1. Chapter 11 - Soil pH and Salinity

2. 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

3. Most agricultural soils Extreme pH range for most mineral soils
pH Range Related to Plant Needs
walnut:
Most desirable
carrot:
cucumber:
spinach:
tomato:
white pine:
Very
strong
Strong
Moderate
Slight
Neutral
Acid
Basic 3 4 5 6 7 8 9 10 11 1 2 12 13 14
Most agricultural soils
Extreme pH range for most mineral soils
cranberry:
apple:

4. pH Values by Region in the US
Why do we see such variation???

5. Causes of Acidity soil water + CO2 (gas)
root growth and decaying plant litter
addition by humans
- commercial fertilizers
Nutrients are added to the soil from several sources. The main sources of nutrients for agricultural soils are:
Nutrients that are recycled to the soil from decaying plant litter,
Minerals in soil that gradually dissolve and release nutrients, and
Nutrients added by humans in the form of fertilizers, manure, limestone, and other materials such as sewage sludge, compost, leaves, and food processing wastes.
The smoke stack shown here is another source of nutrients for soils. Interestingly a lot of sulfur used to be supplied to soils from this source, especially combustion of coal. We are now cleaning up our air and much less sulfur is falling onto our soils. We may eventually see a need for more sulfur fertilization.
Lets look a little more closely at each of these nutrient sources.

6. Nutrient Availability Influenced by pH

7. Improper pH can Lead to Nutrient Deficiencies or Toxicity's
normal leaf
iron deficient leaf

8. 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

9. pH Adjustments 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 H2SO H+ + 2SO42-

10. 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

11. Saline Soils high in salts but low in Na+
high EC values (> 4 mm/cm)
high pH (< 8.5)
correct by leaching

12. Sodic Soils high in Na+ but low in other salts
poor structure and drainage
very high pH (> 8.5)
correct by adding gypsum