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