Fertilizer International 507 Mar-Apr 2022
31 March 2022
Sulphur fertilizers: a growing need
CROP NUTRIENTS
Sulphur fertilizers: a growing need
Sulphur is becoming an increasingly vital crop nutrient – due to a combination of lower sulphur deposition from the atmosphere, the increasing prevalence of high-analysis fertilizers and higher cropping intensity.
The sulphur cycle
Similar to nitrogen and phosphorus, sulphur follows a cycle (Sulphur 392 p16). This cycle (Figure 1) illustrates:
- How sulphur moves between land, air and sea in different forms
- Key processes such as crop uptake, leaching and volatilisation
- The microbial action which makes sulphur plant-available by changing it from organic to inorganic form.
Crops access and remove sulphur through their roots in sulphate form. It can also be taken up as sulphur dioxide gas.
Sulphur typically enters the soil solution by the mineralisation of organic matter. Every one percent of soil organic matter can supply around 1.4-2.3 kilograms of sulphur via mineralisation. Some soil microbes and plants immobilise (fix) sulphur while others mineralise (oxidise) it into sulphate. These mineralisation and immobilisation processes often occur simultaneously within soils as part of sulphur removal and replenishment. Because 95 percent of sulphur found in soils is associated with organic matter, low organic matter soils are typically sulphur deficient (Sulphur 392 p16).
From 2005 to 2014, the average amount of fertilizer sulphur used annually in global crop production was 10.6 million tonnes, with some 53 percent (5.6 million tonnes) of this applied to cereals1 . However, recent calculations suggest that, on average, only one million tonnes of the total sulphur applied in cereal crop production was subsequently recovered in the grain – suggesting a sulphur use efficiency (SUE) of around 18 percent1 . Reasons for low SUE include sulphur:
- Leaching
- Adsorption
- Retention in residues
- Immobilisation
- Failure to adhere to best agronomic practice, e.g. the 4Rs.
The fourth crop nutrient
Sulphur has become increasingly valued by the farm sector in recent years, to the extent that some now describe sulphur as ‘the fourth crop nutrient’ (Fertilizer International 497, p24).
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.
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.
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 later yield losses, 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.
The main signs of sulphur deficiency – pale green colouration, stunted growth and delayed maturity – mirror the symptoms of nitrogen deficiency. However, sulphur deficiency affects in newer growth, typically in the top leaves or whorl, while nitrogen deficiency shows up in the bottom leaves of the plant. A lack of sulphur in corn and wheat slows down photosynthesis and root/plant development as shown by a pale green colour and a lack of vigour (Figures 2 and 3).
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 4). For these crop types, sulphur requirements can match or even exceed demand for phosphorus.
A bushel of corn, for example, removes 77 grams of sulphur in total – 36 grams in the grain and 41 grams in the stalk. That means 200 bushels of corn will remove sulphur at a rate of 38 kg/ha. This is equivalent to an actual removal rate of plant-available sulphate (SO4 ) of 114 kg/ ha (Sulphur 392, p16).
Increasing agricultural value
Sulphur is becoming an increasingly important crop nutrient due to three main factors (Fertilizer International 497, p24):
- 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 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. This long-term buying trend 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 are, however, opening up opportunities for fertilizer producers. A number of leading manufacturers are capitalising on the value of sulphur by broadening their portfolios and supplying sulphur-enhanced fertilizers as premium products to meet growing market demand.
Global and regional consumption
Accurate and up-to-date figures on sulphur fertilizer consumption are hard to come by. Despite this, the market is mainly divided between the following products, in order of popularity:
- Ammonium sulphate
- Single superphosphate (SSP)
- Sulphate of potash (SOP)
- NPS products
- Ammonium thiosulphate (ATS).
The total world market for sulphur-containing fertilizers was estimated at more than 66 million tonnes in 2015. On a nutrient basis this equates to global agricultural sulphur consumption of around 11.1 million tonnes. Sulphur consumption is greatest in Latin America (2.4 million tonnes), East Asia (2.1 million tonnes) and Southeast Asia and Oceania (1.8 million tonnes), with these three regions accounting for 57 percent of global agricultural sulphur usage2 .
SPOTLIGHT ON THIOSULPHATES
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.
Tessenderlo 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 l 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 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.
However, worldwide agricultural consumption of sulphur could be closer to 13.3 million tonnes (Figure 5), according to a first-of-its-kind assessment by the International Fertilizer Association (IFA)3 . This volume is much higher than the frequently quoted estimate of 10-11 million tonnes. However, this latest tonnage is probably still an underestimate, suggests IFA, as it excludes data for some NPK+S products3 .
Market trends
The sulphur fertilizer market divides into two main categories – traditional sulphate fertilizers and sulphur-enhanced fertilizers. These have a wide range of nutrient compositions (Figure 6). Liquid sulphur products – thiosulphates – are also favoured in some countries and regions, particularly in North America and Europe (see box).
- 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.
Rising NPS fertilizer use
The last five to six years has seen the emergence of speciality NPS 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.
Mosaic blazes a trail
Incorporating sulphur into ammonium phosphate fertilizers to create NPS products has become an increasingly popular way of addressing sulphur deficiencies. The Mosaic Company has pioneered this market through MicroEssentials, its successful and pioneering sulphur-enhanced monoammonium phosphate (MAP) product range, These value-added products typically sell at a $45/t premium over standard MAP.
The North American market for 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 in 2019 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 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 of these is a 50:50 mix of elemental sulphur and sulphate.
OCP and PhosAgro look to capture price premiums
Envious of Mosaic’s attractive NPS margins, rival phosphate producers OCP and PhosAgro have also move into this value-added sector – although their NPS products, unlike Mosaic’s, only contain sulphur in sulphate form at present. Both companies have been ramping up their NPS export capabilities to meet market demand.
During the 2020 pandemic, for example, Russia’s January-September NP/NPS exports rose 16 percent year-on-year to exceed 600,000 tonnes. The NPS grade 20-20-0+13S – mainly targeted at India – accounted for 64 percent of NP/NPS exports over this period. While India produces significant volumes of its own 20-20-0+13S domestically, these are topped up with NPS imports from Indonesia, Russia, China and – more recently – Saudi Arabian product4 .
PhosAgro’s Cherepovets plant was Russia’s largest exporter of the NPS grade 20-20-0+13S in 2020, although EuroChem’s Belorechensk plant also contributed significantly to export volumes. PhosAgro has increased its annual NP/NPS sales volumes from around 50,000 tonnes in 2010 to more than 200,000 tonnes currently. This sales growth has been driven by higher 20-20-0+13S deliveries to India, Turkey, Romania and Poland, as well as rising 14-40-0+7S deliveries into Brazil and Mexico4 .
Similarly, OCP has ramped up NPS production and exports at its Jorf Lasfar complex, partly to meet demand from large African tenders. The Moroccan phosphate giant exported 990,000 tonnes of NPS in 2019. Some 70 percent of this volume went to fulfil Ethiopian tenders, with the remainder destined for Brazil, the US, Canada and Australia4 .
Micronised water-dispersible granules
Sulphur Mills Limited (SML) is the world’s largest manufacturer of water-dispersible granules (WDGs). The Mumbai-based company has been expanding rapidly and increasing the availability of these technologically innovative, sulphur-based products across the globe. WDGs are extremely fine, 2-4 micron size granules manufactured by SML using a patented process.
SML’s flagship products, Techno-S ® and Techno-Z ® , are the world’s first low-dose WDG fertilizers. They represent a technological leap forward in soil-based fertilizer applications, according to the company, and are helping to change the way sulphur-based fertilizers are conventionally perceived. Both products have excellent water dispersion properties and are offered as either standalone sulphur microgranules (Techno-S ® ) or zinc-fortified microgranules (Techno-Z ® ),
Techno-S® and Techno-Z® fertilizers function at very low dosages, offer high nutrient use efficiency and undergo quick oxidation – making sulphur and zinc readily available to crops through – out the growing season.”
Techno-S ® is a 90 percent sulphur fertilizer, while Techno-Z ® combines 15 percent zinc with 70 percent sulphur. Both fertilizers function at very low dosages and offer high nutrient use efficiency. They undergo quick oxidation, making S and Zn readily available to the plant throughout the season from early crop stages onwards. SML has thoroughly tested WDGs agronomically – across countries, crop types and on different soil types. This has demonstrated their ability to improve various growth and quality parameters, including: better crop yields, increases in the protein and oil content of oil seed crops, higher starch content in tubers, and prolonging the shelf life of final produce such as onions.
The versatility of both micron-sized granular fertilizers, particularly their flexibility when it comes to the choice of application method, is an added advantage. This means they are suitable for various farming practices across the world – including micro-irrigation (drip) systems, sprinklers, overhead irrigation equipment, or conventional broadcasting and seed drill applications.
“Our R&D is continuously working towards providing more and more innovative solutions in the plant nutrient as well as the crop protection space,” Bimal Shah, SML’s managing director, told Fertilizer International. “Our products reduce the overall impact on the environment while providing the most efficient, efficacious and cost-effective solutions to the farming community.”
SML’s product range is now trusted by farmers in over 80 countries. Advantageously, being OMRI-certified, Techno-S ® and Techno-Z ® are approved for organic farming too.
- 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. Agricultural consumption is 7-8 million tonnes currently. China accounts for more than half of global use 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 is supply-constrained meaning that SOP trades at a premium.
- 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 (APS), 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 5).
Sulphur-enhanced fertilizers
Crop requirements for sulphur were projected to exceed 24 million tonnes by 2020. Fertilizer producers have reacted to this widening demand gap by developing sulphur-enhanced fertilizers. 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 NPS 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:
- Sulphur-bentonite
- Sulphur-coated urea, MAP or TSP
- Sulphur-enriched SSP
- Sulphur-enhanced MAP enriched with sulphate
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 percent urea (36-38% N) with a 14-20 percent 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 rice.
“Speciality NPS products have emerged in recent years. These have established a strong market presence in India, Brazil and the US.”
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 India and is suitable for blending as well as direct application.
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