Ammonium sulfate (AS) is a widely used inorganic salt fertilizer that provides a quick-release source of nitrogen and sulfur. These two elements are necessary for robust plant growth and development. The effectiveness of this fertilizer depends entirely on the timing of its application. Applying it correctly maximizes nutrient absorption, ensuring plant benefit while minimizing waste and environmental impact.
Understanding Ammonium Sulfate’s Chemical Properties
Ammonium sulfate has the chemical formula \((NH_4)_2SO_4\) and is standardized with a guaranteed analysis of 21-0-0+24(S). It contains 21% nitrogen (N) and 24% sulfur (S) by weight. The nitrogen is entirely in the ammonium form (\(NH_4\)), which is immediately available for plant uptake and less prone to leaching than the nitrate form.
The positively charged ammonium ion binds to negatively charged soil particles, holding the nitrogen in the root zone. Soil microbes must convert the ammonium (\(NH_4\)) into nitrate (\(NO_3\)) through nitrification for most plants to fully utilize the nitrogen. This conversion provides a sustained release, offering a quick initial boost followed by a steady supply. Sulfur is supplied as sulfate (\(SO_4\)), the only form plants can absorb, and is immediately available upon application.
Determining the Ideal Application Time
The optimal time to apply ammonium sulfate aligns with the plant’s peak demand for nitrogen and sulfur. For most crops and turf, the best timing is just before planting or at the beginning of the vegetative growth period in early spring. This application supports the rapid development of leaves and stems as the plant emerges from dormancy or is newly established.
For heavy feeders or long-season crops, such as corn or wheat, a split application is common. This involves applying a portion at planting and side-dressing the remainder during maximum vegetative growth. For perennial plants, an application in early spring, just before bud break, provides the necessary boost. Applying it too late in the season, typically late fall, should be avoided as it promotes soft growth susceptible to winter damage.
Soil pH as the Timing Constraint
Ammonium sulfate is a highly acid-forming fertilizer, which is its most significant constraint on timing and use. When soil bacteria convert ammonium to nitrate, hydrogen ions (\(H\)) are released, actively lowering the soil’s pH. This acidifying effect is greater per pound of nitrogen than that of other common nitrogen sources.
The timing of application is highly dependent on the current soil pH, which must be determined through a soil test before applying the product. Applying ammonium sulfate to already acidic soil (pH below 6.0) can quickly reduce the pH further, leading to nutrient deficiencies and potential aluminum toxicity. Conversely, it is ideally used on neutral or alkaline soils (pH 7.0 or higher) to help correct high pH, or specifically for acid-loving plants like blueberries.
Application Techniques and Environmental Timing
Tactical timing related to environmental conditions maximizes fertilizer efficiency and prevents nutrient loss. Because the fertilizer is highly water-soluble, it should be applied to moist soil and immediately watered in deeply. This action dissolves the crystals, moves nutrients into the root zone, and reduces the risk of fertilizer burn on foliage.
Watering in the fertilizer is important because the ammonium form is susceptible to volatilization, or loss as a gas, if left on the soil surface, especially in alkaline conditions. Application should also be avoided during periods of extremely hot temperatures, which increase the risk of volatilization and plant damage. Finally, time the application to avoid heavy rainfall immediately afterward, particularly on sandy or light soils, to prevent the mobile sulfate and converted nitrate from leaching out of the root zone.