Soil acidification Flashcards
(33 cards)
pH i surt nedbør
pH < 5,0
Ofte 4,0–4,5
kan være så lav som 3,0–4,0
What are the 4 most common base cations (BC)?
Calcium (Ca²⁺)
Magnesium (Mg²⁺)
Potassium (K⁺)
Sodium (Na⁺)
What makes Al3+ an acid cation?
Hydrolyses water, releasing 3H+
Reagerer med vann og danner hydroksylkomplekser, som ytterligere senker pH.
pH is a variable for these 6 processes
- [Nutrients i water]
- [Toxic elements in water]
- Plant production and water use;
- Soil biological functions (nitrogen fixation)
- Decomposition
- N turnover
- N2O emission
Acidic soils is common in areas with
precipitation surplus
Lots of rain
Low pH = …. = lots of protons (H⁺)
high acidity
Positive repels positive, so cations become more “active” and available
Since both H⁺ and cations are positively charged, they compete for the same negatively charged binding sites.
H⁺ displaces cations from binding sites
➔ More cations are free in solution, making them more “bioavailable” (to plants, organisms) and potentially more toxic.
Al3+ is solubilized at what pH?
pH < 5
How much of the world’s potential arable land is acidic
50 %
Why worry about acid soils?
(6 points)
o Aluminium (Al) is solubilized in ionic form (Al3+) at soil pH < 5
o P limitation in acid soils
o Decreased microbial biomass and diversity
o Decreased mineralization rate of organic matter
o Decreased nitrification rate (NH4+ + 2O2 = NO3- + 2 H+ + H2O)
o Increased mobility of heavy metals
pH can be expressed as:
- H+ activity
- Exchangeable acidity
- Base saturation (CEC)
- Potential acidity
CEC is operationally defined, with different extractants:
- 1 M NH4Ac (buffered at pH 7)
- 1 M NH4NO3 (neutral salt, measured at soil pH)
Bruker ulike løsninger for å “måle” aciditet basert på hvor man er i verden
NH4Ac is also used to test biavailable fraction of trace metals
Echangeable base cations
Na + K + Ca + Mg (in mmolc/kg)
BS =
BS = ((∑exchangeable base cations) / CEC) * 100%
What are the exchangeble kations H+ and Al3+ called?
Exchangeable acidity
Percentage of Exchangeable acidity =
Percentage of Exchangeable acidity = 100% - BS
100 % = Base cations + Al³⁺ & Fe³⁺
At what pH is base saturation close to 100%.
pH 7
Base saturation is the same as high CEC
Ways to measure pH
– H2O pH(H2O)
– 0.01 M CaCl2 pH(CaCl2)
– 1 M KCl pH(KCl)
meq/kg
milliequivalents per kilogram
represents the amount of an ion that can react with or replace one mole of hydrogen ions (H⁺) in a reaction
Used to express cation exchange capacity (CEC), which determines the soil’s ability to hold and exchange nutrients.
What is soil acidification?
- ↓ Base cations (uptake or leaching):
- ↓ pH
- ↓ Acid neutralising capasity
- ↓ Base saturation (CEC)
- ↑ Exchangeable acidity (Al³⁺ & H⁺)
- ↑ Oxidation
How does soil buffer pH?
Instantaneous:
Quite fast:
Slow, but ultimately:
Instantaneous: Cation exchange
Quite fast: CaCO3 dissolution
Ultimately, but much more slowly
- Mineral weathering (↓ H+ and ↑ base cations)
At what pH are there few exchangeable base cations left?
pH < 5
Al³⁺ is toxic to what?
- Plants
- Aquatic organisms
Which essensial element is limited in acidic soils?
Fosfor (P)
