Soil Acidification

Acid and Alkaline Soils

  • Economic Value of Acid Soils: While often detrimental, acid soils can be economically valuable and support specific crops.
    • Some crops thrive exclusively in acid soils.
    • Examples: Tea, coffee, and rye.

Crops and Acid Soils

  • Tea Plants: Require acid soils for maximum growth and production. All tea production occurs on acid soils.
  • Coffee: Almost exclusively grown on acid soils.
  • Rye: Tolerant to acidic soils but not the most important crop, often grown on acid soils out of necessity.
  • Maize, Rice, and Wheat: The three most grown crops globally. They can grow on acid soils but do not prefer them.

Soil Acidification

  • Inevitable consequence of intensive farming.
  • Input of acidity into soils over time.

Farming Systems and Acidity Input

  • Eucalypt forests: 0.9 (relatively small number of hydrogen ions per space per time).
  • Grazed clover pastures: Broad range depending on clover species and management.
  • Pasture cut for hay:
    • Not fertilized with nitrogen: Broad range.
    • Fertilized with nitrogen: Acidity rate increases substantially, suggesting nitrogen fertilization is not good for avoiding soil acidification.
  • Cereals: Broad range depending on nitrogen fertilization.
  • Legumes: Acidify soil to a greater extent than non-legumes due to nitrogen fixation, which releases hydrogen ions into the soil.
  • Lupines: Example of a legume-based system. Soil acidification occurs but is less severe when lupine is rotated with wheat.
  • Bananas: Extremely acidifying due to high potassium uptake, which results in the release of hydrogen ions into the soil.
  • Tobacco: Negative acidification rates (alkalinization) due to consumption of hydrogen ions.
  • Potassium uptake in bananas leads to high acidification rates, while tobacco cultivation results in alkalinization.

Soil pH and Aluminium Concentration

  • Aluminium concentration is negligible near neutral pH.
  • Starts increasing slightly between pH 5 and 4.5 (in calcium chloride).
  • Increases very strongly below pH 4.5.
  • Aluminium toxicity is generally a problem when pH is between 4 and 5, especially closer to 4.
  • Small changes in pH can cause significant increases in aluminium solubilization and toxicity.

Measuring Soil Acidification

  • Methods: Comparing soil pH over long periods in cultivated vs. uncultivated sites.
  • Agricultural systems tend to acidify soils.
  • Compares sites managed for agriculture with natural sites to determine the rate of acidification over time.
  • Acidification rate formula.

Soil pH Trends

  • Natural sites: Lower pH in topsoil than in subsoil.
  • Farmed sites: pH in topsoil increases due to lime applications, altering the natural tendency.
  • Below 20 cm depth: Acidification trend similar to natural sites but with lower pH values due to farming.
  • Most acidity is observed around 20 cm depth where most roots are present.

Acidification Formula

  • Acidification = \Delta pH \times pH\ Buffering\ Capacity \times Bulk\ Density \times Volume
  • \Delta pH: Change in pH.
  • pH buffering capacity: Soil's resistance to pH change. High buffering capacity requires a large amount of acidity to change pH. Low buffering capacity results in a relatively high pH change with the addition of acidity.
  • Bulk density and volume: Used for unit conversions.

Patricia Hill's Honours Project

  • Used soil acidification formula to produce risk maps for the West River catchment.
  • Examined pH changes over 50 years.
  • The acidification rate is expressed in the amount of lime required to counteract acidity.

Causes of Acidification

  • Input of hydrogen ions into the system.
  • Imbalance between uptake of cations and anions (excess uptake of cations over anions).
  • Cations are associated with exudation of hydrogen, while anions are associated with consumption of hydrogen ions.
  • Ammonium-containing nitrogen fertilizers.
  • Urea breaks down to two ammonium molecules.
  • Disturbance in carbon cycle and nitrogen cycle.

Disturbance in Carbon Cycle

  • If all biomass stays and decomposes in one spot, there is no acidification.
  • Agriculture involves the removal of carbon-containing products, leading to an imbalance in the carbon cycle.
  • Removal of alkalinity results in soil acidification.
  • Soil organic matter may increase or decrease soil pH.
  • Decomposition of organic matter often results in increase in pH, counteracting soil acidification.

H+ ATPase and Ion Transport

  • Primary ion transport: H^+ ATPase pumps hydrogen ions out of the cell, requiring energy from ATP to ADP conversion.
  • Hydrogen ions can enter in co-transport with anions, resulting in consumption of hydrogen ions and increased pH.
  • Cations are taken up as uniport, and the hydrogen ions pumped out by H^+ ATPase stay out, causing acidification.

Legumes, Cations, and Anions

  • Legumes tend to take up more cations than anions due to nitrogen fixation.
  • Biological nitrogen fixation means legumes do not take up as much nitrate as cereals, increasing the cation-anion imbalance.
  • Excess cations in shoot lead to greater acid production.

Root Length and Acidification

  • There is a strong, linear relationship between root length and acidity produced.
  • Increased root length leads to increased acidification.

Legumes vs. Other Crops

  • Legumes are more acidifying due to nitrogen fixation.
  • Clover shows a significant decrease in pH over time compared to ryegrass.

Nitrogen Fertilizers

  • Ammonium-containing fertilizers have a substantial acidification potential, requiring lime to neutralize the acidity produced.
  • Nitrate fertilizers result in increased pH.
  • Ammonium sulfate is highly acidifying due to ammonium and sulfate.

Rothamsted Experiment

  • Long-term experiment showing the effects of nitrogen fertilizers on soil pH.
  • Nitrogen fertilizers contribute to a faster and greater decline in soil pH.
  • Even without nitrogen fertilizers, a slow decline in soil pH can occur over time.