There has been a gradual expansion by conventional fertilizer companies into the territory of slow- and controlled-release fertilizers (SCRFs) and stabilized fertilizers (SFs). Agrium – now combined with PotashCorp as Nutrien – was an early mover in SCRFs, launching its ESN product in the USA in 2002 and acquiring sulphur-coated urea products of Nu-Gro in 2006. Koch made a major move into the SF market with the acquisition in 2011 of the assets of Agrotain International “Agrotain” (NBPT) urease inhibitor. Eurochem has been marketing Entec nitrification inhibitors since acquiring assets from BASF at Antwerp in 2011. The growing force in China is Kingenta, who enlarged its geographical footprint by acquiring the Dutch company Ekompany in 2016, with its E-cote technology. On the technology side, Stamicarbon has teamed up with Pursell Agri-Tech – once a subsidiary of Agrium – to open a new fertilizer coating facility in Sylacauga, Alabama, USA in June 2018. Then Polish fertilizer producer Grupa Azoty acquired Compo Expert in November 2018. This sector has been characterized by much M&A activity. In this section, we take an overview of the product types, the players and the market segments to watch.
Enhanced efficiency fertilizers SCRSFs fall under the umbrella term enhanced efficiency fertilizers (EEF) and include controlled-release fertilizer (CRF), slow-release fertilizer (SRF) and sulphur (S) coated fertilizer, usually sulphur-coated urea (SCU). Enhanced efficiency fertilizers have characteristics that allow increased nutrient availability and reduced potential of nutrient loss to the environment, according to the American Association of Plant Food Control Officials (AAPFCO). The positive impact of fertilizers on global yields has been widely acknowledged, but since the turn of the 21st century, more attention has turned to their environmental impact and efficiency. From an environmental aspect, there is increased scrutiny on water pollution, the reduction in greenhouse gas emissions (GHGs) and soil health, which is now seen as a limiting factor for yield increases. In addition, there is the often-cited driver of population growth, where the world will have to feed an extra two billion people by 2050 from approximately the same area of agricultural land. In order to reduce the environmental impact of fertilizers, the phrase that is frequently used is that more output will need to be achieved from a lower volume of inputs or, simply put, “more for less.” SCRFs and SFs are often seen as part of the solution to this challenge. Previously they were more the preserve of recreational turf and ornamental markets, but increasing they are used for broad-acre applications. The unconventional is on the verge of becoming mainstream.
Managing nitrogen in the field When talking about slow- and controlled-release fertilizer products, the focus tends to be on nitrogen (N) products that are then blended with other nutrient granules, and in some cases with other straight N products. For example, the controlled-release nitrogen product ESN is often applied in a ratio of 85:15 with straight urea. Nitrogen also tends to be the focus for two reasons: firstly, it is applied in volumes significantly greater than either phosphate or potash, and secondly the management of N is difficult owing to the transformations that take place when it is applied to soil. Urea and ammoniacal N fertilizers are the most commonly applied. There will be regional variations. In Europe, more N will be applied in nitrate form, through ammonium nitrate and calcium ammonium nitrate. The dominant market for the direct application of ammonia is the USA. Here, the ammonia is injected directly into the soil. The high N content of these fertilizers (46 per cent and 81 per cent respectively) make them attractive on a $/unit N basis. Plants take up two forms of soil N: ammonium (NH4+) and nitrate (NO3-). N fertilizer needs to be converted to ammonium or nitrate before a plant can use them. Although urea and ammonia are attractive on a $/unit N basis, they are subject to losses through ammonia volatilization that can reduce their efficiency. These losses can vary, depending on soil moisture, pH and temperature. One paper from Purdue University in the USA quotes losses of 15-20 per cent N in the first week after application of urea that has not been incorporated on moist soil in sunny weather conditions. A paper from Montana State University reported larger losses for N: 30-44 per cent when urea is applied to moist soil surface, followed by a period of slow drying and no precipitation to help incorporation. Other papers give even higher loss rates.
Action of enzymes Urea and ammonia are subject to ammonia volatilization through the action of urease enzymes, which are naturally occurring in the soil. The urea is first hydrolysed to ammonium by urease – a process that can take several days and is dependent on temperature. The urease breaks the urea down into an intermediate product ammonium carbamate, which is a salt of ammonia and carbamic acid. The ammonium carbamate is unstable and breaks down to form ammonia gas and carbon dioxide. Additional moisture is then needed to turn this ammonia gas to an ammonium ion, which will then attach to soil particles. Ammonium has a positive charge and is attracted to the clay and organic matter in soil, which has a negative charge. If there is enough water to initiate hydrolysis of the urea but then not convert it to ammonium, this is the point where volatilization can occur if urea is broadcast and not incorporated into the soil. Slow release products work by slowing the action of urease on the urea, preserving the amount of time before the urease unlocks the ammonia for conversion to ammonium.
Nitrate leaching The next stage is for the ammonium to be converted to nitrate, which involves an intermediary step. Ammonium is converted to nitrite, and then to nitrate in a process known as nitrification. This is performed by bacteria in the soil. Nitrosomonas convert ammonia to nitrite, then nitrobacter convert nitrite to nitrate. Collectively, these bacteria are known as nitrifiers. Water content critical Nitrate fertilizer is subject to losses through leaching and denitrification, depending on the water content of the soil and the water movement through the soil. These nitrifiers are “aerobes” which means they need freely dissolved oxygen to do their work. In waterlogged soils the oxidation of ammonium is restricted. Although moisture is needed for the granule to dissolve, too much water and the ammonium will not be converted to nitrate. Ammonium can be taken up by the plant, but by slowing the conversion to nitrate when perhaps not desired limits the availability of N for the plant and lowering the nutrient-use efficiency (NUE).
Leaching After the ammonium forms of N are transformed to nitrate through the activity of soil bacteria there is the potential loss through leaching. Nitrate cannot attach itself to soil particles like ammonium – it has a negative charge – and this makes it vulnerable to leaching. Nitrate molecules are described as moving with the soil water. They are brought to the surface when water evaporates and move downwards after rainfall or irrigation. Some of this nitrate can be leached, causing river pollution. In the denitrification process, N20 is released into the atmosphere. Phosphorus is also subject to leaching and river pollution.
Denitrification If the soil is saturated for a prolonged number of days, another problem is lurking around the corner. The low level of oxygen will already be slowing the rate of conversion to ammonium. But other bacteria will start getting to work and convert the nitrate back to nitrite and the nitrite will convert to nitrous oxide, nitric oxide and nitrogen, which are then all lost to the atmosphere. The nitrate is taken up by the plant through the roots as the plant transpires. If the water leaches below the root system it will carry the nitrate with it, contributing to the loss of N, and the overall NUE.
ph sensitivity The pH value of soil is also an important indicator when determining the rate of nitrate conversion. Published sources say nitrifying bacteria are most effective in a pH window of 5.5-7.5. The Environment Protection Agency (EPA) in the USA gives an optimum range of 7.0-8.0. Values outside of this range can slow nitrifying activity. Low soil pH (more acidic) conditions depress ammonium oxidation because organisms dislike the conditions of low pH. The sensitivity of nitrifying bacteria to acidic conditions is a well-known phenomenon and generally attributed to the lack and/or toxicity of substrates (NH3 and HNO2) with decreasing pHs. (Gieseke 2006).
Slow, controlled or stabilized – what’s the difference? When talking about slow- or controlled-release fertilizer, it is important to consider the mechanism of release. If it cannot be controlled, then it tends to fall into the category of slow release. The products derived from urea, so-called urea reaction products, are what we tend to refer to as slow release, implying the rate is not controlled. Slow-release products tend to rely on the hydrolysis of water-soluble, low-molecular weight compounds. Controlled-release fertilizers tend to be conventional soluble fertilizers with a coating (encapsulation) of a water-insoluble or semipermeable material, or an impermeable material with pores. The stabilized fertilizers refer to nitrogen products that use inhibitors to control the conversion of N in the soil. These are either nitrification inhibitors (NIs) or urease inhibitors (UIs).
Slow release nitrogen The broader internationally accepted definition for a slow release fertilizer, according to the International Standards Organization (ISO), is (ISO 8157:2015): “A fertilizer, of which, by hydrolysis and/or by biodegradation and/or limited solubility, the nutrients available to plants is spread over a period of time, when compared to a 'reference soluble' product such as traditional straight U, AS or AN.” The release of nutrients from slow-release fertilizers is by hydrolysis or biodegradation and is mainly influenced by temperature and water. Slow-release N is usually divided into two forms: slow-release urea (urea reaction products, urea-based low solubility polymers) and sulphur-coated urea (SCU). They work by either delaying the initial amount of N available and/or extending the time of continued availability.
Reaction products Reaction products are produced by reacting urea with aldehyde to create a urea formaldehyde (UF), urea-triazone (UT), methylene urea (MU) and isobutylidine diurea (IBDU). The mechanism for release will depend on the length of polymer chains. The N content for reaction products tends to be lower than urea (46 per cent). The first reaction product, also known as a condensation product, to look at is IBDU (Isobutylidene diurea, 31-0-0). It is formed by reacting isobutyraldehyde with urea. The result is oligomers of similar chain length. The release mechanism is hydrolysis. IBDU is also dependent on temperature but to a lesser degree; but this does mean care is needed when using in greenhouses and that it’s best applied at lower temperatures. The release rate can be determined by particle size. IBDU can be blended or added to the production process for compound NPKs. The result can be fine granules for sports turf – this is because low-cut grass demands fine granulation. IBDU also finds its way to small tablets for potted nursery stock and ornamentals. For this application, release times can be between eight and 12 months. Larger briquettes can be made for forestry and plantations, often placed into the soil, with release from one to three years. Crotonylidene diurea (CDU) 32.5- 0-0 – first produced by Chisso Corporation in Japan reacting acetaldehyde and urea, and then by BASF using crotonaldehyde and urea. The release of N is dependent on hydrolysis and granule size and is also dependent on soil pH. Release rates tend to be longer than IBDU. CDU will break-down more quickly in acidic soil. CDU is used in Japan for turf and specialty fruit crops, while in Europe CDU is used in ornamentals for pot and container stock. Urea-formaldehyde – also known as UF, ureaform, urea-methanal, (38-0-0) is the product of urea condensation with formaldehyde to form a polymer. N release duration can be three to four months. Over 60 per cent of the N is insoluble in cold water. Urea-formaldehyde decomposes by the action of naturally occurring microbes. The activity of the microbes, and therefore the release rate of N, is determined by temperature. The optimum is 20- 30 degrees C. Unlike IBDU and CDU, UF contains methylene units of different chain lengths. The solubility (and hence availability of N) increases with decreased chain length. UF can be mixed with other N types and is used as a raw material in making NPKs. UF products can be in liquid or solid form. Typical commercial products are Nitroform by Koch, which has a 22-week release rate and has been on the market for over 50 years, according to the company. Another is CoRoN (controlled release nitrogen) by Helena AgriEnterprises (formerly Helena Chemical), a liquid product for foliar application. Methylene Urea (MU) typically has nutrient content 39-0-0 with release period of 10-16 weeks. The release mechanism is by microbial action – essentially, the MU provides a source of carbon to feed the microbes (Sadepan). The oligomer chains are uniform in length and of medium length. As a general rule, the longer the chains the more insoluble the product. The insolubility in cold water is often indicated by the vendor. Soil bacteria decompose the longer chains so that they are eventually soluble. A higher cold-water insolubility would likely indicate a longer release period. MU is used as straight N for horticultural crops, recreational turf and amenity turf. MU can be used in NPK blends as granules or used to make tablets.
Sulphur-coated urea Sulphur-coated urea (SCU) needs a mention here for completeness as a form of slow-release N. Indeed, it is one of the earliest forms of a slow-release N being developed by Tennessee Valley Authority (TVA) in the 1960s, by spraying molten sulphur onto granules in a rotating drum. The rate of release is not well controlled, but the sulphur brings the advantage of adding an additional nutrient. The urea granule is coated with sulphur, and usually a second coating consisting of a sealant is then applied. Sulphur is also an inexpensive coating, and in some parts of the world provides an outlet for its use since it is produced as a by-product at refineries (e.g. Iran). The second sealant can fill imperfections in the sulphur coating and provide some protection to the brittle coating when handled. The analysis can vary depending on the thickness of the sulphur coating. Typically, it is 30-40 per cent N. The sulphur coating is impermeable. The mechanism of N release is by the action of moisture and soil microbes on the micropores and cracks in the sulphur coating. The moisture eventually penetrates, dissolving the urea. If there are large cracks in the surface, this can lead to a rapid release of the urea – known as “catastrophic release.” Higher temperatures will increase the action of the microbes.
Coatings for controlled release The addition of a thin polymer coating was a key step in providing greater control over the release rate of fertilizers. Layers of polymer can be used to control the release rate, while only reducing the nutrient analysis by a small amount. The mode of release is different to that of SCU. There is no microbial action. In a two-step action, the release is dependent on moisture and in some cases temperature. In the first stage, water penetrates the semi-permeable polymer coating into the fertilizer granule, creating a salt solution with a high osmotic pressure. Remembering that osmosis is the movement of water across a membrane, the salt – in this case the fertilizer – attracts water across the membrane. The water acts to disperse the concentration of salt, causing osmotic pressure to build. This then drives the fertilizer solution through the pores of the coating. Some of the nutrients might not get pushed out because the pressure decreases when most of the nutrients have been released. There are two main categories of this type of CFR: polymer coated or polymer/sulphur-coated fertilizers, which is an outer coating of polymer over a coating of sulphur. The first category is then subdivided by the type of coating material. At this point, it is helpful to create subcategories based on thermoset or thermoplastic-based SCFs. The Meister method uses thermoplastic resins and manages to control the nutrient release by a blend of thermoplastics rather than relying on varying thickness to achieve varying release rates. Thermoplastic resin can be re-heated, which means they can be melted and then can be sprayed onto the granule as a liquid. This enables the application to be controlled more precisely. Thermoplastics are barely permeable to water, so there will be a low-permeability polyethylene with high-permeability polymer, such as ethylene-vinyl-acetate (EVA). The coatings can be applied to different substrates – urea, diammonium phosphate, potassium sulphate, potassium chloride and ammonium nitrate. Release rates can also be varied by blending talc resin into the coating. This is the method used by Chisso Asahi Fertilizer in Japan to produce Nutricote, a registered trademark of the Japanese company. Nutricote is currently distributed in Canada and in 41 states in the USA by Arysta LifeScience America, which was acquired by UPL in July 2018. Florikan is the exclusive distributor of Nutricote in the states of North Carolina, South Carolina, Georgia, Florida, Alabama, Mississippi, Tennessee, Louisiana and Arkansas.
Thermoset - Osmocote In contrast to thermoplastics, there are thermosetting plastics (or thermoset plastics) that are irreversibly hardened by heat. An example of a thermoset coating is the technology used by Everris (formerly Scotts Company) and now part of ICL Specialty Fertilizers. The technology for Osmocote and Agroblen was developed in the 1960s and is technically classified as a polymeric or alkyd resin. The coating is applied in several layers, with the coating thickness controlling the release. The release periods for this technology can range from three to four months up to 14 to 16 months.
Thermoset coatings – RLC Another subcategory of the polymer coatings is the reactive layer coating (RLC). This is when the coating reacts with the fertilizer core. To achieve a polymer coating are reacted with polyols on the surface of the fertilizer granule. This improves the physical resistance of the granule to attrition. Koch employs a RLC in its Polyon of polyurethane, poly-isocyanates product (originally produced by Pursell and then sold by Agrium). The granules are green, with an ultra-thin, ultra-tough polyurethane coating, states Koch’s website. Polyon uses a polyurethane; so too Haifa with Multicote, where the thickness of the shell determines release rate, and Aglukon’s Plantacote. ESN (environmentally smart nitrogen) is a urea granule covered by a proprietary polyurethane coating. Its analysis is 44-0-0 and its longevity is 40-90 days at 23C (73F). The release of N is driven by temperature and moisture. The higher the temperature the greater the release of N.
Polymer/sulphur coating The second category is polymer/ sulphur coating. These hybrid products use a cheap sulphur coating and a secondary polymer coating. The sulphur is applied first, followed by the polymer coating. The polymer coating adds protection to the sulphur, enabling the sulphur coating to be reduced in thickness. A thinner sulphur coating also lowers the risk of lock-off, which is when a coated granule fails to release its nutrients, often caused by the coating being too thick. The coating is a hard polymer and, unlike the SCU products, does not leave a waxy residue on the equipment. The diffusion rate is controlled by the composition and the thickness of the polymeric film.
Stabilized fertilizers Stabilized fertilizers (SF) are N products that have been treated in some way to extend the time that the N remains in the soil, in either the ureic or ammoniacal form. Stabilised fertilizers can be divided into two groups: nitrification inhibitors (NI) and urease inhibitors (UI).
Urease inhibitors (UI) UIs slow down the activity of a naturally occurring enzyme known as urease, which is responsible for converting urea to ammonium. Nitrification inhibitors delay the conversion of ammonium to nitrate by bacteria in the soil. UIs inactivate the urease enzymes for several weeks. The duration depends on local conditions. By delaying the breakdown of the applied urea, this can give the farmer enough time to wait for rainfall – significant for no-till systems – or incorporate into the soil. UIs allow more flexibility from an application point of view. This is why nitrates predominate in some regions, where it can be applied and then left for rainfall. Particularly in Europe, where there are a number of part-time farmers (in Germany for example), the application of calcium ammonium nitrate is preferred since there is no threat of volatilization (the nitrogen is in ammonium and nitrate form already, in a ratio of 50-50). Urease activity increases with temperature, so UIs can also play a role in warm conditions. This is why in Brazil nitrates are preferred for sugar cane because the warm tropical climate increases volatilization (and hence N loss) through increased urease activity. Urea coated with UI is the subject of research in Brazil (Cancellier 2016). In terms of products, the first to mention is Koch’s Agrotain N-(nbutyl) thiophosphoric triamide (NBPT) which was introduced in 1996 into the USA by IMC-Agrico before being acquired by LangeStegmann in 2000 who marketed the product under the name Agrotain. Agrotain is available as liquid and solid formulations. It can be used to coat urea granules or added to urea melt during the production process. It can also be added to the liquid fertilizers urea ammonium nitrate (UAN) prior to surface spreading in the field. Agrotain is primarily recommended for pre-planting surface application, but can be used for pre-emergence, side-dress, top-dress or other post-planting applications. SKW Piesteritz in Germany uses a urease inhibitor called N-phenylphosphoric triamide (2-NPT) in its product ALZON. It also uses the nitrification inhibito(MPA). NPPT – N-(n-propyl) thiophosphoric triamide (NPPT) is a urease inhibitor developed by BASF, which is combined with NBPT (see BASF interview).
Nitrification inhibitors (NI) The second type of stabilizers are nitrification inhibitors. Ammonium is converted to nitrite then nitrate through a process known as nitrification. NIs inhibit or reduce the bacterial conversion of ammonium in two stages: firstly, to nitrite by Nitrosomas spp. and then from nitrite to nitrate by Nitrobacter spp. These bacteria are known as nitrifiers. The NI can delay nitrification by two to 10 weeks depending on soil pH, temperature and humidity. NIs work by either “bacteriostatic” effect, which means the agent works by preventing the growth of the bacteria; or “bactericidal” mode, which means the NI kills the bacteria. No bacteria have been found that can convert NH3 to NO3directly (Hooper et al., 1997). NIs tend to be more effective in sandy soils, or soil low in organic matter and exposed to low temperatures, according to IPNI. The use of an NI can delay the conversion to nitrate in waterlogged fields, reducing the risk of denitrification and enabling time for the water to clear. Nitrapyrin (2-chloro-6-trichloromethyl pyridine) was first produced and sold as N-Serve by Dow Chemical, which has since undergone various transformations – firstly to DowDupont Agriculture division, and now as Corteva Agriscience. In contrast to DCD and DMMP, it has a bactericidal effect on Nitrosomonas. It was first registered in 1974 and was the first NI to be approved by the EPA. It can be added to fertilizer solutions or ammonia where it is injected into the soil. Nitrapyrin degrades in warm soils, so is suitable for the ammonia application in the fall. Nitrapyrin has a high vapour pressure, which means it easily evaporates from the soil. It has to be handled with care as it is corrosive and explosive. There is a new formulation of Nitrapyrin, “Instinct,” which is an encapsulated (coated) form that counters the evaporation effect. DCD (dicyandiamide) is produced from calcium cyanamide. The mode of action of DCD is a bacteriostatic effect, so rather than killing it suppresses the activity of Nitrosomonas spp. It is non-toxic. First introduced in 1984, DCD can be incorporated into solid, liquid or suspension fertilizer, as well as ammonia although this requires high-pressure equipment. A disadvantage of DCD is that relatively large quantities are required, around five to 10 per cent compared with the N content. It can also degrade quite quickly in soils. To counteract this, DCD is often formulated with other NIs such as 3-methyl pyra zole (3-MP) to reduce incorporation rates and extend the time period of reducing activity, such as “Alzon.” DCD alone can stabilize ammonium-based fertilizers for four to 10 weeks depending on the soil and climate. DMPP (3-4 dimethylpyrazole phosphate) is a nitrification inhibitor, developed by BASF in 1995 and marketed as “Entec” by Eurochem Agro and “NovaTec” by Compo Expert. The mode of action is a bacteriostatic effect – similar to DCD – while the incorporation and application rates are lower than DCD. It is speculated this is probably because DMPP is more stable, having lower mobility after absorption onto soil particles. This makes DMPP more suitable for use at higher temperatures in greenhouses or in irrigation in warm climates (Jaeger, 2012). Ammonium-based fertilizers treated with DMPP often contain a small amount of nitrate-N to give some immediate N for young crops, while the ammonium component is protected for conversion to nitrate at a later time.
Fertilizer Canada told New AG International that data collected from the Canadian Fertilizer Use Survey between 2015-2018 indicated grower use of nitrogen stabilizers on canola in Western Canada ranged from five to 10 per cent, and grower use of nitrogen stabilizers on corn in Eastern Canada ranged from 16 to 23 per cent.
Regulations There are two pieces of legislation that could have implications for the coated and stabilized markets in member states of the European Union. The first relates to an amendment to German domestic law, the so-called “Düngeverordnung” which is part of the German Fertilizer Law (Düngegesetz) which came into effect in June 2017. This piece of legislation essentially brings the German law closer to the Nitrates Directive, the EU legislation. At the heart of the legislation is the desire to reduce ammonia emissions by fertilizers and nitrate leaching. One of the key aspects of the legislation is that nutrient from organic sources, such as manure, are now counted towards the maximum totals that farmers can apply. This requires strict records on the application of all nutrient sources. The upper limit for N application per hectare (ha) is still 170 kg N per calendar year. There is now a requirement to work organic fertilizers into the ground within four hours of application. This four-hour rule also applies to urea unless it has a urease inhibitor. From 2020, the legislation also states that urea without an inhibitor can only be applied on fallow fields and still has to be worked in within four hours. On non-fallow fields, it will be mandatory for a farmer to use an inhibitor when applying urea. The second issue is polymer coatings for CRFs and the proposed regulations for EU member states. Under the EU Fertilizing Products Regulation (2019) the biodegradability criteria are set to be developed by mid-2024, according to Fertilizers Europe, coming into force in mid-2026
