1. Controlled Release Fertiliser technology
2. Characteristics and analysis of Controlled Release Fertilisers
3. Degradation of coatings
4. Impact of EU Fertilising Products Regulation

Controlled Release Fertiliser technology

In general, there are two classes of Controlled Release Fertilisers (CRFs).

The first group of these is that of sulphur coated fertilisers. They were developed in 1960s and 1970s by Tennessee Valley Authority, USA. The products are typically produced by spraying molten sulphur on to the fertiliser granule in a rotating drum. The sulphur acts as an impermeable membrane that water can only penetrate through small imperfections in the coating to dissolve the fertiliser core (generally urea). The advantage of this system is that the sulphur coating material is cheap, it will oxidise in time and it also acts as a nutrient. Its disadvantage is that very precise control over the imperfections in the coating is needed to control the rate of nutrient release over time. This proved to be technically challenging and the industry started looking for other alternate and more controllable technologies. The logical next step was to cover the imperfections either with waxes and/or polymers. Several products were introduced based on this principle. Sulphur remains the principal barrier for the nutrient release, so many of the strengths/weaknesses of the sulphur coated products remain unresolved.

The second class of controlled release fertilisers is that of the polymer coated products. Very different chemistries are used for coating purposes. These chemistries range from cross-linked thermosetting materials to thermoplastic materials. The first system to be commercially introduced was based on an alkyd system, a cross-linked vegetable oil. Several other systems based on reactive monomers such as polyurethanes followed, along with polyolefins. The coating materials must provide an excellent barrier to water vapour entering the granule, which means that sophisticated material properties are required. In Table 1 the different types of technology are shown. For optimal control of nutrient release the coating needs to be stable and not degrade too quickly.

The release mechanism for the polymer coated material is in general water penetration through the coating followed by swelling of the granules and gradual release of the dissolved nutrients over time. The swelling of the granules is caused by water vapour diffusion through the coating, creating a high osmotic pressure that causes micro-fissures in the coating. Due to the elastic properties of the coating these micro-fissures close again when some of the pressure is released by expelling the highly concentrated nutrient solution. After release of the nutrients the thin coating fragments are ultimately degraded. The release period is simply governed by the coating chemistry and coating thickness. A schematic presentation is given in Figure 1.

A second mechanism for the control of the release of nutrients is by the use of a very strong barrier coating in which a minute amount of a pore forming compound is added. In this case the nutrient release is governed by the movement of water and nutrients through these pores. In this case the nutrient release is determined by the pore forming compound and coating thickness.

Another aspect to be mentioned is that currently all kinds of different nutrients and combinations of nutrients are available in coated form on the market. Along with coated urea, ammonium phosphate, potassium sulphate, potassium chloride, potassium nitrate, NPK, NK, PK are also available in a coated form. More recently specialty products have been developed with substrates such as coated calcium nitrate, magnesium sulphate, iron sulphate and aluminium sulphate. For ornamental applications the NPK’s are generally combined with all necessary micro-nutrients (B, Cu, Fe, Mn, Mo, Zn, Co), since the growing media used (peat and/or bark) do not contain these elements in any significant available amount.

For turf and agriculture applications blends of coated and uncoated fertilisers are used. With blends it is important that the size (and density) of the ‘blend partner’ materials are matched with the coated fertilisers. Otherwise, segregation of components will occur during the production, packaging, transport and handling processes. This will result in uneven distribution across fields, resulting in suboptimal agronomic results. Care also needs to be taken during the blending operation to damaging the thin coatings of the CRFs. Sulphur and sulphur/polymer coated fertilisers can be particularly sensitive to mechanical damage during blending.

Characteristics and analysis of Controlled Release Fertilisers

As indicated, the key property of CRFs is their controlled release of nutrients and the ability to match the release pattern with the nutrient demand of a crop. The nutrients are released gradually over a period of between six weeks and about eighteen months for CRF. The release is generally tested in controlled (laboratory) conditions. Several different methods of testing the rate of release are used. These are described in more detail in a recent overview (Terlingen, 2014).

In general, a small sample (10-12.5 g) is placed in 200-500 ml of water at 25-30°C and the release of nutrients is measured over time. For instance, in the European test method (EN 13266) 10 g is placed in 500 ml water at 25°C under mild stirring conditions. An example of the measured release of a fully coated CRF is given in Figure 2.

EN13266 sets certain requirements for such a controlled release product. The first requirement is that the nutrient release during the first 24 hours (called initial release) should be lower than 15% of the total nutrient content.  The reason for this requirement is twofold. Firstly, it is a crop protection requirement. A high initial release can damage crops because application rates for CRF are typically higher than for conventional fertilisers due to the smaller number of applications. The second consideration is economic: the fraction released in the first day acts in the same way as conventional fertiliser and could therefore simply be replaced by such material.

The third requirement is that the release over the first month should be less than 75%. This is closely related to the last, fourth, requirement: that the stated release time or longevity. In the stated release time 75% of the nutrients should be released. Thus the products with a release period of more than one month are considered as true CRF according to this norm.

From Figure 2 it is clear that there are some challenges with these requirements especially for coated NPK products, which are commonly used for ornamental applications. First of all nitrogen, phosphate and potassium have distinctly different patterns. The nitrogen release is slightly faster in time than the rate of potassium release, followed by that of the phosphate. Based on the graph, the longevity according to EN13266 for nitrogen is at least 100 days, potassium is at least 120 days and phosphate more than 200 days. The release of the nutrients is directly related to their water solubility. Faster release is due to a relative higher solubility. In this case the nitrogen ions (ammonium and nitrate) are more soluble than the potash and phosphate components. The solubility of the nutrients is in general very high (>100 g/l) and the different release rate of N, P and K is thus only relevant for coated fertilisers. Upon contact with water uncoated fertilisers will all quickly dissolve.

To create a simpler message to the end user, it is more advisable to use the electrical conductivity (EC) for these types of products and define longevity based on EC. Firstly, as can be seen from Figure 2, it gives an average across the different nutrients. Secondly the measurements are quick and simple, and the equipment needed is affordable. Its weakness is that it can only be used for non-urea type fertilisers, since urea does not provide conductivity in solution. Based on EC the longevity is at least 110 days. This is an easier message to understand than a range of longevities for the different elements.

The 75% release criteria need some explanation. In principle all the soluble nutrients will be released from coated fertilisers. However as a rule of thumb the time to release the first 75% takes as long as that for the release of the remaining 25%. In ornamental applications, the first period (75%) is used for the real growing period of the plant and the rest is used for lower-level base fertilisation.

The laboratory release tests are usually accurate and reproducible; however they are real-time and thus slow. A quicker method is highly desirable and the obvious choice is to test at higher temperature to reduce the time (EN 13266). The results obtained at high temperatures cannot be simply translated to match the release rate at 25°C. In addition, not all coating chemistries can withstand high temperatures.

The release rates tested under controlled conditions and in field conditions are remarkably similar when tested at the same temperature. In Figure 3 the release of a polyurethane coated urea tested at 21°C is compared to field release. Temperature loggers were added to the field test and the temperature of the soil (at the depth where the CRF was applied) was also around 20°C. The CRF was applied in small mesh bags for easy retrieval. However in most field conditions the temperature varies, and advanced simulation models are required to translate the release test under laboratory conditions to agronomically relevant release curves. Several companies have developed these software tools and use them to adjust the release profile/products to the crop requirement. This is of course required to achieve optimal nutrient efficiency.

Degradation of coatings

Coatings that are used for CRF are typically materials with very low water vapour permeability that are able to protect the hydrophilic fertiliser core from attracting moisture. Since CRF coatings other than sulphur coatings do not contribute to the nutrient content, the amount of coating to achieve a desired longevity should be as low as possible, to limit production costs and to provide the highest amount of nutrients per kg of product. From an environmental perspective this approach is also logical and desirable.

However challenges remain. Because of their excellent barrier properties, most of the polymeric coating materials used in CRFs are difficult for microorganisms to assimilate and they biodegrade very slowly. An industrial composting test series performed in 2008 at 58°C showed that the degradation of the most important coating systems in the EU is between 0-15% over a period of 3-4 months. Many coating materials are partially based on modified vegetable oils. Given the hydrolysable triglyceride ester bonds in the vegetable oils it is expected that these coatings will degrade over time or become bound to the soil complexes. The coating fragments that are formed are considered inert and as such become, upon physical disintegration, part of the soil. To ensure the safety of the coating fragments and of the final product, CRF coatings typically undergo thorough phytotoxicity and toxicity testing before release to the market. This is done to satisfy the general safety requirements by law e.g. in Europe (Regulation (EC) No 2003/2003).

Impact of EU Fertilising Products Regulation

The EU Fertilizing Products Regulation 2019/1009 includes a requirement for the coating to be biodegradable in water and soil, meaning 90% degradation of the carbon in the coating into CO₂ within two years after the functional period (longevity of the product) at 25°C in soil (European Commission, 2019). This requirement is a difficult one, since it asks for a product that is not degrading in the beginning (the functional period) and after that degrades for 90% in a maximum of two years. The CRF Industry is working hard to find solutions for these contradicting characteristics within the very narrow time it has got to come with solutions. CRFs are generally seen as fertilisers with high nutrient efficiency and therefore fit well into the new EU legislation “Farm to Fork” strategy which requires a 50% nutrient input reduction by 2030.

References

EN 13266 (2001). Slow release fertilizers –Determination of the release of nutrients- method for coated fertilizers.

European Commission (2019).EU Fertilizing Products Regulation 2019/1009.

Regulation (EC) No 2003/2003 of the European Parliament and of the Council of 13 October 2003, relating to fertilisers. Official Journal of the European Union, L304 (21.11.2003).

GB 954.555 (1964). Patent, Improvements in or related to delayed action granular fertilisers.

Goertz, H.M. (1993). Controlled release technology, Kirk-Othmer encyclopedia of chemical technology, fourth edition, volume 7, 251-274.

ISO 8157:2015 (2015). Fertilizers and soil conditioners: vocabulary. Trenkel, M.E. (2010). Slow- and controlled-release and stabilized fertilizers: An option for enhancing nutrient use efficiency in agriculture, International Fertilizer Industry Association.

Improving fertilizer use efficiency: Controlled-release and stabilized fertilizers in agriculture, M. E. Trenkel, International Fertilizer Association, IFA, Paris, France, 1997.

Slow and controlled release and stabilized fertilizers: A growing market, C. Watson, , pages 32 – 35, 2013.

Smart options, Fertilizer International, No. 455, July – August 2013, pages 24 – 28.

Terlingen, J.G.A., Hojjatie, M. and Carney, F. (2014). Review of analytical methods for slow- and controlled-release fertilizers, International Fertilizer Industry Association.

Links to related IFS Proceedings

  90, (1966), Isobutylidene Diurea as a Slow Acting Nitrogen Fertiliser and the Studies in this Field in Japan, Masao Hamamoto.

153, (1976), Slow Release Fertilisers, Particularly Sulphur-Coated Urea, L H Davies

180, (1979), Practical Experience with Ureaform Slow-Release Nitrogen Fertiliser During the past 20 Years and Outlook for the Future, H Schneider, L Veegans.

268, (1988), Slow Release – True or False? A Case for Control, F N Wilson

431, (1999), Preparation Methods and Release Mechanisms of Controlled Release Fertilisers: Agronomic Efficiency and Environmental Significance, A Shaviv.

469, (2001), Improvement of Fertiliser Efficiency – Product Processing, Positioning and Application Methods, A Shaviv.

773, (2015), Nitrogen Use Efficiency (NUE) – An Indicator for the Utilisation of Nitrogen in Agricultural and Food Systems, O. Oenema.
781, (2016), Current Developments in Controlled Release Fertilisers, J G A. Terlingen, S Radersma, G J J Out, J Hernandez-Martmez and P C Raemakers-Franken.

External Resources

Fertilizers Europe: Explanation of EUFPR