IMPORTANCE FOR PLANT LIFE

Sulfur is essential for plant life as it is a structural component of many fundamental amino acids such as cysteine and methionine. Plants require sulfur from the early stages of development for chlorophyll formation, photosynthesis, and protein synthesis.

Sulfur deficiency often appears as leaf discoloration or yellow spotting in cereals at the end of winter and can easily be confused with nitrogen deficiency.

ABSORPTION MECHANISMS

Plants absorb sulfur through their roots in the form of sulfate (SO₄²⁻) dissolved in the soil solution. Sulfur is mobile in the soil solution, and its uptake occurs through a semi-passive, semi-active process involving both passive and active mechanisms.

INTERACTIONS AND SPECIFIC PROPERTIES

Plants can directly utilize sulfur only in sulfate form. Elemental sulfur (S) must first be oxidized in the soil to sulfate before it can be taken up. Thiosulfate is considered an intermediate form between elemental sulfur and sulfate and is converted to sulfate through oxidation.

SOIL AND CYCLING SCHEME

Most sulfur retained in soil is stored in organic matter. When temperatures rise sufficiently (>12°C), organic matter mineralizes, releasing sulfur in sulfate form and supporting plant uptake, particularly during summer months.

CYCLING SCHEME

1. Nutrient recycling from manure, plant residues, and organic by-products is an important sulfur source.

2. Fertilizer formulations may contain sulfur in sulfate form.

3. Some crop protection products contain sulfate or elemental sulfur, which oxidizes to sulfate in soil.

4. Sulfur emissions from industry and vehicles have decreased significantly over the last 40 years, greatly reducing atmospheric sulfur deposition.

5. When sulfate is incorporated into microbial or plant biomass, it becomes organic sulfur and must be mineralized again for plant uptake.

6. Under aerobic conditions, the final mineral form is sulfate; under anaerobic conditions, sulfate may be reduced to sulfide or hydrogen sulfide (H₂S).

7. Sulfate leaching occurs mainly in winter when excess water transports sulfate beyond the root zone.

8. Plants absorb sulfur exclusively as sulfate (SO₄²⁻) through their roots.

9. Foliar uptake of elemental sulfur vapor is theoretically possible but has limited effectiveness.

10. Sulfur is removed from the field through harvested food and feed products.

SENSITIVITY TABLE & SYMPTOMS

Sulfur is relatively immobile within the plant; therefore, deficiency symptoms typically appear in young leaves. Leaf chlorosis is common and may be confused with nitrogen deficiency. However, sulfur deficiency primarily affects young tissues, whereas nitrogen deficiency usually appears in older leaves.

EXCESS & REQUIREMENT

Excessive sulfur application can cause soil acidification. This may be beneficial in calcareous soils for pH adjustment. Gypsum (calcium sulfate), however, does not directly affect pH and supplies sulfate and calcium without altering soil reaction.

ORIGIN

Sulfur is of natural origin and can be obtained in elemental form from volcanic deposits or as a by-product of natural gas and petroleum refining. During the 20th century, atmospheric sulfur deposition via acid rain was very high; however, in most countries it has decreased by more than 80% today.

KEY FACTORS

SOIL SULFUR CONTENT

Soil sulfur content is closely related to organic matter levels. When evaluating sulfur status, crop type, climate conditions, and soil texture must be considered together.

ORGANIC MATTER

Most sulfur in soil exists in organic form. Higher organic matter content increases sulfate release through mineralization during the growing season. Regular organic fertilization significantly reduces sulfur deficiency risk, provided soil temperature supports mineralization.

TEXTURE

Like nitrogen, sulfur is highly mobile in the soil solution. Heavy autumn and winter rainfall increases the risk of sulfate leaching, especially in sandy and highly permeable soils.

CLIMATE

The frequency and amount of rainfall or irrigation are key drivers of sulfur leaching. The more intense and prolonged winter precipitation is, the greater the expected sulfate loss beyond the root zone. Therefore, sulfur fertilization timing and dosage must be carefully planned according to local climate.