Soil Acidity, Liming Materials and Lime Recommendations

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Highlights

  1. Soil pH indicates whether lime is required but not how much. Lime is needed if the target soil pH, which varies by crop, is 0.2 units higher than the actual soil pH.
  2. Buffer pH indicates how much lime is required. Soils with low buffering capacity (high buffer pH) and a small difference between initial and target pH values require less lime. Conversely, soils with high buffering capacity (low buffer pH) need more lime, even for small pH adjustments.
  3. Liming materials should have more than 80% calcium carbonate equivalent (CCE) and/or 50% effective neutralizing value (ENV), and the recommended lime rate should be adjusted based on these two qualities.
  4. Finer lime particles are more efficient in increasing soil pH by reacting quickly with soils, but liming materials should have both smaller and larger particles so that smaller particles can raise the soil pH quickly and larger particles can have long-term control in neutralizing soil acidity.
  5. The lime recommendations from the LSU AgCenter Soil Testing and Plant Analysis Lab are based on 50% effective CCE (ECCE) or ENV.

Soil Acidity and Soil pH

  1. Soil acidity refers to the concentration of hydrogen ions (H+) in the soil, which determines the soil pH level. The pH scale ranges from 0 to 14, with a pH below 7 indicating acidity, 7 being neutral, and above 7 indicating alkalinity. Soil pH is a crucial factor in agronomic crop production because it significantly influences nutrient availability, microbial activity and overall soil health.
  2. Soil pH is the most important soil quality component that greatly influences soil nutrient availability (see Figure 1). Most nutrients are highly available at the soil pH of 6.5. Therefore, soil pH needs to be adjusted to the target pH either by applying lime for low pH (less than 6.0) soils or by elemental sulfur for high pH (more than 7.5) soils. Increasing soil pH by liming is a more common practice than decreasing soil pH by elemental sulfur.


Causes of Soil Acidity

  1. Weathering of parent material: As rocks and minerals break down, they release hydrogen ions into the soil. Some parent materials (iron and aluminum minerals) are naturally more acidic than others, contributing to the overall acidity of the soil derived from them.
  2. Air pollution and acid rain: Industrial activities release sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) into the atmosphere. These compounds can form sulfuric and nitric acids, which are deposited onto the soil through precipitation, contributing to acidification.
  3. High rainfall: In areas with high rainfall, leaching removes basic cations (calcium, magnesium, potassium and sodium) from the soil, leaving acidic cations (hydrogen and aluminum) behind. This process increases soil acidity.
  4. Ammonium-based fertilizers: Fertilizers such as ammonium sulfate, urea, and ammonium nitrate undergo nitrification when applied to the soil. This process releases hydrogen ions, which increases soil acidity.
  5. Over-fertilization: Excessive use of certain fertilizers such as ammonium (NH4+) containing fertilizer can lead to the accumulation of acids in the soil, further lowering the pH.
  6. Nutrient uptake by plants: As crops grow, they absorb basic cations from the soil, which can gradually increase soil acidity.
  7. Crop harvesting: The removal of crops from the field takes away basic nutrients from the soil, potentially leading to an acidic soil environment.
  8. Organic matter decomposition: As organic matter decomposes, it produces organic acids (e.g., humic acid, fulvic acid) that contribute to soil acidity.
  9. Soil microbes: Microbial activity during decomposition also releases carbon dioxide, which forms carbonic acid when it reacts with water.
  10. Saline irrigation water: Irrigation with water containing high levels of salts can lead to soil acidification. As the water evaporates, salts are left behind, which affects soil chemical balance and increases acidity.
  11. Iron and aluminum ions hydrolysis: In acidic soils, iron (Fe) and aluminum (Al) become more soluble and can hydrolyze, releasing hydrogen ions and further increasing soil acidity.


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Figure 1. Soil pH and nutrient availability. Source: Reitsma et al. (2011). Chapter 2: Soil fertility. In: Alternative practices for agronomic nutrient and pest management in South Dakota. Edition: I. South Dakota State University, College of Agriculture and Biological Sciences.

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7/22/2024 2:20:03 PM
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