Can You Put Too Much Ammonium Sulfate on Your Grass?

Ammonium Sulfate for Lawns

• Every fertilizer is labeled with a nutrient analysis that identifies the percentage by weight of nitrogen, phosphate and potash. The N-P-K analysis for ammonium sulfate is 21-0-0, meaning that it is 21 percent nitrogen by weight but supplies no phosphate or potash. This fertilizer also contains 24 percent sulfur because the manufacturing process uses sulfuric acid. Ammonium sulfate is a dry, crystalline material that is easy to store and spread, and it provokes a rapid growth response because it provides nutrients in a readily available form.

The Benefits of Nitrogen and Sulfur

• Ammonium sulfate supplies two essential plant nutrients. Nitrogen is the most important nutrient for turfgrass because it encourages vigorous, lush vegetative growth and deep-green coloration. Sulfur, though not as widely appreciated as nitrogen, also makes an important contribution to a high-quality lawn. This nutrient improves coloration, enhances winter hardiness and promotes resistance to weeds and diseases. Adequate levels of sulfur allow plants to use nitrogen more efficiently, so ammonium sulfate conveniently supplies two synergistic nutrients in one application.

The Dangers of Over-Fertilization

• Ammonium sulfate is a refined source of readily available nitrogen, which means that excessive amounts will quickly lead to problems. Nitrogen is a growth stimulant, so excess nitrogen encourages vigorous top growth at the expense of root development. This imbalance makes a lawn more susceptible to heat stress, drought, insect pests and diseases. Excessive applications of soluble nitrogen are also wasteful because nitrogen is easily leached out of the root zone by rainfall or irrigation. In extreme cases of over-application, nitrogen fertilizers can directly injure or even kill grass plants; this is known as fertilizer burn. Ammonium sulfate is dangerous also because it has a strong acidifying effect -- in excessive quantities, it can lower soil pH to undesirable levels.

Proper Application Rates

• Do not supply more than 1 pound of elemental nitrogen per 1,000 square feet in a single fertilizer application. This quantity corresponds to 4.75 pounds of ammonium sulfate. You can repeat this application two to five times during the growing season, so the maximum annual nitrogen input for an average lawn is about 5 pounds per 1,000 square feet. Ammonium sulfate also supplies sulfur, so you need to avoid excesses of this nutrient as well. Most lawns do not need more than 3 pounds of sulfur per 1,000 square feet for the entire year, which corresponds to 12.5 pounds of ammonium sulfate.