The Types of Acid in Fertilizer

Too much acid in fertilizer burns plant roots.

Most vegetables and many landscape plants grow best in slightly acidic soil. If soil pH is outside optimal levels, nutrients are unavailable for plants to utilize. Insufficient nutrition compromises plant health and reduces soil microbial synergy. To compensate for undesirable pH levels, two components in fertilizers acidify soil for optimal plant growth.

Types of Acid

Acidity is a measurement of the percentage of hydrogen ions in soil solution, which is the water between soil particles. As the concentration of hydrogen ions increases, the pH decreases and makes soil more acidic. Nitrogen and sulfur are chemical elements found in fertilizers that acidify the soil. Using different types of these acidic agents determines the speed at which you can lower soil pH and improve growing conditions for plants that require acidic soils.

Nitrogen Acid Sources

Fertilizers that contain nitrogen compounds acidify soils. Nitrogen combines with hydrogen to form different ammonium compounds. Types of these compounds in fertilizers include ammonium nitrate, ammonium phosphate and ammonium sulfate. Another nitrogen compound that acidifies soil is urea. Ammonium sulfate provides twice as much acid as urea or ammonium nitrate, and ammonium phosphate is a weaker agent. Aluminum sulfate is a non-nitrogen acid compound found in some fertilizers that has potential metal toxicity to plants and the environment.

Sulfur Acid Source

Sulfur is a natural mineral found in hot springs and volcanic craters. Soil bacteria oxidize elemental sulfur, which lowers soil pH by creating sulfuric acid. Soil test results routinely recommend adding sulfur as a plant nutrient, a different application from using it as a soil acidifier. Sulfur, in sulfate forms, supplies plant nutrition. Elemental sulfur works as an acid to lower soil pH. If soil test results indicate the pH is too high, use elemental sulfur instead of sulfate sulfur.

Organic Acid Sources

Using organic fertilizers not only supplies plants with nutrition, but also builds soil microbial activity to build healthy plants. In addition to using sulfur as an organic acidifying agent, other fertilizers, such as plant- and seed-based meals, work gradually to acidify soil. Alfalfa meal, soybean meal and cottonseed meal decompose in soil and provide sustained, slow-release fertilization and acidification. Alfalfa meal heats up when mixed with water and is not a fertilizer recommended for root zone applications.

References

• Clemson University; Soil Acidity and Liming; Charles C. Mitchell, Jr., et al.
• University of Minnesota; Understanding Phosphorus Fertilizers; George Rehm, et al.; 2002
• Oregon State University; Acidifying Soil; J. Hart, et al.; 2003
• Colorado State University; Organic Fertilizers; Adrian Card, et al.; December 2009