- Sulfate-Sulfur is the only form of S the plant can utilize.
- Elemental S is dependent upon time, temperature and moisture to be available to the plant.
- Sulfate-Sulfur will not acidify the soil.
For various reasons, sulfur (S) deficiencies are increasing in many areas of the country. Consequently, fertility programs use this nutrient more routinely. The most common chemical forms of S used in fertilizers are sulfate S (SO₄²⁻) and elemental S (S⁰). But these two forms of S react quite differently in soils. It’s very important to understand the differences between SO₄²⁻ and S⁰ in order to use these two forms in the most effective manner possible.
Although S exists in many different chemical forms in nature, plants can absorb S only through their root systems in the SO₄²⁻ form. Small amounts of sulfur dioxide gas can be absorbed through leaves, but this is of little consequence in the overall S nutrition of plants. This obviously means that all soil S must be converted to SO₄²⁻ in order to be utilized by plants.
The SO₄²⁻ anion carries two negative charges, so it’s not absorbed by soil colloids to any extent. As soil acidity increases, some positive-charge sites develop on clay particles and organic matter, which allows for a limited amount of SO₄²⁻ absorption. But for the most part, SO₄²⁻ moves freely with soil moisture, especially in the upper levels of the soil profile. It thus reacts much like nitrate nitrogen (N) in soils. As a result, SO₄²⁻ levels frequently increase with increasing depth in the soil profile, at least in humid climates. Table A shows the increase of SO₄²⁻ with depth in several Southeastern U.S. soils.
For this reason, small amounts of SO₄²⁻ applied in a starter fertilizer is sometimes all you need to get young roots off to a fast start, quickly growing down through the profile to greater depths where supplies of SO₄²⁻ are more plentiful. Work in Arkansas has shown that wheat yields were increased from 15 to 44 bushels per acre (bu/A) by the application of just 5 pounds per acre (lbs/A) of S in the potassium sulfate form. However, a more typical recommendation for sulfur falls into the range of 25 to 30 lbs/A of S.
Elemental S is totally unavailable to plants. Plants simply cannot absorb S⁰ through the root system. Elemental S is inert and water insoluble. Commercially, companies can store it in the open, where it can remain in place for years, unaffected by moisture or temperature. However, when farmers add S⁰ to soil, it’s an entirely different matter. In the soil, Soconverts (oxidizes) to the plant-available SO₄²⁻ form, and the rate at which this conversion takes place is the determining factor regarding the effectiveness of S⁰ as a fertilizer source of S. (See Part II.)
Experts recommend the following as the most important considerations regarding the use of these two chemical forms of S in fertility programs:
· When relying on S⁰ for your crop’s total S needs, applying it prior to planting delivers the best results. Broadcasting followed by incorporation is preferable to band placement. Surface applications of S⁰ are not recommended.
· If you apply fertilizer at the time of planting of spring-seeded crops, SO₄²⁻ fertilizers usually give best results. This is especially true if conditions are such that So oxidation rates are depressed — in other words, at cool temperatures or in excessive moisture. In starter or row fertilizers, SO₄²⁻ forms of S generally give better results.
· If you apply fertilizer in the fall for spring-seeded crops, differences between S⁰ and SO₄²⁻ sources become less likely. Elemental sulfur sources usually have better residual effects.
Elemental sulfur sources are highly acidifying. This can benefit your crops under alkaline soil conditions, but hurt them under acid conditions. Sulfate sources can be either acidifying or neutral in reaction. Ammonium sulfate is an acid-forming material; K-Mag, potassium sulfate and calcium sulfate are neutral materials and don’t affect soil pH.