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Acidity, Alkalinity, & Salinity
NRES 201
Soil Acidity, Alkalinity, & Salinity
Acid-base behavior
Definitions
- Acid = H +^ donor
- Base = H +^ acceptor Dissociation of acids
- Weak acid: incomplete dissociation CH 3 COOH + H 2 O CH 3 COO-^ + H 3 O+
- Strong acid: complete dissociation HCl + H 2 O Cl-^ + H 3 O+
Ionization of water 2H 2 O H 3 O+^ + OH- [H 3 O+][OH-^ ] = 0.00000000000001 = 10 -14^ = K (^) w
Acidity, Alkalinity, & Salinity
pH
Fundamental aspects
- Defined as the negative logarithm of H +^ activity
- Expressed as -log [H +] or -log [H 3 O +]
- For a dilute solution, concentration ≈ activity For pure water (^) ____
- [H 3 O+] = [OH-^ ] = √ 10 -14^ = 10 -
- pH = -log(10 -7^ ) = 7.
The pH scale H 3 O+^ concentration _________________( M )^ pH__ 0.1 = 10 -1^1 0.01 = 10 -2^2
- • • •
- • 0.0000001 = 10 -7^7
- • • •
- • 10 -14^14 Soil pH
- Typical range is from 4 to 8 (^) Source: Brady and Weil (2002)
Acidity, Alkalinity, & Salinity
Significance of soil acidity/alkalinity
Range of crop requirements
Source: Brady and Weil (2002)
Nutrient availability
- Phosphorus
- Highest availability at pH 5.5-6.
- Exchangeable bases
- Ca, Mg, or K deficiency in acid soils
- Micronutrients
- Acid soils better for Fe, Mn, B, Cu, and Zn
- Neutral soils better for Mo Microbial activity
- Availability of N, P, and S
- Mineralization of organic forms
- Transformations of inorganic forms
- Symbiotic N 2 fixation
- Reduced in acid soils
Acidity, Alkalinity, & Salinity
Cation exchange
Source: SSSA Slide Set (1974)
Solubilities
- Alkalinity dissolves organic matter
- Acidity dissolves soil minerals
- Al and Mn toxicities in acid soils
Root distribution
- Acidity often inhibits root growth
Soil structure and tilth
Acidity, Alkalinity, & Salinity
Potential acidity
Defined as
- Soil constituents capable of contributing H +^ ions to the soil solution through
- Ionization
- Dissociation
- Hydrolysis The major form of soil acidity
- And the reason why lime is applied with trucks instead of teaspoons
Sources of potential acidity
Ionizable H
- Clay minerals
- Hydrous oxides
• CO 2
- CO 2 + H 2 O ↔ H 2 CO 3
Si
Al
OH
Si
Al
O-^ + H+
Si Al Fe
OH
Si Al Fe
O-^ + H+
Acidity, Alkalinity, & Salinity
- Organic acids
- From soil organic matter
- From residues Exchangeable H
- H+^ (or H 3 O+) held on exchange sites
- Held more tightly than basic cations Soil aluminum
- The key source of soil acidity at pH ≤ 4.
Properties of aluminum
- Hydrated as a soluble ion
Source: Black (1968)
Acidity, Alkalinity, & Salinity
- Amphoteric
- Solubility
- Forms insoluble compounds
- Toxic to plants
- Toxicity is due to soluble Al
Source: McLean (1976)
Acidity, Alkalinity, & Salinity
Sensitivity of various crops to aluminum
Very low pineapple coffee tea rubber cassava tropical grasses rhododendron azaleas blueberries
Low corn turnips redtop swede cauliflower kale potato
Moderate radishes sorghum cabbage oats rye
High lettuce beets timothy barley legumes tomato
Very high celery carrots
Forms of soil aluminum
- Al in minerals
- The major form of soil Al
- Occurs mainly in the octahedral sheet
- Al hydroxides
- Produced by weathering of Al-bearing minerals
- A major form of Al in tropical soils
- Al phosphates
- From weathering or fertilizer reactions
- Important in tropical soils
Acidity, Alkalinity, & Salinity
- Fertilizers
- Hydrolysis of acidic salts (NH 4 ) 2 SO 4 + H 2 O ↔ 2 NH 4 OH + H 2 SO 4
- Effect on leaching losses
- Increased loss of anions and cations
- Reduced losses through anion uptake or precipitation reactions
- Nitrification
- Produces acidity NH 4 +^ + 2 O 2 ↔ NO 3 -^ + 2 H +
- Neutralizing effect of plant uptake
- Excessive NH 4 +^ applications are acidifying
- Liming and N fertilizer usage Fertilizer_______ lb CaCO__________________________ 3 required/lb N applied Ammoniacal fertilizers Anhydrous NH 3 3. Urea 3. NH 4 NO 3 3. (NH 4 ) 2 SO 4 7. NH 4 H 2 PO 4 7. (NH 4 ) 2 HPO 4 5. Nitrate fertilizers Ca(NO 3 ) 2 0 KNO 3 0
Acidity, Alkalinity, & Salinity
Source: Illinois Agronomy Handbook, 23rd edn.
- Organic residues
- Organic acids released during decomposition
- Sulfur oxidation
- Strip mine spoils can become extremely acidic 4 FeS 2 + 15 O 2 + 14 H 2 O ↔ 8 H 2 SO 4 + 4 Fe(OH) 3 ↓
Acidity, Alkalinity, & Salinity
Buffering due to ionizable H
- Clay minerals and hydrous oxides Si-OH 2 +^ Si-OH SiO-^ + H+
- Organic matter R-COOH ↔ R-COO-^ + H+ R-OH ↔ R-O-^ + H+ Buffering due to Al Al(H 2 O) 6 3+^ + H 2 O ↔ AlOH(H 2 O) 5 2+^ + H+
pH↑ pH↓
pH↑ pH↓
Determination of soil acidity
Active acidity
- Measured as soil pH
- Mix soil with water or a neutral salt solution (0.01 M CaCl 2 or 1 M KCl) in a selected ratio
- Salt solutions:
- Reduce variability
- Lower the pH
- –H + KCl ↔ - –K + H+^ + Cl -
Acidity, Alkalinity, & Salinity
Potential acidity
- Measurements needed:
- Soil pH
- CEC
- Total exchangeable bases (TEB)
- Leach soil with neutral, 1 N NH 4 OAc
- Determine Ca, Mg, K, and Na in leachate
- Calculate:
- % Base saturation = 100 × TEB/CEC
- Exchange acidity (meq/100 g) = CEC - TEB
Estimating soil lime requirement:
Titration
Procedure
- Treat soil sample with a known amount of acid (HCl) or base [Ca(OH) 2 ]
- Allow to equilibrate
- Measure the pH
- Treat other soil samples with different amounts of acid or base
- Plot a titration curve
Acidity, Alkalinity, & Salinity
- Stepwise scaling method
- Step 1 1.0 meq CaCO 3 /100 g × 50 = 50 mg CaCO 3 / g soil
50 mg CaCO 3 /meq
- Stepwise scaling method
- Step 1 1.0 meq CaCO 3 /100 g × 50 = 50 mg CaCO 3 / g soil
- Step 2 50 mg CaCO 3 /100 g soil × 4.536 = 226.8 mg CaCO 3 /lb soil
Upscale from 100 g to 1 lb soil
Acidity, Alkalinity, & Salinity
- Stepwise scaling method
- Step 1 1.0 meq CaCO 3 /100 g × 50 = 50 mg CaCO 3 / g soil
- Step 2 50 mg CaCO 3 /100 g soil × 4.536 = 226.8 mg CaCO 3 /lb soil
- Step 3 226.8 mg CaCO 3 /lb soil × 2 × 10 6 = 4.536 × 10 8 mg CaCO 3 /A-6”
Upscale to 2 million lb soil
- Stepwise scaling method
- Step 1 1.0 meq CaCO 3 /100 g × 50 = 50 mg CaCO 3 / g soil
- Step 2 50 mg CaCO 3 /100 g soil × 4.536 = 226.8 mg CaCO 3 /lb soil
- Step 3 226.8 mg CaCO 3 /lb soil × 2 × 10 6 = 4.536 × 10 8 mg CaCO 3 /A-6”
- Step 4 4.536 × 10 8 mg CaCO 3 /A-6” ÷ 453600 = 1000 lb CaCO 3 /A-6”
Convert from mg to lb
