred times less acidic than hydrochloric acid (pH = 1.0)
Sodium hydroxide (NaOH) is strong base with pH of 13.0
Ammonium hydroxide (NH4 OH) is weak base with pH of 11.0
Ammonium hydroxide is one hundred times less alkaline (or one hundred times more acidic) than sodium hydroxide
0 Battery acid
1 Stomach acid (hydrochloric acid)
2 Lemon juice, vinegar
"Coke", "Pepsi" colas
3 Grapefruit juice, orange juice
Apples, "Dr. Pepper" soda
4 Tomato juice, beer
"7-UP" soda
5 Black coffee, "Pepto-Bismol"
Healthy skin
6 Urine, saliva
Milk, cantaloupe
7 Distilled water
Blood
8 Baking soda, sea water
Egg whites
9 Hand soap
Toothpaste
10 Milk of magnesia
11 Household ammonia, cleaners
Soapy water
12 Hair straightener
13 Bleach, oven cleaner, lye
Liquid drain cleaner
14 Caustic soda (sodium hydroxide)
http://www.soils.umn.edu/academics/classes/soil3416/lecture13.htm Rosen. 2008. SOIL 3416: Plant Nutrients in the Environment. Univ. of Minnesota. Lecture 13 outline accessed 1/15/2008
Tisdale, et. al. 1993. Soil Fertility and Fertilizers (5th ed.). MacMillan Publishing, New York. pg. 189-204, 364-378.
Brady. 1974. Nature and Properties of Soil, 8th ed. MacMillan Publishing, New York. pg. 372-396.
Glossary of soil science terms. https://www.soils.org/publications/soils-glossary#
McClellan. 2007. Soil Nutrient Management for Maui County. Univ. of Hawaii at Manoa. www.ctahr.hawaii.edu/mauisoil/c_acidity.aspx
McCauley, Jones, Jacobsen. 2003. Soil pH and Organic Matter. Nutrient Mgmt. Module No. 8. Montana State Univ., Bozeman MT. 12 pg.
Lippert. 1999. Multi-State Internet Inservice Training: Soil acidity and liming. Clemson Univ. Coop. Ext. Serv., Clemson SC.
Comfort, Frank. 2014. Nutrient Management for Agronomic Crops. Pub. EC155. Univ. of Nebraska Coop Ext. Svc., Lincoln NE pg. 51-58.http://extensionpublications.unl.edu/assets/pdf/ec155.pdf
Delhaize, Ryan. 1995. Aluminum toxicity and tolerance in plants. Plant Physiol. 107:315-321.
Lindsay. 1979. Chemical Equilibria of Soils. Wiley InterScience, New York. pg 40, 180-182.
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