Blog post by Indigro independent agronomists
The importance of sulphur in agriculture has been recognised over the last few years. This follows the reduction in sulphur dioxide emissions from industry, leading to the first signs of deficiencies in crops around 30 years ago. Symptoms of sulphur deficiencies are like those of nitrogen, with crops showing chlorosis (yellowing) of the leaves due to low chlorophyll production and stunted growth. Sulphur deficiency is made worse by soils that are light and sandy, have low organic matter, are prone to waterlogging and are acidic.
Sulphur
The main way of differentiating the deficiency is by looking at the location of the affected leaves – sulphur deficiency generally affects the newer leaves and nitrogen the older. In cereals you see reduced growth and tiller numbers, and in oilseed rape the flowers are pale and leaves distorted. Different crops require varying levels of sulphur to maximise their production and quality. The Nutrient Management Guide (RB209) provides a helpful risk table to determine the likely need for sulphur applications based on soil type and rainfall. For cereals for example, RB209 recommends 25-50kg SO3/ha applied in spring when crop requirement is highest. Oilseed rape has a higher requirement of 50-75kgs SO3/ha applied early March.
Sulphur is hugely important within a plant and is essential for the formation of proteins, amino acids, vitamins, and enzymes. Many compound fertilisers containing sulphur also contain nitrogen, and the two elements are intrinsically linked. Sulphur is part of an enzyme required for the uptake of nitrogen and is essential for photosynthesis, overall energy metabolism and carbohydrate production.
The recent escalation in the price of fertiliser has further focused our attention on its efficient use to maximising yield, crop quality and return on investment, whilst minimising the impact on the environment and global warming. It is expected that sulphur supply this year will be very tight, with the announcement of the closure of a major UK fertiliser production plant, where much of the common sulphur products and grades are produced. That, combined with the impact of the war in Ukraine limiting the supply of both nitrogen and sulphur products into the UK, has resulted in many growers now looking for alternative sources of sulphur.
What are the sulphur options?
Mineral Fertilisers
There are several sulphur containing fertilisers available – such as Ammonium Sulphate, Sulphate of Potash, Polysulphate, Kieserite (Magnesium Sulphate), Gypsum (Calcium Sulphate) and elemental sulphur.
The sulphate in fertiliser is immediately available, and liable to leaching if applied in wet conditions and / or before the crop needs it, and therefore should be applied in the spring when crops are actively growing. It is important to note that crop requirements are usually expressed as elemental sulphur (S) whereas fertiliser is declared as sulphur trioxide S03. To convert from one to the other requires the use of a factor of 2.5 i.e., 25kgs/ha of S becomes 60kgs/ha of S03.
There is an option to apply elemental sulphur which is less likely to leach, as it needs to be oxidized to sulphate by soil microbes before it becomes available to the plant. For this reason, elemental sulphur is mainly applied in the autumn.
Organic Manures
Solid organic material, such as farmyard manures and biosolids contain significant amounts of sulphur, but are slowly released as the organic sulphur must be mineralised to sulphate before being available to the plant. Sulphur content in the soil rises naturally from the breakdown of organic matter. Soils with high organic content, or that are heavy textured, are more able to supply adequate sulphur than light and inorganic soils.
Liquid organic material, such as slurries and digestates, supply more available sulphur, so should be applied in the spring ideally, preventing leaching and allowing maximum uptake. It is important to follow the Defra guidelines to comply with the Farming Rules for Water when considering application and timings.
Understanding sulphur and choosing the correct product and application timings will be essential to maximise crop potential; continual monitoring and measuring of soils nutritional status, including sulphur and organic matter, will provide useful baseline information. Regular testing of organic manures and other adjuncts for their nutrient content, and adjusting levels of inorganic inputs to maximise soil and plant health throughout the growing season will also maximise profitability. There are various methods of tissue testing within the growing season to identify deficiencies and continue the monitoring, measuring and management of plant health. Also, we advise growers to take grain samples, sending them to the lab for N:S ratio analysis. The results provide a valuable summary of nutrient levels within the grain over its growing season. A good sulphur supply would mean an N:S ratio lower than 17:1 and anything greater would suggest that the crop may have suffered from sulphur deficiency. This is particularly important for milling wheats where a ratio of 15:1 would be ideal.
Looking to an uncertain season for input pricing and availability, on farm sulphur management this coming year will be essential, not only as an important crop nutrient but also to maximise return from expensive nitrogen fertiliser. Indigro can help growers navigate these problems and please do get in touch for more information.
To find out more about Indigro independent agronomists follow this link https://indigro.co.uk/
