SOIL pH SOIL REACTION is the relative ACIDITY or

ALKALINITY of the SOIL

In WATER molecules are in constant FLUX

H2O →→←← H+ + OH- (hydrogen ion) (hydroxyl ion)

pH scale MEASURES the concentration of H+ ions 0 – 1 – 2 – 3 – 4 – 5 – 6 – 7 – 8 – 9 – 10 – 11 – 12 – 13 – 14

Acidic Neutral Alkaline (Basic)

LOGARITHMIC SCALE

Typical solutions

SOIL pH

TYPICAL SOILS - 5.0 - 8.0 pH

0 – 1 – 2 – 3 – 4 – 5 – 6 – 7 – 8 – 9 – 10 – 11 – 12 – 13 – 14

Acidic Neutral Alkaline (Basic)

The most ACIDIC soils ?

The most ALKALINE soils ?

CAUSES of ACIDITY / ALKALINITY

PARENT MATERIAL

YOUNG SOILS retain the pH of the parent material

ACIDIC pH - Granite, Sandstone, Shale, Basalt (slightly acidic)

ALKALINE pH – Limestone CALCAREOUS soils typically have pH of 8.0 – 8.5

As SOILS AGE the pH can CHANGE due to other factors

CAUSES of ACIDITY / ALKALINITY PERCOLATION of RAINFALL

RAINWATER - 5.6 pH (slightly acidic) LEACHES away BASES (Ca+2, etc) and REPLACES with H+ and

Al+3 ions (both acidifiers)

As pH < 6.0 Al+3 HYDROLYSIS occurs which LOWERS pH to ~ 4.0

Al+3 + 2 H2O →→→→ Al (OH-)2+ + 2H+

(hydrolysis)

SEMI-ARID and ARID soils have little or no precipitation, more EVAPORATION means SALTS can accumulate and tend to have ALKALINE soils

CAUSES of ACIDITY / ALKALINITY RESPIRATION (by plant roots and biology)

SUGARS + O2 →→→→ CO2 + H2O

(respiration)

CO2 + H2O →→→→ H2CO3

(Carbonic acid)

H2CO3 →→→→ H+ + HCO3- (acidifiers)

Same thing happens with CO2 and WATER passing through the ATMOSPHERE causing acidic rainwater

CAUSES of ACIDITY / ALKALINITY EXCHANGE of NUTRIENTS by ROOTS

Take in BASE CATIONS (Ca+2, etc) and EXCHANGE with H+ ions (acidifier)

NITRIFICATION (bacteria)

2NH4+ + 3O2 →→→→ 2NO3

- + 8H+

(ammonium) (nitrate)

EFFECTS of pH PLANTS have pH preferences depending on their NATIVE

ENVIRONMENT and what they have EVOLVED with

EFFECTS of pH NUTRIENT

AVAILABILITY

TOXICITIES

SOIL BIOLOGY

Best GROWTH for all organisms near NEUTRAL pH

LIMING SOIL LIMING soil raises pH and

makes NUTRIENTS more available

UPTAKE of nutrients is

INCREASED Effectiveness of FERTILIZER

is ENHANCED Promotes greater

MICROBIAL GROWTH SUPPLIES Ca and/or Mg

LIMING SOIL LIMING MATERIALS

CALCITIC lime - CaCO3 DOLOMITIC lime - CaCO3 + MgCO3 BURNED lime – CaO HYDRATED lime – Ca(OH)2

WOOD ASHES

HOW LIME WORKS LIME interacts with soils in 2 WAYS

Replaces Al+3 + H+ IONS on EXCHANGE sites by MASS ACTION (high concentration)

CaCO3 + 2H2O →→→→ Ca+2 + H2CO3 + 2OH- (hydrolysis)

CONVERTS H+ ions to H2O taking H+ ions out of solution becoming more NEUTRAL pH

2OH- + 2H+ →→→→ 2H2O

APPLICATION of LIME

4 FACTORS effect APPLICATION RATE

PRESENT pH (from soil TEST)

ACTIVE ACIDITY is the pH of the SOIL SOLUTION or how many H+ ions in solution

MEASURED by pH METERS and STRIPS Tells you whether you NEED to LIME

DESIRED pH – depends on CROP grown

APPLICATION of LIME CEC - GREATER the

CEC the MORE LIME is NEEDED to raise pH – called BUFFERING CAPACITY

RESERVE ACIDITY - how many H+ IONS are on the exchange sites

Measured by SMP BUFFER TEST

Tells you HOW MUCH LIME to apply

APPLICATION of LIME LIME MATERIAL USED (See Handout)

TYPE of LIME USED

CALCIUM CARBONATE EQUIVALENT measures the neutralizing power in comparison to pure Calcium Carbonate

PURITY of mined limestone

The FINER it’s ground, the FASTER WORKING it is