Ammonium-based fertilizers are the primary type of fertilizer that contributes to soil acidity.
Here's a breakdown of why and how this happens:
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Nitrification Process: Ammonium (NH₄⁺) in the fertilizer undergoes a process called nitrification, where soil bacteria convert it into nitrate (NO₃⁻). This process releases hydrogen ions (H⁺) into the soil, which increases acidity.
NH₄⁺ + 2O₂ → NO₃⁻ + 2H⁺ + H₂O -
Leaching of Nitrate: When nitrate isn't immediately taken up by plants, it can be leached from the soil profile by rainwater. This carries away essential nutrients and contributes to increased soil acidity because the hydrogen ions remain.
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Plant Uptake of Nitrate vs. Ammonium: Plants preferentially taking up nitrate over ammonium can also lead to acidification. To maintain charge balance, plants release hydroxide ions (OH⁻) when taking up nitrate, which can temporarily raise pH. However, the overall effect of nitrification and subsequent processes generally leads to acidification.
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Examples of Ammonium-Based Fertilizers: Common examples include:
- Ammonium sulfate ((NH₄)₂SO₄)
- Ammonium nitrate (NH₄NO₃)
- Urea (CO(NH₂)₂) - which is converted to ammonium in the soil
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Impact of Plant Decomposition: As the reference mentions, acidification is somewhat mitigated when plants decompose and return the nitrogen to the soil. However, modern agricultural practices often involve harvesting and removing plant material, preventing this natural recycling process.
In summary, while nitrogen is essential for plant growth, the use of ammonium-based fertilizers can unintentionally lead to soil acidification due to the nitrification process and the subsequent leaching of nitrate. Management practices, such as efficient fertilizer application and crop residue management, are crucial to minimizing this effect.
