Soil acidity

Question 1

what is an acid?

Answer 1

any substance that increases the hydrogen ion concentration in the soil solution

Question 2

how are H+ ions provided?

Answer 2

through the ionization of water (dividing hydroxides and hydrogen ions)

Question 3

what is pH?

Answer 3

the negative logarithm of the H+ ion concentration

Question 4

how does pH determine nutrient availability in soil?

Answer 4

nutrients’ presence depends on the pH (usually cant handle very acidic), especially phosphorus which is most available in pH 6-7, otherwise it reacts strongly with cations (Ca, Mg) in alkaline conditions and Fe and Al in acidic conditions

Question 5

what dangerous thing happens when soil is acidic?

Answer 5

aluminum concentration increases and causes toxicity for plants

Question 6

what 5 influences does soil pH have?

Answer 6

1. determines nutrient availability, in particular, phosphorus
2. influences cation exchange capacity
3. influences earth worm activity and other fauna in soils
4. strong influence on the microbial community in soil
5. low pH can lead to aluminum toxicity for plants

Question 7

what is the relationship between Al and pH?

Answer 7

Aluminum concentration goes to 0 at pH of 5 until pH 6.2 when it increases again

Question 8

what is the relationship of Fe and pH?

Answer 8

decreases until 6 then it is not soluble in soil anymore (really high at low pH)

Question 9

what are 6 sources of hydrogen ions?

Answer 9

• carbonic acid
• organic acids
• accumulation of organic matter
• oxidation of nitrogen and sulfur
• acid rain
• plant uptake of cations

Question 10

how is carbonic acid formed?

Answer 10

when CO2 is dissolved in water

Question 11

how are organic acids formed?

Answer 11

microbes generate organic acids as they break down organic matter in the soil

Question 12

what does having low molecular weight mean?

Answer 12

dissociate less frequently

Question 13

how can accumulation of organic matter influence soil pH?

Answer 13

accumulation of dissolved organic matter which form complexes with cations, those cations are then leached out of the system

Question 14

how does the oxidation of nitrogen influence soil pH?

Answer 14

nitrification releases 2 hydrogens for each ammonium molecule

Question 15

how does acid rain affect soil pH?

Answer 15

combustion of fossil fuels leads to nitrogen/sulfur oxides that react with oxygen and UV to create sulfuric acid and nitric acid in water droplets which dissociate and release hydrogen ions

Question 16

how does aluminum toxicity work?

Answer 16

• at edge of mineral, there is an aluminum octahedral
• when this mineral is exposed to H, it will react with the edge of the molecule to create water molecules -> exposes aluminum at the edge of the molecule
• aluminum at the edge will be in equilibrium with aluminum in soil solution -> the more al at edge, the larger the concentration of al in soil solution
• al can hydrolyze water and produce 3 H+ ions

Question 17

what are the three pools of acidity?

Answer 17

1. active acidity (amount of H+ ions in soil solution)
2. exchangeable acidity (exchangeable al and H+ ions on surface of clay minerals or organic matter)
3. residual acidity (Al and H ions that are bound in nonexchangeable forms)

Question 18

what does soil pH look like geographically?

Answer 18

• low pH in rainy places (leaching of base cations)
• low pH in temperate areas (organic acids dissociated due to accumulation of OM and low temperatures -> humus)