Soil pH and pH Management

© 2007 Donald G. McGahan (aka soilman) All Rights Reserved

Of all soil chemical properties, pH is the easiest to measure and it provides much information about soil properties. Soil pH is a master variable that affects a wide range of chemical, biological and physical soil properties. Soil pH is easily measured using a pH meter (about $50 for a hand-held unit).

Soil Acidity and Basicity

The two components regulating soil acidity/basicity are the hydrogen ion (H⁺) and the hydroxide ion (OH⁻).

H₂O → H⁺ + OH¯
or
HOH → H⁺ + OH¯

When the pH is 7 the hydrogen concentration (H⁺) is 0.0000001 and the pOH, or hydroxyl ion concentration, is 7 and are therefore equal. This is then refered to as nutural.

The pH scale:

  • pH = -log H⁺
  • pH = -log 10-7
  • pH = 7

pH is reported on a log scale; thus pH = 4 is 10 times more acidic than pH = 5.

H⁺ > OH¯ ∴ pH < 7 and the solution is considered acid
H⁺ = OH¯ ∴ pH = 7 and the solution is considered neutral
H⁺ < OH¯ ∴ pH > 7 and the solution is considered basic or alkaline

Soil Reactivity Classes

Reactivity is pH of the soil solution taken as 1 part soil to 1 part water (1:1) on a mass basis.

The scaler values of pH are placed into classes and the classes named according to the following list.

  • <3.5 ultra acid
  • 3.5–4.4 extremely acidic
  • 4.5–5.0 very strongly acidic
  • 5.1–5.5 strongly acidic
  • 5.6–6.0 medium acidic
  • 6.1–6.5 slightly acidic
  • 6.6–7.3 neutral
  • 7.4–7.8 mildly alkaline
  • 7.9–8.4 moderately alkaline
  • 8.5–9.0 strongly alkaline
  • >9.0 very strongly alkaline

These classes have practical applications.

Sources of Acid (H⁺)

  1. CO₂ - carbonic acid (CO₂↑ + H₂O ⥂ H⁺ + HCO₃¯; pH about 5.7 non-polluted rain water in equilibrium with atmospheric CO₂).
  2. Organic acids -produced by vegetation (root exudates) and microbial processes.
  3. Acid rain - primarily nitric acid (HNO₃), sulfuric acid (H₂SO₄), and ammonium (NH₄⁺).
  4. Nitrification: NH₄⁺ → H⁺ + NO₃¯ + energy (reaction is performed by bacteria to obtain energy).
    1. natural sources of NH₄⁺ from microbial decomposition of organic matter.
    2. anthropogenic sources from NH₄⁺ fertilizer and cattle manure.

Sources of Base (OH¯)

  1. Chemical weathering reactions (Hydrolysis) K-feldspar + H₂O → H-feldspar + K⁺ + OH¯.
  2. Irrigation water may have OH¯ > H⁺ (therefore pH > 7).
  3. Additions of lime CaCO₃ + H₂O + Ca²⁺ + HCO₃¯ + OH¯.

What controls the pH of a non-managed soil?

Primarily the degree of leaching which is affected by climate (leaching = rainfall – evapotranspiration) and time. Water moving through soil that contains hydrogen cations from various weak acids, such as carbonic acid and organic acids, bases (Ca, Mg, K & Na) are leached from the soil, the soil becomes more acidic. In dry climates, bases accumulate and the soil pH becomes basic. Parent material also influences soil pH through its ability to supply bases to the soil.

How does pH affect soil chemical & biological properties!

  1. Supply of Ca, Mg and K (a function of base saturation).
    1. These elements are low in acid soils that have low base saturation (they have been leached away).
  2. Aluminum toxicity.
    1. Occurs at low pH values
      • Generally pH < 5.0.
    2. Can add lime CaCO₃ to soils to eliminate Al toxicity.
  3. Next to N, P is generally the most limiting nutrient with regard to plant growth. P is most available at pH values near 6.5.
    1. High pH values forms a solid.
      • Ca²⁺ + PO₄³¯ → Ca₃(PO₄)₂ (solid).
    2. Low pH values forms a solid.
      • Al + ⅓PO₄³¯ + 2 H₂O = AlPO₄ ⋅ 2 H₂O (solid).
      • Fe + ⅓PO₄³¯ + 2 H₂O = FePO₄ ⋅ 2 H₂O (solid).
  4. Micronutrient availability (e.g., Fe, Cu, Zn, Mn)
    1. Forms insoluble hydroxides (-OH) at high pH values.
    2. Mrak Test
      • saturate string with metal; if leaf greens up at contact with string, that metal nutrient is limiting.
    3. Can adjust pH (acidify) to make micronutrients more available or fertilize with micronutrients.
  5. pH affects the composition and activity of soil flora and fauna.
    1. fungi populations are relatively constant across the pH range common to most soils.
    2. at low pH, bacteria populations are decreased.
    3. at low pH, earthworm activity is greatly decreased.
  6. Cation and anion exchange capacity (CEC and AEC).
    1. change due to pH dependent charge.
    2. the higher the pH, the greater the CEC: and the lower the AEC.

pH Management

TOO ACID

  1. Add lime, CaCO₃, or CaO (fly ash).
    1. Removes H⁺ and adds Ca²⁺; thus lime (i) increases base saturation, (ii) increases calcium nutrient availability, and (iii) decreases aluminum toxicity potential.

Soil behaves like a weak acid

In acid soils the clay, OM, roots, and cation exchange complex absorb Al³⁺ and the absorbed Al³⁺ is in equilibrium with [Al³⁺] solution. Where [ ] is depicting concentration.

Hydrolysis is what happens if the Al³⁺ comes off the exchange complex.

Aluminum hydrolysis:

  • Al³⁺ + H₂O → Al(OH)²⁺ + H⁺ (pH about 4.5–5)
  • Al(OH)²⁺ + H₂O → Al(OH)₂⁺ + H⁺ (pH about 5–6.5)
  • Al(OH)₂⁺ + H₂O → Al(OH)₃⁰ + H⁺ (pH about 6.5–8.5)
  • Net reaction: Al³⁺ + 3H₂O → Al(OH)₃⁰ + 3H⁺

We add the lime (CaCO₃) as a base.

Lime hydrolyzes in the soil to form OH¯.

CaCO₃ + H₂O = Ca²⁺ + HCO₃¯ + OH¯

The hydroxyls, OH¯, neutralize the H⁺ that are produced from aluminum hydrolysis.

If sufficient aluminum hydroxide exist it can exceed the solubility product of a solid form and thereby precipite out of solution to a form solid: Al(OH)3(l) → Al(OH)3(s)

As the concentration of Al³⁺ in solution decreases, more Al³⁺ desorbs to replace the solution Al³⁺ and attempt to come to the new equilibrium. Then the Ca²⁺, which has a higher concentration since adding CaCO₃, is now free to replace the Al³⁺ on the exchange complex. Do note that not only the concentration of the constituent elements are important, but also the relative activity of H⁺ and OH¯.

Each cation in solution is represented on the exchange complex according to its concentration in solution, [ ], modified by selectivity on the exchange complex. Selectivity for representation on the exchange complex is termed the lyotropic series.

The lyotropic series encapsulates that when two ions with the (i) same concentration are in the soil solution (ii) the ion with the smaller hydrated radii will be preferred on the exchange complex and that if the hydrated radii is similar and the concentration in solution is similar that the ion with (iii) the greater charge is preferred on the exchange complex.

Lyotropic series: Al³⁺ > H⁺ > Ca²⁺ > Mg²⁺ > K⁺ = NH₄⁺ > Na⁺

When you think buffering capacity of soils, you should think of it as being tied to CEC; therefore to organic material, mineral type, mineral amounts, and mineral size.

So, we can think of acidity as being placed into two broad bins (1) some in solution, and (2) some on the exchange complex. Two terms are of importance:

  • Potential Acidity = Exchange Complex (for short term reactions)
  • Active Acidity = Solution

Soil Buffering Capacity

The buffer capacity is the ability of ions associated with the solid phase to buffer changes in ions concentration in the solution phase. In acid soils, buffering refers to the ability of the exchangeable-Al³⁺, exchangeable-H⁺, and hydroxy-Al³⁺ (Al(OH)²⁺, Al(OH)₂⁺, Al(OH)₄¯) to maintain a certain concentration of H⁺ in solution. The amount of H⁺ in the soil solution of a soil with a pH of 6.0, for example, is extremely small compared with the non-dissociated H⁺ adsorbed and the amount of aluminum that can hydrolyze to produce H⁺. Neutralization of the active, or solution, H⁺ results in rapid replacement of H⁺ from the relatively large amount of H⁺ associated with the solid phase. Thus, the soil exhibits great resistance to undergo a pH change.

Rules of thumb:

  • Soils with more clay size separates lead to more buffering capacity.
  • Soils with more organic matter lead to more buffering capacity.

Reactivity Inferences

With ample time and intensity of weathering such as seen in soils in many humid regions changes in both the mineralogical and chemical properties and changes in soil pH are evident. Minimally weathered soils tend to be alkaline or slightly acid and fertile. By contrast, intensively weathered soils are very acid and infertile. Some 'Rules of Thumb' of the major processes and changes that produce various ranges in soil pH and accompanying properties is as follows.

  • In the pH range 7.5 to 8.3, and possibly somewhat above, soil pH is controlled mainly by carbonate hydrolysis.
  • Where soils are calcareous carbonates must be removed by leaching prior to the soil having a neutral or acid reactivity.
  • Soils that are naturally in the neutral range tend to live in regions of limited precipitation and the reactivity is a result of a relative balance between H⁺ and OH¯. The acid inputs are from biological respiration, organic matter mineralization, and precipitation. These acid inputs are offset, or balanced, by the basic inputs as a result of mineral weathering.
  • In humid regions, basic cations tend to decrease over time and acid cations increase.
    • Between 7 and 5.5 pH, the H⁺ is mainly supplied by hydroxy-Al³⁺ hydrolysis.
    • Between 5.5 and 4 pH, the H⁺ is mainly supplied by exchangeable-Al³⁺ hydrolysis
    • As organic matter increases the potential for significant contributions of H⁺ from organic matter exchangeable hydrogen.
  • Advanced stages of soils intensely weathered can be expected to exhibit:
    1. Low Cation Exchange Capacity
    2. High Cation Exchange Complex saturation with Al³⁺
    3. Low base cation (Ca²⁺, Mg²⁺, K⁺, Na⁺) Cation Exchange Complex occupancy
    4. Intensively weathered soils tend to produce aluminum toxicity and deficiencies of magnesium and calcium for plant growth.
  • In the most acid soils, where the pH < 4, H⁺ strongly exists as potential and active acidity.