lecture 8 Flashcards
What is an acid?
Any substance that increases the hydrogen ion concentration in the soil solution
What is pH?
The ion product of the concentration of the H+ and OH- ions is a constant (Kw), which at 25C is known to be [H+] x [OH-] = Kw = 10^-14 mol^2L^-2
The very small concentrations of H+ and OH- is expressed by using the negative logarithm of the H+ ion concentration, termed the pH.
What is the importance of soil pH?
- Determines nutrient availability, in particular phosphorus
- Influences cation exchange capacity
- Influences earth worm activity and other fauna in soils
- Strong influence on the microbial community in the soil
- Low soil pH can lead to aluminum toxicity in plants
What is the effect of pH on the surface in weathering alumino-silicates under neutral, acidic, and alkaline conditions?
Neutral conditions (pH 5-6.2): retention of Al octahedra sheet, there are no exchanges
Acid conditions (pH < 5): removal of Al octahedra sheet, there is cation exchange which is driven by H+ ions –> it replaces the Al. In acidic environments (low pH), the weathering of alumino-silicates tends to be more pronounced and accelerated. Hydrogen ions (H⁺) from acidic solutions can replace cations in the mineral structure through cation exchange, leading to leaching of essential elements like calcium, magnesium, and potassium.
Alkaline conditions (pH 9): removal of Al octahedra sheet. Exchange driven by OH-, which reacts with Al surfaces, where the Al can be removed. Produces a negatively charged molecule.
What are the sources of hydrogen ions? Why is this important?
- Carbonic acid (H2CO3)
- Organic acids
- Accumulation of organic matter
- Oxidation of Nitrogen and Sulfur
- Acid rain
- Plant uptake of cations
The pH depends on the H+ concentration in the soil. These are the sources.
Explain the formation of carbonic acid (H2CO3) and the subsequent dissociation:
- When CO2 dissolves in water carbonic acid is formed
- The subsequent dissociation leads to the formation of bicarbonate (HCO3-) and H+ ions
This is important because root respiration and the decomposition of soil organic matter by microorganisms produce high levels of CO2 in soil air. This will lead to the formation of carbonic acid and H+, making the soil more acidic.
Explain the formation of organic acids:
- Microbes generate organic acids as they break down organic matter in the soil
[organic matter] + O2 + H2O –> RCOOH –> RCOO- + H+
- Some are low molecular weight organic acids, such as citric or malic acids, that only weakly dissociate
- Others are more complex and stronger acids, such as the carboxylic and phenolic acid groups in humus
Explain the process of accumulation of organic matter:
Connected to the presence of organic acids: the more organic matter you have, the more organic acids there are
- Organic matter forms soluble complexes with base cations such as Ca2+ and Mg2+
- These complexes can be leached out of the soil profile leading to the loss of nutrients
- Organic matter is a source of H+ ions because it contains numerous acid functional groups from which these ions can dissociate
Explain the process of oxidation of nitrogen:
Oxidation leads to an increase of the acidity
Oxidation reactions generally produce H+ ions, and reduction reactions, tend to consume H+ ions!
This process occurs with nitrogen and is termed nitrification.
NH4+ + 2O2 –> H2O + H+ + H+ + NO3-
How do plants balance the uptake of cations?
Plants uptake cations (such as ammonium molecules) and gain a positive charge. To balance the uptake of cations, plants release H+ ions from their roots, which will have an acidifying effect.
Some plants can balance the uptake of cations by uptaking anions, which does not affect the pH.
What are the characteristics of Al for plants?
Aluminum is toxic for plants. It also has the ability to hydrolyze H2O and reacts with resulting OH- Therefore, it will release a lot of H+ ions into the soil solution - one aluminum can produce 3H+ ions.
What are the 3 principle pools of soil acidity and their characteristics?
- Active acidity: defined by the H+ ion activity in the soil solution; it has a direct affect on plant uptake, plant roots, earthworm activity; very small pool; extremely important, determines the solubility of many substances
- Exchangeable acidity: ions on the surface of the clay mineral; associates with exchangeable aluminum and hydrogen ions; large pool; ions (such as Al) can be released into the soil solution by cation exchange
- Residual acidity: associated with hydrogen and aluminum ions that are bound in nonexchangeable forms by organic matter and clays; important for the formation of secondary minerals
What is the pH range of soils in humid regions?
pH 5 - pH 7
Explain in what pH ranges the following nutrients are most available:
1. Fungi
2. Bacteria
3. Nitrogen
4. Phosphorus
5. Iron
- Fungi: they can occur at any pH
- Bacteria: prefer slightly acidic or alkaline soils, but don’t thrive in very acidic conditions.
- Nitrogen, same as bacteria
- Phosphorus: highest availability at a pH between 6-7 and declines in availability if it is too acidic or too alkaline
- Iron: most available in acidic condition, less so the more alkaline it gets
What is the impact of low or high pH on the CEC?
- At low pH (acidic), the CEC decreases
- H+ ions displace exchangeable cations attached to the negative parts of clay colloids
- the cation bonds with the anions from the acid
- The nutrients from the clay colloids are leached from the soil and become unavailable to be taken up by plants - At high pH (alkaline), the CEC increases
- The OH- ions remove the H+ ions from the hydroxides groups on the clay
- A negative charge is gained which increases the CEC in the soil
