SOIL CHEMISTRY

SOIL pH

A measure of the degree to which the soil is

Acidic or Basic; also known as . . .

Soil Reaction

SOIL pH

Technically:

expressed as the negative logarithm of the hydrogen ion (H+) concentration

www.johnkyrk.com/pH.html

SOIL pH

At pH 7:

- neutral pH- indicates H+ concentration of . . .

10-7 mol/L

SOIL pH

Since the product of normalities of H+ and OH- ions is always 10-14, OH- is also 10-7 mol/L

mathematically . . .

(10-7)(10-7) = 10-14

pH scale: Relative H+ and OH- ions

SOIL pH

therefore . . .

at neutral pH (pH 7 on a scale from 0 to 14)

amount of H+ ions = amount of OH- ions

solution is neither acidic nor basic

SOIL pH

Most agricultural soils pH range from 4 to 9 . . .

common productive agriculture soils fall

between pH 5 to 8

SOIL pH

2 factors affecting pH are:

1) Amount of precipitation

2) Type of vegetation

SOIL pH

AMOUNT OF PRECIPITATION –

HUMID AREAS (heavy rainfall) ACID SOILS

Ca+, Mg+, and Na+ are leached downward and replaced by H+ ions

Percolation of Acidic Water

Precipitation vs. Evaporation

SOIL pH

TYPE OF VEGETATION –

Vegetation affects pH over period of years:

grasslands – higher pH

forests – lower pH

SOIL pH

The most critical effects of soil pH are indirect . . .

the availability (unavailability) of certain nutrients is greatly influenced by soil pH

SOIL pH

Nutrients UNAVAILABLE at higher pH:

Cu – Copper

Fe – Iron

Mn – Manganese

Zn – Zinc

SOIL pH

additionally . . .

nitrogen-fixing bacteria associated with

legumes are seriously hindered in acid soils

Critical with beans and peas

SOIL pH

ADJUSTING SOIL pH

- Crops vary considerably regarding soil pH preferences

SOIL pH

AZALEA

BLUEBERRY

RHODODENDRON

ACID SOILS

SOIL pH

Mildly Acid Soils:

WheatCorn

SoybeanApple

Tomato

SOIL pH

Neutral to Slightly Acid Soils:

Alfalfa

Lettuce

Muskmelon

pH Range of Crops

SOIL pH

RAISING Soil pH:

ADD LIME

Factors to consider before applying- soil type- how much to raise the pH- fineness of lime- type of lime

How Much Lime?

Four Factors:

1) Present pH

2) Desired pH

3) Cation Exchange Capacity of the soil

4) Liming material to be used

SOIL pH

AMOUNT OF LIME NEEDED TO RAISE pH:

Sandy Loam < Clay LoamWHY?

Cation Exchange Capacity (CEC) of clay particles

Buffering

Effect of Cation Exchange Capacity on Liming:

pH measures only the

Active Acidity in soil solution

When lime is added, the H+ ions neutralized are quickly replaced from the Reserve Acidity

SOIL pH

LOWERING Soil pH:

Add Sulfur

- May be necessary for acid-loving crops- May also benefit potato production by reducing potato scab

pH reactions

CO2 + H2O → H2CO3 → HCO3- + H+

CaCO3 + 2H2O → Ca²+ + H2CO3 + 2OH-

Al³+ + 2H2O → Al(OH)2+ + 2H+

NH4+ + 2O2 → NO3 - + H2O + 2H+

2S + 3O2 + 2H2O → 2 H2SO4 + energy

Soil Salinity

Saline Soils- have high levels of soluble salts (except Na)

- pH is 8.5 or less

- SAR < 13

Sodic Soils- high in sodium

- exchangeable sodium > 15

- pH 8.5-10

Soil Salinity

Saline-Sodic Soils- high levels of soluble salts and sodium

- SAR > 13

- pH < 8.5

Reclaiming Salted Soils

Saline Soils are the most easily reclaimed

- flood to leach out salts

Sodic Soils cannot usually be reclaimed

- first must treat with Gypsum

to remove sodium

- calcium replaces sodium

- sodium sulfate leaches out