Sulfur fertilization

The seven misconceptions

How is your knowledge of sulfur fertilization? The different states and properties of sulfur cause a lot of misconceptions when it comes to plant supply.

Sulfur is one of six macronutrients and has numerous important functions in plant nutrition. In nature, sulfur occurs in many different state forms, each with different properties. The different modes of action of these state forms lead to misconceptions in the goal of ideal plant nutrition. This article will focus on the misconceptions and provide corrective action. Before the KALI Academy lists the seven most common misconceptions about sulfur fertilization, the following video compares the properties of sulfate sulfur and elemental sulfur:

Explanatory video

What form of sulfur do plants absorb?

The seven misconceptions of sulfur fertilization

Misconceptions about the properties of sulfur are presented, examined and corrected here. The effect of the different state forms on soil and plant physiology is explained. 

Misconception 1: All forms of sulfur in fertilizers have a soil acidifying effect.
Only elemental sulfur is oxidized to sulfate sulfur by soil bacteria. Sulfate sulfur is the only form in which sulfur can be absorbed by plants. This transformation produces sulfuric acid, which causes the acidifying effect. If sulfate sulfur is fertilized, acidification of the soil does not occur. 

Misconception 2: Fertilizers with sulfate sulfur always appear acidic.
Neutral salts such as magnesium or potassium sulfate cannot have an acidic effect because they neither contain hydrogen ions nor release protons (H+) after their application. When fertilizer is applied with ammonium sulfate, on the other hand, four hydrogen ions are introduced into the soil via the ammonium. When converted to nitrate nitrogen, these are released and have a strong acidifying effect. The sulfate component itself is neutral.

Misconception 3: Soil acidification releases nutrients.
Crops develop best at an optimum pH value in the soil. Nutrients are mobile within a certain pH range in the soil and this can be influenced by fertilization. In acidified soils, nutrients are fixed and can be released by liming. The proverb: "Lime makes rich fathers - but poor sons", is characteristic for a nutrient mobilization by lime and warns against a depletion of the nutrient stock without balancing it by a proper mineral fertilization. 

Misconception 4: Fertilization with elemental sulfur lowers the pH-value in calcareous soils.
Soils with pH values above 7 contain free lime and are generally problematic for plant growth. Despite the strong acidifying effect, fertilized elemental sulfur does not have the potential to significantly lower the pH on such soils. The sulfuric acid formed during oxidation of elemental sulfur is immediately neutralized by the lime to form gypsum.    The application of ammonium nitrogen also cannot lower the pH of a calcareous soil. When plants take up nitrogen as ammonium, however, they have to excrete acids through the roots into the surrounding soil as a charge compensation. This results in a pH effect in the immediate root area and this can lead to a mobilization of nutrients (e.g. P, Fe, Zn etc.) in this limited area. These two effects are completely different, however; elemental nitrogen cannot replace sulfur in this role. In soils with free lime, a pH reduction can be caused neither with elemental sulfur nor with ammonium nitrogen.

Misconception 5: Sulfate is leached out and must therefore be constantly fertilized again.
During the summer months, evaporation is higher than precipitation. Unlike in winter, the water flow is therefore directed from the bottom to the top. Consequently, when fertilizer is applied as needed, the sulfur is not leached out but used for growth. Plants do not engage in luxury consumption of sulfur, so that a single fertilization with sulfate in the spring ensures supply until the fall.

Misconception 6: Fertilization of elemental sulfur in fall prevents sulfate leaching over the winter.
Elementary sulfur is basically not soluble in water. Sulfate formed from elemental sulfur as late as fall is displaced in the same way as sulfate from other sources. In spring, the bacterial conversion of elemental sulfur to sulfate starts only haltingly and is therefore not yet available for winter crops at the start of vegetation. This also applies to sulfur from organic matter.

Misconception 7: Excessive sulfur fertilization has no effect on plant growth.
Excessive fertilization should be avoided - if it nevertheless occurs, its effect depends on the form of fertilizer applied. In the case of sulfate, with a few exceptions in the case of extremely high sulfur applications (e.g., molybdenum), no antagonistic effects to other nutrients were observed, even when the application was far above the withdrawal. In contrast, elemental sulfur has fungicidal and bactericidal effects. In this capacity, it is used specifically for crop protection as a foliar spray. In contrast to sulfate, larger amounts of elemental sulfur damage the microorganisms in the soil and this has negative effects on nutrient dynamics and availability.


Our advice: Fertilize with sulfate

Sulfur is taken up by the plant in the form of sulfate. Potassium or magnesium sulfate is pH-neutral, while elemental sulfur has a soil acidifying effect. Furthermore, sulfate sulfur is immediately available to plants and therefore also provides the nutrients needed in this important phase immediately at the beginning of vegetation.