14. Soil Acidity and Adjusting Soil pH Managing soil pH is essential to creating ideal growth conditions for most plants. This is because the pH of the soil controls the solubility of nutrients as well as toxic metals. Because of this, most plants have a preferred range in soil pH. Under most cases, liming agents are added to soil to raise pH to the desired range. However, in some cases, lower soil pH is desired, which can be achieved using soil amendments such as elemental sulfur (S), or aluminum sulfate (more commonly referred to as “alum”). In either case, the pH tolerance of the target plant species, the properties of the soil, and the properties of the soil amendment must be considered to achieve the desired change in soil pH. Learning Objectives • Understand the origin of soil acidity. • Measure soil pH with field and laboratory techniques. • Determine the role of aluminum in soil acidity. • State the relationship between cation exchange capacity, buffering capacity, and potential acidity. • Write chemical reactions related to soil pH and liming. • Know the objectives of liming and the factors affecting lime requirement. • Measure limestone requirement. • Determine limestone quality. Materials • Soils with low pH • pH meter with a pH electrode • pH test strips • Beakers • Glass stir rods • Pure reagent grade calcium carbonate • Pure reagent grade calcium oxide • Pure reagent grade calcium sulfate (gypsum) • Dolomitic limestone, coarse (sieved using a 20-40 mesh sieve) • Dolomitic limestone, fine (sieved using a 100+ mesh sieve) 122 | Soil Acidity and Adjusting Soil pH
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14. Soil Acidity and Adjusting Soil pH
Managing soil pH is essential to creating ideal growth conditions for most plants. This is because the pH of the soil
controls the solubility of nutrients as well as toxic metals. Because of this, most plants have a preferred range in soil
pH. Under most cases, liming agents are added to soil to raise pH to the desired range. However, in some cases, lower
soil pH is desired, which can be achieved using soil amendments such as elemental sulfur (S), or aluminum sulfate (more
commonly referred to as “alum”). In either case, the pH tolerance of the target plant species, the properties of the soil,
and the properties of the soil amendment must be considered to achieve the desired change in soil pH.
Learning Objectives
• Understand the origin of soil acidity.
• Measure soil pH with field and laboratory techniques.
• Determine the role of aluminum in soil acidity.
• State the relationship between cation exchange capacity, buffering capacity, and potential acidity.
• Write chemical reactions related to soil pH and liming.
• Know the objectives of liming and the factors affecting lime requirement.
• Measure limestone requirement.
• Determine limestone quality.
Materials
• Soils with low pH
• pH meter with a pH electrode
• pH test strips
• Beakers
• Glass stir rods
• Pure reagent grade calcium carbonate
• Pure reagent grade calcium oxide
• Pure reagent grade calcium sulfate (gypsum)
• Dolomitic limestone, coarse (sieved using a 20-40 mesh sieve)
• Dolomitic limestone, fine (sieved using a 100+ mesh sieve)
122 | Soil Acidity and Adjusting Soil pH
Recommended Reading & Viewing
• Soil pH Overview (CropWatch – Youth, 2013c)
• Soil pH Test (CropWatch – Youth, 2013d)
• Soil pH (USDA-NRCS, 2011)
• Soil pH (USDA-NRCS, 2014a)
• Liming Acid Soils (Whitney and Lamond, 1993)
Prelab Assignment
Using the recommended reading and the introduction to this lab, consider the questions listed below. These
definitions/questions will provide a concise summary of the major concepts addressed in the lab. They are also
useful as study notes for exams.
1. Define pH and pOH. Show the formulas used to calculate both.
2. Why is aluminum considered an acidic cation? Show the relevant reactions.
3. List four negative consequences of low pH (acid) conditions in soil. List two negative consequences of
high pH (basic) conditions in soil.
4. Define buffering capacity. How does buffering capacity relate to cation exchange capacity?
5. Define active acidity, salt-replaceable (exchangeable) acidity, and residual acidity.
6. List various soil amendments that are used to increase the pH of an acid soil?
7. Write a chemical reaction depicting how a typical liming material (calcitic limestone, burned lime, etc.)
act to neutralize acidity.
8. List various soil amendments used to acidify a soil and reduce the pH.
9. What influences how much material is needed to increase the pH of an acid soil?
Introduction
Soil acidity is largely controlled by the composition of ions on exchange sites on the colloidal fraction. The H+ cations
are acidic by definition, and Al3+ cations are considered acidic because they react with H2O to produce Al(OH)3 and
3H+. The Ca2+, Mg2+, K+, Na+ cations, among others, are considered basic because they form strongly dissociated bases
by reacting with OH–. These exchangeable cations on the exchange complex are in equilibrium with the cations in soil
solution. Therefore, the nature of the exchangeable cations influences the composition of the soil solution.