Sulfur is an essential secondary macronutrient required for protein synthesis, enzyme activation, and chlorophyll formation. Beyond its role as a direct nutrient, sulfur is widely used as a soil amendment to modify soil chemistry, primarily by lowering the soil’s pH level. The correct amount of sulfur to add depends entirely on the intended goal: nutrient correction or pH adjustment.
Determining Soil Sulfur Needs
Before any application, a soil test is necessary to establish current conditions and define the specific goal. A comprehensive soil analysis provides two critical data points: the concentration of plant-available sulfate-sulfur (\(\text{SO}_{4}^{2-}\)) and the existing soil pH. The sulfate-sulfur concentration reveals if the soil is deficient in this mobile nutrient, guiding the need for nutrient application.
The current pH level determines the type and rate of sulfur needed for acidification, which is required for acid-loving plants or to increase the availability of micronutrients in alkaline soils. If the soil test shows low sulfur levels, the goal is nutrient correction, requiring a readily available form of sulfur. If the soil is alkaline (e.g., pH 7.5 or higher), the goal shifts to pH reduction, requiring a chemically different form of sulfur and a much higher application rate.
Selecting the Correct Sulfur Type for Application
The choice of sulfur product must align with the intended purpose, as different forms react with the soil in fundamentally different ways. The two primary commercial forms are elemental sulfur and sulfate-based compounds.
Elemental sulfur (\(\text{S}^{0}\)) is the preferred choice for lowering soil pH. It is not immediately plant-available and requires a slow, biological oxidation process by soil microbes. This process converts the sulfur into sulfuric acid, releasing hydrogen ions that increase soil acidity and effectively lower the pH.
In contrast, sulfate forms, such as gypsum or ammonium sulfate, are used exclusively for correcting nutrient deficiencies. Sulfate (\(\text{SO}_{4}^{2-}\)) is the only form plants can absorb through their roots, making these compounds immediately effective as fertilizer. These sulfate salts have a minimal long-term impact on soil pH and are not suitable for significant acidification.
Calculating Application Rates for Soil Health
The calculation for sulfur application depends entirely on whether the goal is to adjust the soil pH or to supplement nutrient levels.
Calculation for pH Reduction
Lowering the soil pH requires applying elemental sulfur, and the necessary amount depends heavily on the soil’s buffering capacity, texture, and organic matter content. Clay soils and those high in organic matter resist pH change, requiring significantly more sulfur than sandy soils to achieve the same reduction. For example, to lower the pH by one unit (e.g., 7.5 to 6.5), sandy soils may require 150 to 300 pounds of elemental sulfur per acre, while clay soils may need 300 to 600 pounds per acre.
For residential applications, a general guideline to lower the pH by about 0.5 unit suggests applying 1 to 2 pounds of elemental sulfur per 100 square feet. The maximum single application should not exceed 5 to 10 pounds per 1,000 square feet. Applying excessive amounts at once can cause severe soil acidification and potential plant toxicity. If a large pH change is required, the total amount should be split into multiple smaller applications over several years.
Calculation for Nutrient Deficiency
To correct a sulfur deficiency, the application rate is determined by the soil test recommendation for \(\text{SO}_{4}^{2-}\) and the percentage of sulfur in the chosen fertilizer product. If a soil test recommends adding a specific amount of actual sulfur (e.g., 20 pounds of \(\text{S}\) per acre), the required fertilizer material is calculated using the product’s guaranteed analysis. For instance, using ammonium sulfate (24\% sulfur), the calculation involves dividing the recommended pounds of nutrient by the percentage of the nutrient (20 lbs \(\text{S}\) / 0.24). This shows that approximately 83 pounds of ammonium sulfate product would be needed to supply 20 pounds of actual sulfur.
Timing and Methods for Applying Sulfur
The timing and method of application must correspond to the sulfur form used to maximize effectiveness.
Elemental sulfur, used for pH adjustment, relies on soil bacteria that are most active when the soil is warm and moist (typically above \(55^\circ\text{F}\)). It is best applied in the fall or early spring to allow the slow oxidation process to occur over several months before the main growing season. For maximum contact with microbes, elemental sulfur must be incorporated into the top 6 to 8 inches of the soil rather than simply broadcast on the surface.
Sulfate-based fertilizers, intended for nutrient delivery, are immediately available to plants and are often applied closer to or at the time of planting. Because the sulfate form is mobile and can leach through the soil, particularly in sandy environments, it may be applied in a band near the seed (side-dressing) or split into multiple applications throughout the growing season. It is recommended to re-test the soil pH 6 to 12 months after applying elemental sulfur to monitor the change and determine if a follow-up application is necessary.