Don’t rely on organic manures for sulphur

• Soil sulphur deficiency increasing

• Supplement for high yielding crops

• Mineral polysulphate ideal solution

Growers should take care to ensure sulphur deficiency is rectified to optimise plant health, yields and crop quality, says a leading academic.

Deficient sulphur levels have become a widespread issue in modern crop production, says Jorgen Eriksen, of the Department of Agroecology at Aarhus University, Denmark.

Addressing the International Fertiliser Society Conference in Cambridge, Professor Eriksen said lower atmospheric sulphur, increasingly wet weather and poor nutrient availability in organic fertiliser were a concern.

“Sulphur is often called the forgotten nutrient, yet it is one of the most important in terms of optimising plant health, making sure nitrogen is used by plants as effectively as possible and ensuring the best yields and quality.

“With reduced emissions from industry and power generation, there is now no more than a few kg/ha of sulphur available from the atmosphere and any sulphate present in the soil is vulnerable to leaching in much the same way nitrates are.

“With the increasingly wetter conditions over the winter and spring now frequently seen as result of climate change, there is virtually no carry over of sulphate from one year to the next in many areas.”

Prof Eriksen stresses that while growers are being encouraged to make greater use of organic sources of fertiliser, in reality they contain very little plant available sulphur, so timely and appropriate supplementation is essential.

“Animal manure is not a great source of sulphur as a huge part of it is in the organic form which is not immediately available to the plant.”

Storage

Another problem is that legislation around nitrogen use often results in manure being stored for a long time – often many months and in some cases up to a year, said Prof Eriksen.

“During that time plant available sulphate will turn into either the organic form or sulphide, which is extremely volatile and is basically the smell of manure, but neither of these are readily available to the plant.

“When you apply fresh manure, there is some content of plant available sulphur, but the longer it is stored the less of this there is and the more of the unavailable organic forms there are.

“These other non-sulphate forms will of course contribute in the longer term when they become mineralised in the soil, but they are not immediate sources and it could take several years before plants are able to utilise the sulphur they contain.”

Digestate from biogas production performs little better in terms of sulphur availability, he adds.

Availability

“When organic manures are used in digesters, plant availability of nitrogen in the resulting digestate increases compared to the raw manure or slurry. There is a decomposition process taking place that means what is left becomes more easily available.

“But this is not the case with sulphur. The result of the decomposition process produces volatile forms of sulphur that become emissions and when the methane goes to the grid, these have to be collected in filters.

“Furthermore, the solid material leaving the digester is not only very low in sulphur, it creates immobilisation of sulphur in the soil, so not only does it not contribute to plant availability, it actually fixes some of the sulphate that was otherwise available.”

“All in all, I would say organic sources offer the potential to supply 10% of the sulphur crops require, but no more,”explains Prof Eriksen.

“Growers relying on manures and slurries for their sulphur will soon run into deficiencies, so supplementation is essential.”

Supplementation

Independent soil analysis suggests some 85% of UK arable land is now deficient in sulphur and unless corrected this can have a serious effect on plant growth, final yields and grain quality, says Richard Ward of ICL Growing Solutions.

“With an exceptionally high sulphur content, naturally occurring polysulphate is a proven and reliable means of ensuring nitrogen is used as efficiently as possible and nutrient use efficiency is maximised.

“Polysulphate has vital prolonged release characteristics, with University of Nottingham trials showing over 50% of the sulphur contained is available in the first 12 days after application with the remainder released over the following 6-8 weeks.

“This matches nutrient availability precisely to crop needs through the growing cycle, which is in contrast to traditional NS products, where 100% of the sulphate is released within just 5 – 6 days after application.

“Plus, its unique composition of 48% SO3, 14% K2O, 17% CaO and 6% MgO means it also provides many of the other the essential building blocks required for optimum plant growth and health.

“It’s a naturally occurring multi-nutrient sulphate fertiliser mined from under the North Sea and processed into an easy-to-apply product with excellent physical properties and performance right up to spreading widths of 36m and more.”