Sulfur in the soil

The sulfur cycle in the soil shows some similarities with the nitrogen cycle.

Sulfur resides in different, interconnected pools. Only a minor part of it is immediately available for plant uptake. The rest needs to undergo transformation processes first. What are the specificities of sulfur?

Crops prefer sulfate

Plant roots can take up sulfur only as sulfate ions (SO42-). Plant leaves can also take up sulfur from the air as sulfur dioxide (SO₂), but this contribution is now minor. All elementary soil sulfur must first be mineralized before it becomes available to plants.

Sources of sulfur

All sulfur in the soil, whether it is applied as elementary sulfur, manure or sulfate, ends up as sulfate before plants take it up. If sulfate is applied directly, losses are avoided.See the illustration below.

Sulfur from mineral fertilizers

Mineral fertilizer contains sulfur as sulfat. Sulfate from fertilizer is immediately available as a nutrient and easily absorbed by plants. Sulfate is highly mobile in the soil and reaches the plant roots quickly. The application of sulfur during an early stage and during intensive plant gowth makes it suitable for combination with other fertilizers, especially nitrogen. Applied as elementary sulfur, it needs to be oxidized to sulfate by soil microbes, which takes time. Elementary sulfur also has a strong acidifying effect.

Depostitions from the air

Sulfur is present in the atmosphere mainly as sulfur dioxide (SO₂) from natural events (volcanic eruptions) or manmade activities (burning of fossil fuels). It can enter the leaves of plants from the air as sulfur dioxide gas. Most of the atmospheric sulfur, however, enters the soil as acid rain. Depositions have dropped significantly and hardly exceed 10 kg/ha today.

Manure

Manure contains sulfur mainly as organic matter and therefore needs to be mineralized before it can be take up.

Sulfur leaching

Sulfur behaves similar to nitrogen in the soil. Sulfate ions, as nitrogen ions, are dissolved and prone to leaching. Fertilization shall therefore be matched to plant growth in order to ensure rapid uptake. Application in the early phase of plant growth is most efficient. Stock fertilization in the autumn is generally not recommended.