Don't forget about sulphur

Why is sulphur such an important nutrient?

Many agronomists now consider sulphur to be the second most important nutrient after nitrogen. Certainly sulphur is an essential nutrient, closely linked with nitrogen in biological processes with both elements forming an inseparable team.

Previously crop requirements were generally met from atmospheric deposition so sulphur was confined to a secondary role, however today it is back in its rightful place as an essential component of optimum nitrogen management.

Sulphur is now the second most important nutrient

Sulphur is a fundamental ingredient of life on earth. Sulphur is present in all crops and plays an important role in plant metabolism. Sulphur is essential for the formation of plant proteins, amino acids, some vitamins and enzymes.

Most compound fertilisers containing sulphur also contain nitrogen, highlighting the close link between these two elements. Sulphur is part of an enzyme required for nitrogen uptake and lack of it can severely hamper nitrogen metabolism. Together with nitrogen, sulphur enables the formations of amino acids needed for protein synthesis. It is found in fatty acids and vitamins and has an important impact on quality and taste or smell of crops.

Sulphur is also essentially involved in photosynthesis, overall energy metabolism and carbohydrate production.

Sulphur deficiency – often neglected

Until the 1990s, sulphur availability was not a matter of concern, since sulphur dioxide emissions from industrial origins guaranteed a sufficient and automatic supply. Environmental regulation and low sulphur fuel in particular has strongly reduced sulphur emissions. At the same time, higher yield and quality expectations have increased sulphur withdrawal from the field. If sulphur deficiencies were fairly rare 20 years ago, they are commonly encountered today. Sulphur deficiencies are more likely to occur under the following conditions:

• Light and sandy soils with little soil organic matter (> low sulphur content)
• High rainfall during winter (>sulphur leaching)
• Dry spring (>low mobility of sulphates)
• Low temperature (>low mineralisation rate)
• Low input of organic matter and mineral sulphur (>low input)
• Distance from industrial sites (>low sulphur depositions)

Sulphur deficiency – symptoms

Sulphur deficiency is often hard to distinguish from nitrogen deficiency, to which it may be linked. Symptoms include a yellowing of the younger leaves, as a result of low chlorophyll production. Growth is generally reduced. In cereals, tillering is reduced. In oilseed rape, flowers are pale and leafs are distorted. In most cases, symptoms appear too late for effective treatment. Hidden deficiency is by far more frequent than acute deficiency. 

Where does the sulphur in soil come from?

The sulphur cycle in the soil shows some similarities with the nitrogen cycle. Sulphur exists in soil in different, interconnected pools. Only a minor part of it is immediately available for plant uptake. The remainder needs to undergo transformation processes first.

Crops prefer sulphate

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

Sources of sulphur

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

Sulphur from mineral fertilisers

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

Sulphur depositions from the atmosphere

Sulphur is present in the atmosphere mainly as sulphur dioxide (SO2) from natural events (volcanic eruptions) or man-made activities (burning of fossil fuels). Sulphur can enter the leaves of plants from the air as sulphur dioxide gas but this contribution is now relatively minor.Most of the atmospheric sulphur, however, enters the soil as acid rain. Over the last 25 years emissions of sulphur have dropped by over 82% so as a result depositions have also significantly reduced and now hardly exceed 5 kg/ha.

Sulphur from organic manures

Manure contains sulphur mainly as organic matter and therefore needs to be mineralised before it can be taken up. A recent study from ADAS found that only 5-10% of the sulphur in cattle manure was available to crops in the spring following an autumn application.

Sulphur leaching 

Sulphur behaves similarly to nitrogen in the soil. Sulphate ions, as nitrate ions, are dissolved and very prone to leaching. fertiliser application should therefore be matched to plant growth in order to ensure rapid uptake. Application in the main phase of plant growth is the most efficient strategy. Autumn applications of sulphur are not recommended.

How much sulphur do crops actually need?

When it comes to sulphur, crops are not equal. For some crops, soil supply can be sufficient while severe losses in quantity and yield are to be expected for others without appropriate sulfur fertilisation. Sulphur fertilisation is often guesswork. How much is enough?

Sulphur requirements

Some crops need more sulphur than others. Generally the higher the sulphur demand, the higher the sensitivity to deficiency. Oilseed rape has a very high uptake, but most of the sulphur remains in plant residues.

Uptake dynamics

Accumulated uptake is only one aspect of sulphur needs. Uptake dynamics are the other important aspect. Crops with a short vegetative period need high amounts of sulphur in a short time. Plants with a longer vegetation cycle have more time to recover sulphur from the soil and are therefore less dependent on external supply. Oilseed rape is specifically demanding with regard to sulphur, due to its short vegetation cycle and high uptake. Sulphur deficiency can therefore cause yield losses of up to 1 or 2 t/ha.

During the growing season, sulphur requirements for oilseed rape and wheat exceed by far the S supplied by soil. During May, typically half of sulphur needs are missing for wheat and two thirds for oilseed rape

Soil analysis

As with nitrogen, soil sampling and analysis can be conducted to quantify of actual amount of available sulphur in the soil. However, results can be variable and change rapidly due to plant uptake, soil mineralisation, capillary rise and leaching. Soil sampling is therefore very rarely used.

Tissue analysis

Tissue analysis to determine the concentration of sulphur in dry matter is a more reliable indicator of sulphur deficiency. Generally sulphur levels should exceed 0.3% of dry matter for most crops and 0.45% for oilseed rape.

Because of the close relationship between nitrogen and sulphur the calculation of the N:S ratio is often used and generally found to be a more meaningful indicator of sulphur deficiency for most crops.

Sulphur recommendations

The actual recommendations for sulphur depend on several factors and should always be checked with a FACTS Qualified Agronomist (FQA) and if manures are applied a full nutrient management plan should be conducted.

These are Yara's general sulphur recommendations for the major crops.

 Right nutrient rate, right nutrient source and right nutrient timing

 Not all fertilisers are the same so it is important to choose the right product to avoid compromising. As ever the three main factors to consider are: right nutrient rate, right nutrient source and right nutrient timing.

Right nutrient source

Plants need sulphur in the form of sulphate. This is the only form of sulphur which can be taken up by the roots and is the form in which it is used within the the plants. Plants cannot take up elemental sulphur which must first be converted into sulphate by soil microbe; a process which can take anything from days to weeks depending on soil temperature and moisture conditions.

Similarly plants prefer nitrogen in the form of nitrate. This is the form most readily taken up by plants and whilst plants can also take up nitrogen as ammonium this is less efficient and before it can be used the plant has to convert this into nitrate. Plants cannot take up urea which must also be converted by soil microbes into available forms.

If an efficient nutrient uptake and an immediate crop response is important to you then you should choose a fertiliser which contains sulphur as sulphate with a high proportion of nitrogen in the nitrate form. Sulphur fertilisers which are based on ammonium sulphate inevitably have a higher proportion of nitrogen in the ammonium form. Most YaraBela N+S fertilisers contain sulphur as calcium sulphate which is highly soluble so reaches plant roots quickly and calcium sulphate, does not reduce the proportion of available nitrate in the formulation, unlike ammonium sulphate.

Right nutrient timing

When it comes to the timing, the key message is to apply sulphur 'Little and Often'. This avoids all of the issues caused by applying all the sulphur in a single application early in the season. The 'little and often' approach reduces the risk of leaching and ensures the sulphur will be available for uptake when needed during periods of rapid growth. It also avoids problems of distribution of sulphur within the plant when the sulphur is bound up in older tissues making it unavailable when needed to support new growth and also avoids the risk of interactions with other nutrients such as molybdenum. To be able to achieve this it is necessary to choose a fertiliser with an appropriate N:S ratio to match the crops nutrient requirement.

Right nutrient rate

Finally there is no point in undoing all the good decisions made above by choosing a poor blended product which segregates during handling and application leading to an uneven application of nutrients to the crop. It is also worth avoiding low bulk density fertiliser which may not capable of being spread evenly over the entire working bout widths, especially if conditions are not ideal. To avoid this choose a compound fertilisers such as Yara Bela Axan and YaraBela Sulphan which contain both nitrogen and sulphur in every granule so eliminating segregation and allowing an even application of all nutrients.

Read more about YaraBela nitrate + sulphur fertilisers