Fertilizer International 497 Jul-Aug 2020
31 July 2020
Sulphur: the fourth crop nutrient
SULPHUR FERTILIZERS
Sulphur: the fourth crop nutrient
Sulphur is becoming an increasingly important crop nutrient – due to a combination of lower sulphur emissions, the increasing prevalence of high-analysis fertilizers and higher cropping intensity.
Sulphur is present in all crops and plays an important metabolic role. It is essential for the formation of proteins, amino acids, vitamins and enzymes, and vital for photosynthesis, energy metabolism and carbohydrate production. Sulphur also contributes to the flavour and aroma of crops such as onions and can therefore influence the quality of farm produce.
Back in its rightful place
Importantly, sulphur does not act alone as a plant nutrient, as it works in tandem with nitrogen to enable the formation of amino acids during protein synthesis. Sulphur is also part of the plant enzyme required for nitrogen uptake. Sulphur and nitrogen are inseparable nutrients because of this, according to, fertilizer producer Yara International:
“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 team. Previously, crop requirements were generally met from atmospheric deposition, so sulphur was confined to a secondary role. However, today it is back in its rightful place as an essential component of optimum nitrogen management.”
In crop nutrition, sulphur plays a critical role in early crop establishment and improves resistance to environmental stress. Deficiency stunts early plant growth, leading to yield losses later on, and is exacerbated by the following conditions:
- Light and sandy soils with low soil organic matter
- Sulphur leaching during high winter rainfall
- Low sulphate mobility during dry spring conditions
- Slower mineralisation at low temperatures
- Low input of organic matter and mineral sulphur
- Low atmospheric deposition of sulphur to soils.
Crops can typically remove between 15 to 30 kilograms of sulphur per hectare from soil. Root vegetables, onions and brassica, especially oilseed rape (canola), have a particularly high demand for sulphur. Pasture and other widely-grown crops such as coffee, corn, cotton, rice, soybean, sugarcane and wheat also require moderately high sulphur applications (Figure 1). For these crop types, sulphur requirements can match or even exceed demand for phosphorus.
Increasing agricultural value
Sulphur is becoming an increasingly important crop nutrient due to three main factors:
- Falling atmospheric deposition. Soil sulphur deficiency, a relative rarity 20 years ago, is becoming more common. The deposition of sulphur dioxide emissions from the atmosphere used to guarantee that soils in many regions were automatically enriched and replenished with sulphur. This is no longer the case, however, as increasingly stringent environmental regulations and the introduction of low-sulphur fuels have sharply cut emissions
- The prevalence of high-analysis fertilizers. Farmers are continuing to switch to high-analysis products, containing little or no sulphur, at the expense of sulphur-rich, lower analysis products (Figure 2), a long-term consumption trend that has also put sulphur replenishment on a downward path.
- Rising cropping intensity. Improving crop yields are withdrawing ever larger amounts of sulphur from the field.
These three factors combined are opening up opportunities for fertilizer manufacturers to capitalise on the value of sulphur by broadening their portfolios and supplying sulphur-enhanced fertilizers as premium products to meet growing demand. Sulphur has certainly become increasingly valued by the farm sector in recent years, to the extent that some have even started to call sulphur ‘the fourth crop nutrient’.
Higher-analysis, lower sulphur
Worldwide, the amount of sulphur added to soils through the application of fertilizers has diminished in recent decades. This is partly because the use of sulphate fertilizers, mainly single superphosphate (SSP), ammonium sulphate (AS) and sulphate of potash (SOP), has either fallen or been outpaced by the rising consumption of higher-analysis alternatives such as urea, diammonium phosphate (DAP), monoammonium phosphate (MAP) and muriate of potash (MOP), which are largely sulphur-free. (Figure 2).
Traditional sulphate fertilizers
Total world demand for sulphur-containing fertilizers is more than 66 million tonnes. Consumption is greatest in Latin America (15.2 million tonnes), East Asia (14.3 million tonnes) and Southeast Asia and Oceania (10.3 million tonnes), with these three regions accounting for 60 percent of global demand for these products1 .
On a nutrient basis, global agricultural consumption of sulphur is estimated at 13.3 million tonnes, according to a firstof-its-kind assessment by the International Fertilizer Association (IFA)2 . This value is much higher than the frequently quoted estimate of 10-11 million tonnes1 . However, this latest value is probably still an underestimate, suggests IFA, as it excludes data for some NPK+S products2 .
The sulphur fertilizer market divides into two main categories – traditional sulphate fertilizers and sulphur-enhanced fertilizers. These have a range of nutrient compositions (Figure 3). Liquid sulphur products – thiosulphates – are also favoured in some countries and regions.
Traditional sulphate fertilizers have long dominated global demand (Fertilizer International 476, p19). They include:
- Single superphosphate (SSP) is the second largest-selling phosphate fertilizer on the market after diammonium phosphate (DAP). Consumption is concentrated in four main markets, China, Brazil, India and Australia, which collectively account for around 85 percent of total global demand. SSP is a low-analysis fertilizer with a nutrient content of around one-fifth (18-22% P2 O5 ). Because of this, it tends to be consumed in the country of origin, and export volumes have declined due to increasing competition from more economic high-analysis phosphates. SSP consumption has contracted by almost a third in the past twenty years.
- Ammonium sulphate (AS) consumption, in contrast, is on the rise even though its nitrogen content is much lower than urea and ammonium nitrate. World supply (26.4 million tonnes) has been boosted by the massive growth of ‘involuntary’ production capacity in China. Consumption of AS is concentrated in the Americas (the US, Brazil, Mexico and Canada) and East and Southeast Asia (China, Indonesia, Vietnam and Malaysia). Turkey and Germany also offer sizeable markets for AS (Fertilizer International 469, p20). The use of AS in NPK blends has become increasingly popular as awareness of sulphur deficiency in soils has become more widespread. Rapid growth in world oilseed rape (canola) production has been a notable factor behind the rise in AS demand.
- Sulphate of potash (SOP) is valued as a chloride-free source of potash for lucrative cash crops such as tobacco, tree nuts and citrus fruits. World demand is 7-8 million tonnes currently. China accounts for more than half of global consumption and has been responsible for much of the expansion in SOP demand globally in recent years. North America and Europe are also sizable markets accounting for some 60 percent of demand outside of China (Fertilizer International 475, p49).
- Global demand for sulphate of potash magnesia (SOPM) has grown strongly in recent years. The market for SOPM, similar to SOP and SSP, is highly concentrated with just four countries, China, the US, Canada and Germany, accounting for the lion’s share of consumption. Production is also mainly located in three countries, China, the US and Germany.
- Ammonium phosphate sulphate, a fertilizer with a 60 percent ammonium sulphate and 40 percent MAP composition, is a commonly produced grade of NP+S fertilizer (16-20-0-14S). It is directly applied to forage crops in many countries, particularly legumes, and is also a popular choice of fertilizer for small grains and oilseed rape (canola).
The global sulphur fertilizer market is still dominated by AS, SSP and SOP, with these three products combined being responsible for 70 percent of agricultural sulphur consumption (Figure 4).
Sulphur-enhanced fertilizers
Crop requirements for sulphur are projected to exceed 24 million tonnes by 2020. Fertilizer producers have reacted to the widening demand gap by developing sulphur-enhanced fertilizers (Figure 3). Many of these premium products are manufactured by incorporating elemental sulphur into high-analysis fertilizers, either within granules or as an external coating. Introducing a liquid sulphur spray to Urea, TSP, MAP or DAP during drum or pan granulation, for example, results in N and P products with a 5-20 percent elemental sulphur content.
Sulphur-enhanced fertilizers combine nutrient availability with high use-efficiency, and also have good storage and handling properties. Examples include: l Sulphur-bentonite l Sulphur-coated urea, MAP or TSP l Fortified SSP l Sulphur-enhanced DAP l Sulphur-enhanced MAP enriched with sulphate
The market for sulphur-enhanced NP+S products is developing particularly quickly in the US, Brazil and Africa1 .
Sulphur-enriched SSP is popular in countries such as New Zealand and can contain twice as much sulphur as ordinary SSP. Added elemental sulphur complements SSP’s existing sulphate content and helps meet crop needs during the whole growing season by providing both immediate and reserve stores of sulphur. This makes it particularly suitable for applications in areas with high leaching losses.
Controlled release fertilizers (CRFs) can be produced by coating highly-soluble nutrients with relatively insoluble elemental sulphur. Sulphur-coated urea (SCU), for example, combines 77-82% urea (36-38% N) with a 14-20% sulphur coating. SCU is suitable for multiple nitrogen applications on sandy soils under high rainfall or irrigation conditions. It is marketed as a CRF for grass forage, turf, sugarcane, pineapple, cranberries, strawberries and intermittently-flooded rice.
Sulphur-bentonite
To be of value to crops as a nutrient, the elemental sulphur (S8 ) present in sulphur-enhanced fertilizers firstly needs to be oxidised into plant-available sulphate by thiobacillus soil bacteria. This process requires the availability of oxygen and moisture and only occurs within a certain temperature range.
Fine elemental sulphur (40-150 microns) can be combined with 5-10 percent swelling clay to form sulphur-bentonite pastilles. The minor clay component promotes microbial conversion into sulphate early in the growing season by dispersing and releasing sulphur particles into the soil. This helps guarantee the supply of sulphur throughout the season and minimises leaching losses. Sulphur-bentonite is widely-used to treat sulphur deficiency in the US and India1 and is suitable for blending as well as direct application.
Tiger-Sul Products, a Canadian subsidiary of Connecticut-based HJ Baker & Bro, Inc, is a leading and long-standing global sulphur-bentonite supplier. The firm is the world’s largest producer of 90% sulphur pastille fertilizer, Tiger 90CR, a product that has been exported throughout South America, New Zealand, Australia, Europe, and China for more than 20 years.
Additionally, Tiger-Sul sells the granular sulphate/sulphur fertilizer Tiger 50CR (50% S + 12% N) which combines 36% elemental sulphur and 60% ammonium sulphate with 4% bentonite. The combination of immediately available sulphate and the slow-release sulphur make it an ideal product for colder Northern climates, according to Tiger-Sul.
The company also manufactures and markets Tiger Micronutrients, a range of premium sulphur-enhanced fertilizers. These combine Tiger 90 CR sulphur-bentonite with micronutrients using proprietary Microsite Enhanced technology.
HJ Baker expanded into the Chinese market in 2014 by opening a sulphur-bentonite plant in Lianyungang, China. This manufactures Tiger 90CR and a zinc-enhanced micronutrient product to meet the country’s growing demand for sulphur fertilizers. The company also opened a new Canadian Tiger-Sul sulphur-bentonite production plant at Irricana, Alberta, 46 miles north of its existing Calgary manufacturing plant, at the end of 2014.
In 2015, HJ Baker launched Tiger XP, a new sulphur-bentonite product with a higher release rate to target early-season sulphur deficiency in crops. This makes sulphur available to plants more rapidly by using a proprietary activator to speed up oxidation to sulphate.
Water-dispersible granules
A new sales agreement between Tiger-Sul, the world’s largest sulphur-bentonite producer, and Sulphur Mills Limited (SML), the world’s largest manufacturer of water-dispersible granules (WDGs), was announced in August last year This provides Tiger-Sul with exclusive North American rights to distribute and sell Sulphur Mills’ Techno-S® and Techno-Z® sulphur- and zinc-based products.
SML uses a patented process to manufacture 2-4 micron-size sulphur and zinc granules with excellent water dispersion properties. Its WDG products are used to supply sulphur and zinc to deficient crops via low-dose fertigation. Techno-S® is a 90 percent sulphur fertilizer, while Techno Z® combines 15 percent zinc with 70 percent sulphur. Both products undergo extremely quick oxidation, rapidly providing S and Zn in plant-available form.
Mumbai-based SML currently supplies products to over 80 countries. Its chief operating officer, Bimal Shah, said: “This venture to work together in the US and Canadian market brings a great value proposition of these two important nutrients, sulphur and zinc, to the farming community.”
Murat Kamisli, the general manager of SML’s international crop nutrition business, added, “The same technology and delivery system of these two patented nutrition products have been great successes in many other countries and we are looking forward to even greater successes in the US and Canada, with this partnership with Tiger-Sul Products.”
Thiosulfate products
Fertigation, the application of nutrients via an irrigation system, is a niche but high-value agricultural market for sulphur. Thiosulphates, in particular, are widely-used as liquid sulphur fertilizers in the broad acre and speciality crop market in North America and Europe. Their use is also on the increase in Latin America.
Tessenderl o Group is a global leader in speciality liquid fertilizers and manufactures four main thiosulphate products:
- Ammonium thiosulfate, Thio-Sul (12% N + 26% S)
- Potassium thiosulphate, KTS (25% K + 17% S)
- Calcium thiosulfate, CaTs (6% Ca +10% S)
- Magnesium thiosulfate, MagThio (4% Mg + 10% S)
Thio-Sul is suitable for most irrigation systems and, alongside nitrogen, delivers sulphur in both elemental and sulphate form. It also improves phosphorus uptake, and can be added to urea ammonium nitrate (UAN) as a nitrification inhibitor to reduce nitrogen losses. KTS, another of Tessenderlo’s leading thiosulphate products, is marketed as a high-analysis potassium and sulphur fertilizer for fertigation. It is suitable for booster or starter formulations and can also be applied as a foliar fertilizer when crop demand for potassium is high.
Thiosulphates offer sulphur in both immediately plant-available form and in slower-release form available to plants over a longer period of time. Thiosulfates also have a modest acidification effect, benefitting crops growing on alkaline (calcareous) soils.
Providing sulphur to crops by applying thiosulphates offers a number of specific benefits:
- Enhances crop protein and chlorophyll content
- Assists the synthesis and functioning of enzymes in the plant
- Optimises fertilizer efficiency by stabilising nitrogen
- Improves availability of nutrients in the soil, particularly phosphorus and micronutrients and their uptake by the crop
- Energy efficient assimilation in the plant
- Beneficial soil microorganism interaction
- Provides prolonged sulphur nutrition
- A controlled and localised pH adjustment effect in the soil.
Thio-Sul has the most powerful acidification effect because it combines the ammonium cation with thiosulfate. Thio-Sul can be combined with UAN solutions to provide two main benefits:
- It brings sulphur as a nutrient into the mix – the correct N/S ratio being very important for most crops
- It acts as nitrogen stabiliser improving nitrogen use efficiency.
Thio-Sul and CaTs both have the ability to inhibit the urease reaction, thereby reducing nitrogen loss through ammonia volatilisation, as well as slowing down nitrification by reducing the loss of nitrogen though nitrate leaching.
CaTs, as well as offering a nitrate- and chloride-free source of calcium, and providing thiosulfate sulphur, also acts as a soil-conditioner. Being a highly-soluble liquid form of calcium – unlike gypsum – CaTs is effective at penetrating the soil profile where it acts as a flocculant, opening up soil pores and improving soil structure and drainage. It can also help displace undesirably high levels of sodium in soils.
KTS is perhaps one of the most concentrated forms of liquid potassium and sulphur available in the market. When combined with liquid ammonium polyphosphate (APP), it can be applied as a very effective starter fertilizer early in the plant’s growth cycle. The presence of KTS improves phosphorus use efficiency by effectively regulating the rate at which polyphosphates turn into orthophosphates and becomes plant-available. (Note: Thio-Sul, CaTs, MagThio and KTS are registered trademarks of Tessenderlo Group NV/SA.)
Product and process innovation
The last five to six years has seen the emergence of speciality NP+S products. These have established a strong market presence in India, Brazil and the US during the last decade. Demand from Australia and Ethiopia is also on the increase.
The North American market for The Mosaic Company’s successful and pioneering sulphur-enhanced MAP product range, MicroEssentials, broke through the one million t/a barrier at the end of 2013. Mosaic’s sales of 8.2 million tonnes of finished phosphates last year included 3.2 million tonnes of its MicroEssentials speciality product. These versatile premium fertilizers are now applied to more than 10 percent of US farmland. They are suitable for both direct application and bulk blending and their increasing use is backed by more than a decade of field data and over 1,200 crop trials globally.
The proprietary Fusion process used in the manufacture of MicroEssentials joins together nitrogen, phosphorus, sulphur and zinc to create a nutritionally-balanced granule capable of boosting crop yields by 3-7 percent, compared to conventional MAP or DAP. Mosaic’s innovative fertilizer technology also provides nutrient use efficiency gains. MicroEssentials SZ formulations, for example, improve plant uptake of P by up to 30 percent and Zn uptake by up to 45 percent compared to a typical blend.
The company offers three main formulations:
- MicroEssentials SZ with 12% N, 40% P, 10% S and 1% Zn (12-40-0 10S 1Zn)
- MicroEssentials S15 with13% N, 33% P and 15% S (13-33-0 15S)
- MicroEssentials S10 with 12% N, 40% P and 10% S (12-40-0 10S)
The sulphur content in all three formulations is a 50:50 mix of elemental sulphur and sulphate.
Shell Sulphur Solutions has developed its own micronised sulphur product, Thiogro, which it licenses to key fertilizer producers around the world. The company successfully commercialised and patented sulphur technology for incorporating sulphur into ammonium phosphates in the early 2000s. Sulphur-enhanced phosphate lines have subsequently been licensed and installed at fertilizer plants in Asia, North America and Australia. This include a major collaboration with SinoChem in China (Sulphur 381, p24).
Shell also landed a major licensing deal with OCP Group in 2016. This allows the Moroccan phosphate giant to produce its own range of highly-concentrated sulphur-enhanced fertilizers by using Thiogro technology to incorporate elemental sulphur into ammonium phosphates, NPKs and other products manufactured at its Jorf Lasfar site. By adding new and potentially highly-lucrative premium products to its existing fertilizer range, the licencing deal with Shell is an important long-term strategic move by OCP.
A more recent breakthrough was Shell’s development of Urea-ES (enhanced sulphur), a dispersion of micronised sulphur in urea. This innovative technology – introduced to the market by Shell in 2016 – suspends 7-20 percent elemental sulphur in a urea matrix with a nitrogen content of 43-37 percent. Shell subsequently introduced Special-S, a further refinement of the technology, in 2017. This produces a co-granulated high sulphur content (11-0-0-75ES) urea product (Fertilizer International 492, p44).
Shell has successfully collaborated with both thyssenkrupp (Uhde Fertilizer Technologies) and IPCO, leading providers of fluid bed granulation and Rotoform finishing technology (see box), respectively. These partnerships mean Urea-ES and Special-S technologies are now widely available to producers wishing to expand their portfolios to include sulphur-enhanced fertilizers.
H Sulphur Corp, one of Asia’s leading sulphur suppliers and sulphur-bentonite producers, has licensed Shell’s Special-S technology, commissioning the first ever production plant in South Korea in February last year. H Sulphur subsequently begun manufacturing and selling Special-S under its own Super S brand name. This product has been successfully sold and shipped to customers in Canada, Australia and Brazil (Fertilizer International 492, p44).
Russia’s PhosAgro increased its sulphur fertilizer production capabilities by launching a new 100,000 t/a capacity production line at its Metachem site in 2015. This will manufacture sulphur-containing phosphate-potash fertilizers specifically formulated for priority markets such as Brazil.
EuroChem began production at Russia’s first urea ammonium sulphate (UAS) unit two years ago. The RUB 5.4 billion ($84 million) unit is located at EuroChem’s Novomoskovskiy Azot complex, about 200 kilometres south of Moscow. The 600 t/d unit was designed and built in partnership with Stamicarbon. Stamicarbon’s UAS process allows a very wide range of ammonium sulphate concentrations (0-50 weight percent) to be handled, without modifying the granulation plant, or affecting the quality of UAS granules produced.
EuroChem started UAS production at the end of 2018 following the commissioning of granulation equipment. The ability to manufacture UAS will add to EuroChem’s existing sulphur fertilizer portfolio, which already includes ammonium sulphate and ammonium sulphate-nitrate.
References
IPCO’s Rotoform process
Sulphur-bentonite pastilles
Sulphur-bentonite pastilles combine elemental sulphur with 10 percent bentonite. This special clay swells in wet soil, breaking apart the pastilles and releasing dustsize particles of elemental sulphur. These can then be easily oxidised into plant-available sulphate by soil microbes.
Rotoform pastillation
IPCO’s Rotoform pastillation process is ideal for the production of sulphur fertilizer products. It is simple and versatile, with low investment and operating costs, and minimal environmental impacts.
Multi-nutrient fertilizers can be created by combining sulphur with macronutrients (such as nitrogen from urea) or micronutrients, opening up new opportunities for producers in the speciality fertilizer market. The range of suitable speciality products includes:
- Sulphur-bentonite + micronutrients, e.g. zinc, iron, boron
- Urea + sulphur
- Urea + ammonium sulphate.
The Rotoform process, in partnership with Shell Thiogro, has been used to successfully mix sulphur with urea to produce Special S and Urea-ES products (see main article).
The Rotoform process, when linked to an upstream dosing and mixing plant, can be delivered as an automated continuous process for fertilizer production. An example of an IPCO plant for blending and mixing urea with sulphur is shown in Figure 5.
Additional benefits provided by the IPCO Rotoform process include:
- High-quality uniform and free-flowing pastilles with high crushing strength
- Very low dust, vapour and gas emissions
- Low power consumption.
The superior performance and operational flexibility offered by IPCO’s Rotoform process allows fertilizer producers to develop and launch innovative new sulphur-enhanced fertilizers. These can boost profit margins by adding value to existing commodity products, while at the same time helping farmers to achieve the higher crop yields they need.
