Ammonium sulfate (AS) is an inorganic chemical compound widely used in agriculture, primarily as a high-nitrogen fertilizer and as a performance-enhancing additive for certain spray solutions. This crystalline or granular salt dissolves completely in water, making it suitable for liquid applications. Determining the correct amount to mix per gallon depends entirely on the intended use, either supplying nutrients to plants or optimizing the effectiveness of other chemicals. This guide provides practical mixing instructions for these two distinct applications.
Understanding Ammonium Sulfate’s Role
The primary function of ammonium sulfate in plant care is to deliver two specific nutrients: nitrogen and sulfur. Its composition is typically labeled as 21-0-0, signifying that it contains 21% nitrogen by weight, with no phosphorus or potassium present. The remaining portion includes 24% sulfur, which is immediately available to plants as sulfate.
The nitrogen in ammonium sulfate is in the ammoniacal form, which is rapidly taken up by plants, producing a quick greening effect. This nitrogen source is beneficial for turfgrass and acid-loving plants, as its continuous use can gently lower the soil’s pH over time. The sulfur component supports the synthesis of proteins and enzymes.
Ammonium sulfate exhibits a high solubility in water, with a saturation limit of approximately 76.7 grams per 100 grams of water at room temperature. This means the compound can be easily dissolved without leaving residue, which is necessary for consistent application through sprayers or irrigation systems. The maximum amount that can be dissolved in one gallon far exceeds the concentrations needed for practical agricultural use.
The dual nature of this compound means that mixing rates for fertilization are different from those used for water conditioning. As a fertilizer, the goal is to deliver a precise amount of nitrogen to the plant or soil. As a water conditioner, the focus shifts to a much lower concentration intended to bind problematic mineral ions in the mixing water.
Mixing Rates for Nitrogen Fertilization
The most common application for ammonium sulfate is as a liquid nitrogen feed for plants, requiring a specific dilution to deliver the desired amount of nutrient without causing plant injury. For the average home gardener, one level tablespoon of granular ammonium sulfate weighs approximately 0.425 ounces. Since the product is 21% nitrogen, one tablespoon delivers about 0.0056 pounds of actual nitrogen.
Standard Liquid Feed
For general-purpose feeding of garden plants, shrubs, and trees, a dilute solution is appropriate for regular watering. A common rate for this standard liquid feed is 1 to 2 tablespoons of ammonium sulfate per gallon of water. This concentration provides a steady supply of nitrogen that can be applied every four to six weeks during the active growing season.
This dilution is appropriate for root-zone application, ensuring the nitrogen is available to the plant’s root system. Applying it to small house plants requires an even lower concentration, typically 1 teaspoon per gallon of water, to avoid over-fertilization. Medium to large container plants can tolerate the higher concentration of 1 to 2 tablespoons per gallon.
High-Nitrogen Application
For turfgrass or plants with high nitrogen demands, such as acid-loving plants like blueberries and azaleas, a more concentrated solution may be used. A typical target for turf is to apply approximately 0.2 pounds of actual nitrogen per 1,000 square feet. To achieve this, a common measurement is 90 grams, or about 6 to 7 tablespoons, dissolved in one gallon of water, then applied evenly across that area.
This high-nitrogen rate is based on the final application rate over a specific area, not simply the concentration in the water. The water used is merely the carrier, and turf applications require a large volume of water to dissolve the granules and prevent leaf burn. Always follow up this application with thorough irrigation to wash the fertilizer off the foliage and into the soil.
Foliar Feeding Rates
Foliar feeding, which involves spraying a solution directly onto the leaves for rapid nutrient absorption, requires a dilute mixture. The recommended rate for foliar application is 1 to 3 tablespoons of ammonium sulfate dissolved in one gallon of water.
The lower end of this range is safer, especially in warmer conditions. Foliar solutions must be applied when temperatures are cool and the plant is not under direct sunlight to prevent the concentrated salts from damaging the leaf tissue. Applying the solution in the early morning or late evening maximizes absorption and minimizes the risk of leaf burn.
Mixing Rates for Herbicide Adjuvancy
Ammonium sulfate is used as a water conditioner, or adjuvant, to enhance the performance of certain herbicides, particularly glyphosate-based products. Hard water contains mineral cations like calcium and magnesium, which can chemically bind to the herbicide and reduce its effectiveness.
The purpose of the ammonium sulfate is to preferentially bind to these antagonistic ions in the spray tank, leaving the herbicide free to be absorbed by the target weeds. This action is separate from its nutritional role and requires a specific, standardized mixing rate to condition the water.
In commercial and agricultural settings, the standard rate for using dry, spray-grade ammonium sulfate as an adjuvant is 8.5 to 17 pounds per 100 gallons of spray solution. For smaller applications, this translates to a rate of approximately 1.36 to 2.72 ounces of ammonium sulfate per gallon of water.
Some commercial adjuvant products may recommend a slightly higher rate, ranging from 1 to 6 ounces per gallon of spray solution, with the higher rate reserved for extremely hard water conditions. The amount required depends on the hardness of the water source; harder water requires a greater quantity of ammonium sulfate to neutralize the mineral content.
When mixing for adjuvancy, the amount of ammonium sulfate used is a function of the water quality, not the acreage being treated. Use the higher end of the rate range when the water source is very hard or when the target weeds are under stress due to drought or other environmental factors.
Essential Safety and Mixing Procedures
Handling and mixing ammonium sulfate solutions requires adherence to specific procedures and safety measures. Personal protective equipment (PPE) is necessary, including chemical-resistant gloves, protective eyewear, and long sleeves, to prevent contact with the concentrated chemical. Although considered non-toxic, the powder can cause slight abrasion or irritation upon prolonged contact with skin or eyes.
The correct order of adding ingredients to the spray tank is important to ensure a fully dissolved solution and maximum efficacy. Ammonium sulfate should always be added to the water first. The tank should be filled at least halfway with water before the ammonium sulfate is slowly added, and complete dissolution must be achieved before any other product is introduced.
Continuous agitation is necessary during the mixing process and while the solution is being applied, especially for high-concentration solutions. Agitation helps the granular product dissolve fully and prevents it from settling out of the solution, which could lead to inconsistent application or nozzle clogging.
Storage precautions include keeping the granular product in a sealed container in a cool, dry, and well-ventilated location. The compound is highly soluble and will absorb moisture from the air, which can cause caking and make it difficult to measure and dissolve accurately. Avoid mixing or storing ammonium sulfate with alkaline substances, as this can cause a chemical reaction that releases ammonia gas.