Use of sulphur fertilisers for organic crops

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Sulphur deficiency is likely to cost yields in organic crops. Apply sulphur to clover grass and winter oilseed rape, but provide 'windows' in the field to monitor the effects on growth and yield. The only fertilisers specifically aimed at covering sulphur deficiencies in Appendix 2 of the Guidance on organic agricultural production are free sulphur and gypsum, both of which are available on the Danish market.

In appendix 2.1 of the Guidance on organic agricultural production a supplementary text has been added, which means that if you need to apply sulphur, you can use a sulphur-containing fertiliser from appendix 2, such as patent potash or potassium sulphate 41.

The use of patent potash and potassium sulphate 41 is generally linked to the soil's potassium number (Kt below 8) and kiserite is linked to the soil's magnesium number (Mgt below 6). If you are not over-fertilising with another nutrient at the same time, causing imbalances, these products can be used. Remember to obtain/complete a declaration of fertiliser requirements for substances from Appendix 2.

Sulphur fertilisers permitted for organic crops

The total nutrient requirements of the crop - both sulphur and other nutrients - should determine the choice of sulphur fertiliser, as the ratio of nutrients in different fertilisers is very different. The plant availability of sulphur is estimated to be fairly similar for all the products shown in Table 1. However, free sulphur must first be converted to sulphate ions and the rate depends on the particle size.

The plant availability of powdered sulphur is estimated to be in line with the other sulphur fertiliser sources.

The nutrient content of sulphurous fertilisers can be seen in Table 1. You can also read more on the organic fertiliser exchange, where you can get an overview of availability and current regulations for fertilisers with and without sulphur.

Table 1. Sulphurous fertilisers and their nutrient content. Content may vary from batch to batch.

^1Annex2 and 2.1 of the Guidance on organic agricultural production, July 2025
^2Naturallyoccurring mineral with 18-23% sulphur, mined in Austria.
^3WigorS contains 90% naturally occurring elemental sulphur mixed with 10% bentonite (clay type).

Declaration of fertiliser requirements

Find the Declaration of fertiliser requirements for substances from Appendix 2 on this page: Link to crop production reporting if you are organic.

Sulphur can also be used for plant protection

"Spray sulphur" such as Kumulus S or Fungus Free are plant protection products that can be used in organic production to combat fungal diseases. This means that it is important to be able to document from e.g. the allocation of the products that the use of "fertiliser sulphur" such as Wigor S is solely as fertiliser and not as plant protection products.

Plant protection products for organic production are listed in Appendix 1 of the Guidance on organic agricultural production and can be found in Middeldatabasen.dk.

Sulphur fertilisers that may be permitted for organic crops

Fertilisation with soybean molasses

The residual product soybean molasses can be used as fertiliser, as GMO-free soy is used in some of the production. The significant content of nitrogen, potassium and sulphur as well as an affordable price make soy molasses an interesting fertiliser product. Be aware of getting GMO-free soya molasses.

Desulphurisation products from biogas plants

Most biogas plants have some form of desulphurisation plant installed. However, these are limited quantities, which are also in demand by fertiliser suppliers. In some cases, the technologies do not comply with organic regulations, so it is important to be aware of what kind of product you want to buy. Trials are underway to develop liquid fertiliser from desulphurisation plants from biogas plants that can supply fertiliser to organic farms. Such fertiliser has a high ammonium content and a high sulphate content and will have a rapid fertilising effect. The method has been approved for use in organic farming by the Danish Agency for Green Land Restructuring and the Aquatic Environment, but the fertiliser is still at the experimental stage.

Sulphur fertilisers that are not permitted for organic crops

TASP (Dry-Af-Sulphurisation-Product) comes from flue gas cleaning at power plants

Waste product from power plant flue gas cleaning (TASP), which contains approximately 16% sulphur and has the same effect as sulphur fertilisers such as gypsum and potassium sulphate. The main ingredient in the product is calcium sulphite (_CaSO3) with a certain amount of calcium sulphate (gypsum, _CaSO4). The product also contains substances from the power plant smoke, including heavy metals.

Fertigro®

Fertigro® contains 11 kg N, 1.5 kg P, 1.5 kg K and 6 kg S per tonne of product, making it an interesting source of sulphur. The product comes from one of the world's largest producers of heparin, which is a blood-thinning drug. Heparin is made from "mucosa", which is derived from pig intestines (mucus layer on the inside of pig intestines and the periosteum, which is connective tissue from the outside of pig intestines). No environmentally harmful substances are added in the process. Almost 100,000 tonnes of Fertigro® are produced per year.

Crop needs for sulphur

The sulphur requirements of conventional crops are shown in Table 2. As fertiliser levels and yields are generally lower in organic fields than conventional ones, the sulphur requirement can probably be reduced by 20-50%, depending on the crop and yield level.

The table is not soil type dependent, as the need does not vary significantly. Sandy soils are often more sensitive to sulphur deficiency, but often have lower yields. The sulphur requirement is therefore roughly the same.

Table 2. Indicative need for sulphur fertiliser in conventional crops. The need is likely to be lower in organic crops.

Trial results with sulphur fertilisers

We lack experimental documentation under organic growing conditions, but the response of organic clover grass to different sulphur fertilisers is highlighted in the Overview of National Trials 2016 under the title: "Clover grass - varieties and cultivation".

In 2017, Organic VKST on Zealand conducted a small trial with the application of several types of sulphur fertiliser to organic winter oilseed rape. The fertiliser types used were vinasse, kiserite, patent potash, spray sulphur and gypsum. The trial tested two different application times: before sowing and in the spring.

The results showed significantly higher yields for the application of sulphur fertiliser at both application times (min. 600 kg/ha). Especially the early spring application gave good results for all fertiliser types tested.

The Innovation Centre for Organic Agriculture has conducted organic national trials with sulphur application to winter oilseed rape in a series of trials that were completed in 2022. In 2021, a significant response was found for the application of sulphur to winter oilseed rape in the spring (See the Overview of the National Trials 2021).

Based on both years' trials, it is concluded that when sufficient nitrogen is available, a significant response is achieved for the application of sulphur in gypsum at sowing compared to no sulphur application.

Read more about the trial and its results here: Overview of the 2022 National Trials (pages 281-282).

The background input

The input comes from the combustion of fossil fuels in connection with transport, energy production, industry etc. The results in 2013 from the Danish main stations showed an annual background input (deposition) of 3.8-5.7 kg S per ha.

The rest of the deposition comes from natural sources, of which sulphate from sea salt is the most important. The sulphur that falls with precipitation during the winter months is just as prone to leaching as nitrate-N if there is no effective plant cover on the soil.

Low content and availability of sulphur in manure

The content of S in manure is low, less than 0.5 kg per tonne in slurry, around 1 kg per tonne in solid manure from cattle and pigs and around 3 kg per tonne in deep litter from chickens. Only a small part of the sulphur in applied manure is plant-available.

When manure is applied, the first-year effect is unlikely to be higher than 1 kg of sulphur per ha. In fields where manure is continuously applied, a release of 3-5 kg S per ha per year can be expected.

Sulphur in irrigation water

The sulphur content in groundwater varies, but is typically around 15 mg per litre. When irrigating with 30 mm, for example, 4.5 kg S per hectare is added. Sulphur from groundwater is directly available to plants, but typically the crop is only irrigated after the critical period of sulphur deficiency when a water deficit occurs.

Sulphur in manganese sulphate

When spraying manganese sulphate for manganese deficiency, a maximum of 5 kg manganese sulphate per hectare can be applied. In this amount, less than 1 kg of sulphur is applied, so spraying manganese sulphate only adds a very small amount of sulphur.

After-effect of cover crops

Sulphur uptake of up to 36 kg per hectare has been measured in cruciferous cover crops on a clay soil, but typically the uptake is much lower. High uptake requires a good supply of nitrogen. In another series of trials on the same clay soil, the best cover crops absorbed 10-12 kg of sulphur per hectare.

On the coarse sandy soil at Jyndevad, no more than 5 kg of sulphur per hectare can be expected. As with nitrogen, the mineralisation of organically bound sulphur depends on the ratio to carbon, the C:S ratio. In trials, the C:S ratios in different cover crops varied from 40 to 329.

Typically, the C:S ratio must be below 200 for there to be a net release of sulphur. The highest mineralisation of 57-85% has been measured after cruciferous cover crops, and the lowest mineralisation of up to 46% has been measured after legumes. Cover crops contribute a certain amount of sulphur to subsequent crops, but it is rare that ploughing down a cover crop can cover the entire crop's sulphur needs.

It's a good idea to always check the effect of a fertiliser if in doubt. Therefore, use the very short time it takes to create one or more "windows" in the field when no sulphur fertiliser is applied.

References

• Bertelsen, I. 2016. "Clover grass - varieties and cultivation". Overview of the National Trials 2016, pp. 290-296.

• Eriksen, J., Thorup-Kristensen, K., Askegaard, M. 2004. Plant availability of catch crop sulphur following spring incorporation. J. Plant Nutr. Soil Sci. 167, 609-615.

• Eriksen, J. 2009. Soil Sulphur Cycling in Temperate Agricultural Systems. Advances in Agronomy, Volume 102, 55-89.

• Knudsen, L. 2009. Cultivation guide sulphur. LandbrugsInfo.dk

• Laursen, C., Bertelsen, I. 2020. "Overview of the National Trials 2020", pp. 278, 280-281.

• Thomsen, D. A. 2017. "Large additional yields for sulphur fertilisation of organic oilseed rape". Økologisk Planteavlsberetning 2017, Økologisk VKST, pp. 19-20.