11/12: Sources of Acidity and Alkalinity in Soils

Question 1

What 4 general processes produce H+ in soils?

Answer 1

Chemical reactions
Biological activity
Plant nutrient uptake
Presence of acid cations

Question 2

Chemical reactions

In regards to producing ions in soils

Answer 2

Changes in pH are regulated by chemical reactions that involve the release of H+ and OH- ions

Question 3

Biological activity

In regards to producing H+ in soils

Answer 3

Most soil organisms respire oxygen and release CO2
Are reversible reactions

Question 4

Plant nutrient uptake

In regards to producing H+ in soils

Answer 4

Plant roots release charged ions to take up other ions

Question 5

Presence of acid cations

In regards to producing H+ in soils

Answer 5

Al3+ and Fe3+ ions have a strong tendency to hydrolyze water and release H+ causing soil acidity

(one Al3+ can release up to 3 H+ ions when it reacts with water)

Question 6

Acid cation examples

Answer 6

Al3+ and Fe3+
Great hydrolyzing power (make the soil more acidic)

Question 7

Basic cation examples

Answer 7

Ca2+, Mg2+, K+, and Na+
No hydrolyzing power

Question 8

Natural causes of soil acidity

Answer 8

High rainfall
Weathering
Parent material

Question 9

Human-induced causes of soil acidity

Answer 9

Nitrogen fertilization
Acid deposition from the atmosphere
Exposure of potential acid materials

Question 10

High rainfall impact on soil acidity

Answer 10

A natural cause of soil acidity

Normally slightly acidic because the atmosphere is also slightly acidic

Areas with high annual precipitation have lower soil pH

Question 11

Weathering impact on soil acidity

Answer 11

A natural cause of soil acidity

Results in the loss of basic cations (thereby acid cations increase in relative concentration)

Although weathering by itself consumes acidity, leaching will lead to acidity

Question 12

Parent material impact on soil acidity

Answer 12

A natural cause of soil acidity

If there are more acid vs. basic cations, more acidic

Question 13

Nitrogen fertilization impact on soil acidity

Answer 13

A human-induced cause of soil acidity

Applying too much nitrogen to the soil will increase acidity

Normally plants can neutralize the acidity when they take up a NO3-

Question 14

Acid deposition from the atmosphere impact on soil acidity

Answer 14

A human-induced cause of soil acidity

Acid rain. N and S gasses emitted after the combustion of fossil fuels combine with water and create acidity

Question 15

Natural causes of soil alkalinity

Answer 15

Low rainfall
Calcareous parent material (CaCO3)

Question 16

Human-induced causes of soil alkalinity

Answer 16

Overliming

Question 17

Exposure of potential acid sulfate material

Answer 17

A human-induced cause of soil acidity

Drainage of wetlands can create acid mine drainage, which is extremely acid and toxic