Soil Acidification

Soil acidification is when soil pH gets lower (below pH 7) over time. It occurs naturally as soils weather and “base cations” such as calcium (Ca2+) and magnesium (Mg2+) are leached or lost through plant removal, but acidification can be exacerbated with continuous use of ammonium-based fertilizers because ammonium produces H+ ions as it converts to nitrate. When soil pH drops to 5, aluminum toxicity becomes a major issue.

The pH level of soil has a significant impact on nutrient availability, toxicity, and microbial populations and even alters the effectiveness of certain pesticides. Soils with a pH below 7 are classified as acidic, while pH levels above 7 are considered alkaline.

A graphic showing the nitrification process.

When ammonium-based fertilizers are applied to the soil, they quickly convert to nitrate. This nitrification process releases excess hydrogen ions (H+) that make soil more acidic. Figure from WSU Extension Publication: Soil pH and implications for management: an introduction

Managing Soil Acidification

Amending soils with agricultural lime is the most common and effective long- and short-term strategy to correct soil acidity. Other short-term strategies that can mitigate the impact of low soil pH on crop yield include planting aluminum-tolerant varieties of wheat or aluminum-tolerant crops such as triticale.

Strategies for reducing the rate of soil acidification and restoring the health of low pH soils include:

Leaving crop residues in the field to minimize the removal of base cations from the soil and to increase the soil organic matter content, which helps buffer pH changes;
Use crop rotations that incorporate legumes because, while legumes still produce some acidity with N fixation, they do not require supplemental nitrogen fertilizers that accelerate acidification.
Use alternative nitrogen fertilizers. Unfortunately, non acidifying fertilizers, such as calcium nitrate, are not commonly used because they are not economically competitive, compared to commonly used ammonia-based fertilizers.
Applying lime that compensates for the acidification occurring from annual N fertilizer applications; and
Implementing practices that optimize nitrogen use efficiency in the cropping system. Before taking management action, it is critical to measure and understand the extent of the soil pH problems within fields and across farms, as well as in the region.