Why is sulphur such an important nutrient?
Many agronomists now consider sulphur to be the second most important nutrient after nitrogen. It is closely linked with nitrogen in biological processes. It plays an important role in plant metabolism, 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.
Sulphur is also essentially involved in photosynthesis, overall energy metabolism and carbohydrate production.
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
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 and impaired growth .
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.
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 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 too.
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.
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.
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.
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