Ammonium Sulfate to Acidify Soil

You may have heard or read a statement like "Add sulfur to your soil to acidify it." This is generally true but determining what form of sulfur product to use and how much to use can be a bit challenging. Ammonium sulfate is generally a safe and easy chemical option to lower your soil's pH, either to prepare your soil for planting or to maintain a lower pH in naturally alkaline or neutral soils. It's a good idea to conduct a thorough soil test before applying ammonium sulfate.

Soil Testing

Before applying ammonium sulfate to your lawn or garden soil, conduct a thorough soil test. Purchase a kit from a local home and garden center or consult a landscape service in your area that offers soil testing. Adding ammonium sulfate to untested, naturally acidic soil can lower its pH to the point that it negatively impacts plant health. Neutral to alkaline soils (those with a pH of 6.5 or higher) may benefit from ammonium sulfate applications, especially if you would like to grow acid-loving plants.

Basic Facts

Ammonium sulfate has been produced since the 19th century as a horticultural fertilizer and soil treatment. It can derived from a number of sources but most forms available today are made from the chemical byproducts of various manufacturing processes. It is usually sold as a white or beige-colored, granular crystals that are highly soluble in water and are often included with other compounds in commercial fertilizer blends. Ammonium sulfate crystals can be incorporated into soil mixes for lawns and gardens or broadcast as a top dressing then watered in. Apply it at rates and in amounts recommended by your soil test or according to label instructions, in the absence of a soil test report.

How It Works

In the presence of water and oxygen in the soil, ammonium and sulfate are decomposed by naturally occurring microbes. During this process, acids are released into the soil, along with nitrogen and sulfur compounds. Nitrogen and sulfur are elemental nutrients required by most plants. As a result, ammonium sulfate also acts as a fertilizer. Ammonium sulfate is a weaker source of acidic compounds than elemental sulfur or aluminum sulfate. Thus, the risk of accidental over-application is reduced when compared to the latter two. It generally takes a year or two for the acidification process to complete itself.

Maintenance and Precautions

Since ammonium sulfate gradually lowers soil pH, periodic applications may be required over several seasons, especially if your soil is significantly alkaline in its natural state. To maintain a desired soil pH over a long period of time or for specific crops, repeated periodic soil testing is recommended. Ammonium sulfate is labeled 21-0-0, meaning that it has a high nitrogen content and no phosphorus or potassium. Avoid overusing it as excess nitrogen can accumulate in the soil and leach into storm drain systems and watersheds, posing an environmental hazard.