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Soil pH (Acidity measurement - soil pH test (Common extractants (Distilled…

- - - - Aids exchange of cations and protons
      - Aids adsorption of anions (OH- more easily adsorbed)
      - pH decreases and soil becomes more acidic
- - - - New Zealand soils have a natural pH between 4.5-7
      - Different arable and horticultural crops require different optimum pH
      - Optimum for pasture about 5.5-6.5
    - - At neutral pH Al(OH) 2+ or Al(OH)2 + adsorbed to clay minerals
      - As soil becomes acidic Al3+ released into soil solution
      - Toxic to root development
- - - - Increase ionic strength decreases pH
      - Nitrification by microbes
  - - - Limestone CaCO3 CaCO3 + H2O -> Ca2+ HCO3- + OH-
      - Burnt lime CaO CaO + H2O -> Ca(OH)2
      - Slaked lime Ca(OH)2 Ca(OH)2 -> Ca2+ + 2OH-
    - - Neutralisation of acidity in solution (active acidity) H+ + OH- -> H2O
      - Displacement of acidity off surfaces - followed by neutralization in solution or leached out of soil - decreases reserve acidity
    - - Target pH for mineral soils is 5.8-6.1 based on lime trials over many years on different soils
      - Lime responses depend on many factors - temporal variability, limiting nutrients etc.
      - A well-buffered soil becomes acidic more slowly than a weakly buffered soil but will require more lime to increase its pH
      - 1 tonne lime per ha will raise the pH of pasture by approx 0.1 units
      - Lime is not a substitute for other fertilisers - but changes the availability of nutrients
      - Amount of lime to add
        
        Soil pH
        
        Target pH
        
        pH buffer capacity
        
        Soil management
        
        Money to spend
      - Benefits
        
        Improved structure - by increasing activity of earthworms and formation of aggregates
        
        Microbial activity - nutrient cycling, legume functioning etc
        
        Nutrient availability