Nutrients such as nitrogen, phosphorus, and potassium are applied each year to our crops, based on soil lab test results. However, there is a nutrient we might be overlooking when making our fertiliser decisions. Sulphur is considered the fourth most important nutrient needed by plants. ⁽²⁾ Sulphur is required for many plant functions including photosynthesis, chlorophyl formation and nitrogen fixation. ⁽¹⁾

Crops such as maize, legumes, members of the mustard and onion families have a particularly high sulphur requirement. ⁽²⁾ Potatoes are also a crop that requires an adequate supply of sulphur.

Sufficient sulphur in plants is important for nitrogen use efficiency and grain quality. Sulphur is also involved in protein synthesis and as a vital component of several essential amino acids and vitamins. Sulphur is required for chlorophyll production and, in legumes, has a role in nitrogen fixation. ⁽²⁾

In plants, sulphur is bound up in older tissue making it unavailable when needed to support new growth. To ensures sulphur will be available during periods of rapid growth apply sulphur 'little and often', this reduces the risk of leaching and avoids the risk of interactions with other nutrients such as molybdenum. Application in the main phase of plant growth is the most efficient strategy. Autumn applications of sulphur are not recommended. ⁽³⁾

Sulphur deficiencies are commonly seen in plants early in the season, when the soil is cool, roots systems are small, and organic matter mineralization is minimal. In these cases, crops will often overcome sulphur deficiency later in the season when organic matter mineralization rates increase, and larger root systems are able to access sulphur deeper in the soil profile. ⁽²⁾

As an example, in maize, sulphur deficiency is displayed as an overall yellow appearance, similar to nitrogen deficiency. However, because sulphur is immobile in the plant, symptoms appear first on younger leaves. ⁽¹⁾

Sulphur deficiencies are more likely to occur under the following conditions:

• Light and sandy soils with little soil organic matter (increasing low sulphur content)

• High rainfall during winter (increases sulphur leaching)

• Dry spring (increases low mobility of sulphates)

• Low temperature (increases low mineralisation rate)

• Low input of organic matter and mineral sulphur (lowers the available sulphate for plant uptake) ⁽³⁾

Most sulphur in the soil is found in organic matter and crop residue; however, it is not readily available for plant uptake until converted to a soluble form of sulphate, this mineralization process is dependent on temperature, moisture, and other environmental factors. Sulphate in the soil is soluble and will move easily to the root with soil water, however, it will also move past the root zone with excessive rainfall or irrigation. ⁽²⁾

Gypsum is a cost-effective sulphur fertiliser supplying readily available sulphate that is easily absorbed by plants. Gypsum can be applied as required during active growth periods of a crop.

Sulphur fertilizers added to the soil that are not already in the sulphate form need to undergo chemical transformation to sulphate before the sulphur will be plant available. ⁽²⁾    

By understanding the behaviour of sulphur in the soil, knowing how to monitor for sulphur deficiencies through soil and plant tissue testing, as well as familiarity with the properties of different sulphur fertilisers will help you make informed decisions about sulphur management in your fields. ⁽²⁾

For more information on applying gypsum as a cost-effective sulphur source on your crop, talk to your local fertiliser advisor. Application rates will be based on a recent soil test.

Gypsum is BioGro certified. Gypsum, it all starts with healthy soil.

References

1. The Importance of Sulfur for Corn and Soybeans | Crop Science US (bayer.us)
2. Sulfur Fertility Management for Grain and Forage Crops (psu.edu)
3. Agronomy Advice | Don't forget about sulphur | Yara UK
4. Sulfur deficiency in corn can lead to lower yields | Illinois Extension | UIUC