35 Liming: Raising the pH of Acid Soils

Liming

The pH of problematic acid soils (pH < 5.5) can be raised with alkaline materials called liming agents. The names “liming agent” and “agricultural lime” come from limestone, which is alkaline. Limestone is alkaline because it contains the base carbonate (CO₃^2-), found in two common limestone minerals: calcite (CaCO₃) and dolomite ((Ca,Mg)CO₃). The most common liming agent is agricultural lime , a ground-up limestone product rich in calcite and/or dolomite. Besides adding agricultural lime to raise pH, other alkaline substances can be added, including quicklime (CaO), wood ash, and slag leftover from fossil fuel combustion.

Liming agents are all alkalis (bases containing alkali metals and/or alkaline earth metals) that work by accepting H⁺. For example, each mole of carbonate (CO₃^2-) from calcite (CaCO₃) can accept 2 moles of H⁺ in an acid-base reaction:

CaCO_3(s) + 2H⁺_(aq) ⇌ H₂CO_{3 (aq)} + Ca²⁺_(aq)

The carbonic acid then turns into CO₂ and H₂O in a reaction called decarbonation (the reverse of carbonation):

H₂CO_3(aq) ⇌ ↑CO_2(g) + H₂O_(l)

Notice how the CO₂ is in the gas phase – this means it can easily escape the soil, decreasing the concentrations of the products in the soil and speeding up the reaction (aka “right shift”) via Le Chatelier’s Principle. Although largely beyond the scope of introductory soil science, briefly, Le Chatelier’s Principle states that a chemical reaction experiencing a change in reactant or product concentrations (or temperature or pressure changes) will shift oppositely from the change, back towards equilibrium.

Meanwhile, the Ca²⁺ ions do not directly raise the pH but rather help force more colloid-adsorbed H⁺ into the solution:

H⁺[Colloid]H⁺ + Ca²⁺_(aq) ⇌ [Colloid]Ca²⁺ + 2H⁺_(aq)

Notice how one Ca²⁺ from calcite replaces two H⁺ on the colloidal surfaces to maintain equivalent charges. Similarly, 3 Ca²⁺ ions from the solution replace 2 adsorbed Al³⁺:

2[Colloid]Al³⁺ + 3Ca²⁺_(aq) ⇌ 3[Colloid]Ca²⁺ + 2Al³⁺_(aq)

…and this Al³⁺ in solution then reacts with H₂O to form H⁺ through hydrolysis:

Al³⁺_(aq) + 3H₂O_(l) ⇌ Al(OH)_3(s) + 3H⁺_(aq)

Once in solution, the H⁺ can then react with more carbonate from the calcite (see the acid-base reaction above). Incidentally, adding liming agents like limestone help add Ca and Mg (Ca from calcite and dolomite found in limestone, and Mg from dolomite only). This is beneficial because, as mentioned earlier, problematic acid soils are commonly deficient in these nutrients.

Thus calcite and other liming agents raise the pH of acid soils by accepting H⁺ (i.e. acting as bases), and the presence of alkali and/or alkaline earth metals like Ca²⁺ therein both augment their efficiency and provide plant nutrients.

For optimal results, soil in most cases should be limed to pH ~6.5. Doing so requires neutralizing not only the acid cations (H⁺ and Al³⁺) in solution but also those adsorbed to surfaces (acid cations inside particles do not require neutralization because they affect pH very slowly, over the course of many years). Liming to pH ~6.5 instead of ~7 reduces the risk of overshooting and causing the soil to become alkaline (pH > 7). Check out the next chapter on alkaline soils for more information about how high pH can be bad for plant growth.

Estimating the Limestone Requirement Using pH and Soil Texture

The amount of agricultural lime needed to raise the pH of a soil is known as the limestone requirement, and can be calculated based on its pH and CEC. However, sometimes CEC isn’t known. In this case, we can estimate the limestone requirement based on initial soil pH and texture instead using the below graph.

To use the graph, find the initial soil pH on the y-axis and draw a line to the curve pertaining to your soil’s texture. Then, we draw another line straight down, parallel to the y-axis. Then when this vertical line reaches the x-axis, the x-axis value is the limestone requirement.

Example limestone requirement estimation

When a soil with a sand texture has a pH of 4, we find 4 on the y-axis, draw a horizontal line to the sand curve, and then draw a vertical line straight down. This intercepts the x-axis at 2, so we know that 2 Mg/ha is the liming requirement.

Using Acid Saturation to Calculate Liming Requirements

Beyond estimation, how can you calculate liming requirements more precisely?

To lime an acid soil, you must neutralize both the acid cations in solution (called active acidity) exchangeable acid cations adsorbed to surfaces (called exchangeable acidity). In an acidic soil, the amount of exchangeable acid cations (mainly adsorbed to colloids) far outweighs the amount of acid cations in solution. Therefore, if we know the CEC and the acid saturation (AS) of a soil, we can estimate the liming requirement following the steps below.

1. First, calculate the exchangeable acidity by multiplying the acid saturation (as a fraction) by the cation exchange capacity:

EA = AS*CEC

2. Then, calculate the mass of the soil:

M_s = D_b*V_s

3. Next, multiply the mass of the soil by the exchangeable acidity in cmolc/kg to get the total exchangeable acid cations in cmol_c:

   TEA = EA*M_s

4. Then, convert the units to kg of CaCO₃ to get the calcite requirement (CR):

CR = TEA*(100 g CaCO₃/mol)

5. Finally, most agricultural liming products do not have 100% effectiveness, or effective calcium carbonate equivalent (ECCE). ECCE measures how much acid agricultural liming products can neutralize relative to pure, powdered calcite. The higher ECCE, the more effective the liming agent is, and the less needs to be applied; the lower the ECCE, the less effective the liming agent is, and the more needs to be applied. So, when ECCE < 100% (as is almost always the case), we have to calculate the liming requirement (LR), by dividing the calcite requirement (CR) by the ECCE:

LR = CR ÷ ECCE

This calculation assumes that the amount of acid cations in solution is negligible compared with the amount on exchange sites. Refer to the examples below for how to adjust the steps based on the volume of soil, provided units, and desired output units.

You can find three methods for determining liming requirements, including one nearly identical to this one, in the Soil & Plant Growth Laboratory Manual – Exercise 6A. Note that some definitions of ECCE differ from what’s used here. In this chapter, ECCE is a percent or fraction, but other definitions use absolute units instead.

Benefits of Liming Acid Soils

From the soil acidity impacts outlined in the acid soils chapter, we can infer that raising problematic acid soil pH to ~6-6.5 would normally result in:

• Reduced aluminum toxicity due to decreased solubility of Al³⁺
• Reduced risk of transition metal micronutrient toxicity (Fe²⁺, Mn²⁺, Cu²⁺ and Zn²⁺)
• Increased cationic macronutrients: Ca²⁺, Mg²⁺, and K⁺, because:
  • Calcite directly adds Ca²⁺, and dolomite adds both Ca²⁺ & Mg²⁺
  • Increasing pH increases CEC, enhancing cation retention
• Increased plant-available P due to less with Al and Fe precipitating with PO₄^3-
• Increased activity of potentially beneficial microbes, as well as earthworms

Remember to avoid overshooting – these benefits will be overshadowed by alkaline soil problems if a pH of 7 is exceeded. That is why we aim for a pH of 6 to 6.5.