Chapter 11 - Soil pH and Salinity

Soil pH - WHAT is it and WHY is it Important?

• pH is the concentration of hydrogen ions (H+) in the soil solution

• pH affects the availability of nutrients for plant uptake

pH Range Related to Plant Needs

walnut: 6.0-8.0

Most desirable

carrot: 5.5-7.0cucumber: 5.5-7.0

spinach: 6.0-7.5tomato: 5.5-7.5

white pine: 4.5-6.0

Verystrong Strong Moderate Slight Slight Moderate Strong

Verystrong

NeutralAcid Basic

3 4 5 6 7 8 9 10 111 2 12 13 14

Most agricultural soils

Extreme pH range for most mineral soils

cranberry:4.2-5.0

apple: 5.0-6.5

pH Values by Region in the US

Why do we see such variation???

Causes of Acidity

• soil water + CO2 (gas)

• root growth and decaying plant litter

• addition by humans

- commercial fertilizers

Nutrient Availability Influenced by pH

Improper pH can Lead to Nutrient Deficiencies or Toxicity's

normal leaf

iron deficient leaf

Active vs. Reserve Acidity

• active acidity

- H+ ions in soil solution

- what is actually measured by a pH test

• reserve acidity

- H+ and Al+++ ions adsorbed to soil colloids

• add a liming material to raise the pH

CaCO3 + 2H+ Ca2+ + H2O + CO2 (gas)

- calcitic limestone

- dolomitic limestone

- quicklime

- slaked lime

• add sulfur to lower the pH

2S + 3O2 + 2H2O 2H2SO4 4H+ + 2SO42-

pH Adjustments

Benefits of Liming

• reduces the solubility and potential toxicity of Al and Mn

• supplies the essential elements Ca and/or Mg, which are generally low in very acid soils

• increases the availability of several essential nutrients

• stimulates microbial activity (such as nitrification) in the soil

• improves symbiotic nitrogen fixation by legumes

• high in salts but low in Na+

• high EC values (> 4 mm/cm)

• high pH (< 8.5)

• correct by leaching

Saline Soils

Sodic Soils

• high in Na+ but low in other salts

• poor structure and drainage

• very high pH (> 8.5)

• correct by adding gypsum