Don't forget about sulphur

The focus on sulphur nutrition has changed many times in the past but today it is recognised as an essential nutrient and considered just as important as nitrogen, so not to be forgotten.

Don't forget about sulphur

Don't forget about sulphur

Is sulphur now the second most 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 bond and should be considered as an essential component of optimum nitrogen management.

Why is sulphur such an 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 

Until the 1980's, sulphur availability in cropping and pasture enterprises was not a matter for concern since phosphorus application was mainly by super phosphate, which supplied S in excess to crop needs and building soil S levels. The 1980’s saw the introduction of high analysis fertilisers, containing little S and the switch to canola production. Soils were mined of sulphur and sulphur deficiencies appeared. Blanket recommendations of 20-30 Kg/ha S were implemented for canola production and this has been followed for many years. However, this has been questioned in recent years as high application rates of S has built up the adequacy levels of our soils. Many current crop fertiliser programs are applying little or no sulphur which may lead to sulphur deficiencies. 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 canola, 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.
Sulphur deficiency in wheat

Sulphur deficiency in in a wheat with typical pale chlorosis on newer leaves and stunted growth

Sulphur deficiency in canola

Canola with advanced sulphur deficiency (left). Growth is reduced, flowering is sparse and white.

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 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. Sulphur application strategies that split or use multiple applications, particularly for lighter soil types and areas that receive large rainfall events which can leach sulphur before plant uptake occurs are preferred.

Sulphur cycle

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. Canola 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. Canola 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.

Sulphur requirements for canola and wheat

During the growing season, sulphur requirements for canola and wheat exceed by far the S supplied by soil.

Soil analysis

As with nitrogen, soil sampling and analysis can be conducted to quantify of actual amount of available sulphur in the soil. Sulphur availability can change rapidly due to plant uptake, soil mineralisation, capillary rise and leaching. Just like nitrogen, this factors need to be considered throughout the season and fertiliser programs adjusted to take into account changes.

Tissue analysis

Typical Crop NS ratiosTissue 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 canola. 

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

Sulphur RecomendationsThe actual recommendations for sulphur depend on several factors and should always be checked with a Qualified Agronomist and if manures are applied a full nutrient management plan should be conducted.

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

How to choose a sulphur fertiliser?

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

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 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 for root uptake. 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 roots cannot take up urea which must also be converted by soil microbes into available forms.

Choosing a sulphur fertilizerFor soil applications, 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 such as YaraMila or Yara Liquids Sulsa products. For foliar applications, if quick crop response is required then you should choose a fertiliser that contains sulphate sulphur and urea nitrogen such as Yara Liquids Sulsa 27-0-0+7S.

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

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 fertilizer which may not capable of being spread evenly over the entire working bout widths, especially if conditions are not ideal. To avoid this choose compound fertilisers such as YaraMila 17-5-10(4) or YaraMila 21-7-3(4) which contain both nitrogen and sulphur in every granule/ prill so eliminating segregation and allowing an even application of all nutrients, alternatively choose, Yara Liquids Sulsa 36-0-0+3S or Yara Liquids Sulsa 27-0-0+7S through boomspray equipment for even application with unbeatable accuracy.

Right Placement

Finally, nutrients need to be placed in the active root zone for plant uptake. When planning fertiliser programs, consider the mobility of the nutrients to be applied by the soil type in the field for application when determining where to position the fertiliser product, eg. mobile nutrients such as nitrogen & sulphur can be both surface applied or incorporated into the soil.