It’s a Coating Matter: Controlled-Release Fertilisers in Agriculture
Controlled-release fertilisers (CRF) are considered the fertiliser technology with the highest nutrient use efficiency (NUE).
Over 50 years ago, ICL’s Osmocote Controlled-Release Fertilisers (CRF) introduced the first polymer-coated fertilisers in the USA: an NPK granule with an organic resin coating, ensuring that one application of nutrients would be enough for a long time. It marked the start of a whole new principle of fertiliser application. This concept was originally developed for high-value ornamental plants grown in pots, where loss of nutrients directly affect the quality of the crop. The CRF concept was quickly adopted in other high-value crops such as strawberries and fruit trees, where efficiency and simplicity of fertiliser application made a lot of sense.
Adoption in more extensive horticultural and agricultural crops was not yet the case, mainly because of the price gap between CRF and conventional fertilisers. In the last 10–15 years, new technical developments have enabled more economical and more efficient CRF products, creating possibilities for a more widespread use of controlled-release fertilisers in agriculture.
With a global population expected to rise even further until 2050 and arable land decreasing, farmers need to use the land available to them more efficiently. At the same time, global pressures and climate change also increase the need to apply fertilisers more sensibly and use technologies that help to lower the environmental impact of agriculture worldwide.
The Highest Nutrient Use Efficiency
CRF is considered as the fertiliser technology with the highest nutrient use efficiency. It is widely recognised that mineral fertilisers, including CRF, are responsible for feeding about half of the world’s population. Conventional fertilisers dissolve in the soil immediately after application, only providing nutrition for a short time. Multiple fertiliser applications are necessary to ensure that the crop gets the nutrition it needs. Multiple applications can also be practically complex for big farms as they are costly and, as we know from recent studies, increase problems with soil compaction.
Thanks to their special coatings, CRF granules release their nutrients gradually over an extended time. Depending on a CRF product’s longevity, nutrient release takes several weeks up to many months.
CRFs support better nutrient use efficiency, thanks to a reduction of nutrient losses to the environment and new application methods, such as plant-hole application.
Increased nutrient efficiency is achieved by closely matching the nutrient needs of crops with the CRF’s nutrient release. This not only leads to optimal growth, but also reduces nutrient leaching, volatilization, denitrification, and run-off.
CRF can maintain or increase crop yields at lower total nutrient inputs—an important benefit in regions where regulations have started to limit total nitrogen inputs, for example.
CRFs reduce the number of applications needed: a major advantage in regions where manual labour costs are high or in (tropical) regions where mechanisation and fertiliser transport is complicated due to steep slopes or poor infrastructure.
Depending on circumstances in cultivation, CRFs can significantly reduce:
- total fertiliser use: by a rate of 20%, even up to 50%;
- leaching: by rates up to 55%;
- denitrification: by an average rate of up to 40%;
- volatilisation: by rates up to 40%.
(Numbers based on independent research with ICL CRF technologies)
What is a Controlled-Release Fertiliser?
It is important to note that not all CRFs are the same, and often the term CRF is – incorrectly – used when slow-release fertilisers (SRF) is meant. Let’s look at the official definitions of these two types of fertilisers:
CRF: “A fertiliser in which nutrient release is controlled, meeting the stated release rate of nutrient and the stated release time at a specified temperature.” (ISO 8157:2015)
SRF: “A fertiliser of which, by hydrolysis and/or by biodegradation and/or by limited solubility, the nutrients available to plants is spread over a period of time, when compared to a ‘reference soluble’ product e.g., ammonium sulphate, ammonium nitrate, and urea.” (ISO 8517:2015)
So, the nutrient release mechanism for the two fertilisers is completely different. SRFs are minerals that take a certain time to dissolve or to release their nutrients. CRFs require a good-quality coating and a consistent manufacturing process. While the release of SRF nutrients is influenced by temperature, water, soil pH, and microorganisms, CRFs are only influenced by temperature and water.
There are differences to be found in the materials applied to create the coating as well. What is important to understand, is that these differences exist and that they directly impact the nutrient release efficiency. The most efficient coating technology is the most recently developed one—the polymer coating. This allows the best control over the nutrient release, and so matches the plant demands most closely without any peaks or dips in release.
|Technology||Coating type||Release Mechanism||Advantage||Disadvantage|
|Sulfur-Coated Urea (SCU)||Sulfur||Liquid water diffusion through micro fissures||Low temperature sensitivity||Difficult to steer release|
|Polymer Sulfur-Coated Urea||Sulfur + polymer||Liquid water diffusion through micro fissures||Low temperature sensitivity||Better control of release|
|Polymer-Coated Urea/NPK||Polymer||Water (vapor) diffusion through coating||Excellent control of release||Higher costs|
|Source: Terlingen, J.G.A., Radersma, S., Out, G.J.J., Hernández-Martínez, J., Ramaekers-Franken, P.C. (2016) Current developments in controlled-release fertilisers, International Fertiliser Society, Proceedings 781, p 6.|
Another aspect to take into consideration when choosing a CRF is the different nutrients, combination of nutrients, and even their coating level. A fertiliser blend that consists of a 10% coated fraction is often called a controlled-release fertiliser, but as you can imagine, its longevity characteristics, efficiency, and effect on plant safety are very different to a CRF that is 100% coated. The closer to a crop’s roots you apply the fertiliser, the more important it is to use a CRF product that is fully coated. For example, in forestry, where fertilisers are applied in the planting hole and the young tree or seedling is planted straight onto that, a good-quality, 100% coated CRF is essential.
A fully coated CRF is also the most efficient option to reduce nutrient losses, which in turn increases the fertiliser’s nutrient use efficiency and lowers emissions to the environment.
Controlled Release Prevents Nutrient Losses
Nitrogen (N) is especially prone to losses through nitrate leaching, mainly in light (sandy) soils. In tropical climates, total nitrogen losses are highest. Controlled-release nitrogen is often incorporated into the soil, so that run-off is automatically minimised. The coating around the N ensures that high NO3 levels in the soil shortly after application are avoided, so that leaching can also be prevented.
Phosphorus (P) suffers from fixation in soil. It can happen on alkaline/calcareous P-fixing soils and acidic P-fixing soils which are rich in alluminium and iron (hydr)oxides. Large parts of the humid tropics are covered with acidic highly weathered clay-type soils which are predominantly P-fixating. CRFs can prevent soil fixation: phosphorus in CRFs can keep nutrient concentration in the soils low, and thus limit precipitation with, or adsorption to, reactive minerals.
Potassium (K) can leach easily on sandy soils in heavy rainfall areas and highly weathered tropical soils. CRFs can prevent leaching. Because of complicated infrastructures in many tropical countries, lowering the input and transport of nutrients to plantations can be very important to improve the crop’s carbon footprint.
CRF efficiency and reduced nutrient losses mean that a farmer can lower the application rates and still maintain a healthy yield level. Lower rates reduce total fertiliser costs and makes fertiliser applications easier. At same rate input, CRFs have shown to increase yields per ha.
“At ICL we have used our coating expertise and experience in the past decade to develop special formulations with specific coating percentages and longevities, which create a higher nutrient use efficiency in a specific crop or crop group.”
Success Stories in Agriculture
CRFs are already being used successfully in several agricultural crops in which nutrient efficiency, reducing fertiliser inputs, and applications are very important (for example bananas, coffee, palm oil, rice, sugarcane, and potatoes). But there is no such thing as a one-size-fits-all CRF that will realise higher nutrient use efficiency and higher yields in every crop it is applied to, in every region. It is vital to understand the crop needs and match the best possible CRF to that, and the growing circumstances in the region.
At ICL we have used our coating expertise and experience in the past decade to develop special formulations with specific coating percentages and longevities, which create a higher nutrient use efficiency in a specific crop or crop group. The effectiveness of these CRF concepts has been demonstrated in our extensive trial program and independent research, which has resulted in tailored CRF programs.
The Future of CRF?
Technological developments in the past decade have made more economical controlled-release fertilisers possible, which are changing agricultural practices. CRFs now play a key role in reducing nutrient losses and in maintaining or increasing crop yields at lower or similar nutrient inputs than conventional grower practice.
Life cycle analyses will provide agricultural producers, trade, and consumers with further insights into the environmental benefits and the lowered carbon footprint of this technology, compared with conventional agricultural fertilisers.
Independent research on the efficiency of CRFs is ongoing and it is important to convince policymakers to adopt the technology in any new legislation to reduce the environmental impact of fertiliser practices.
Current polymer-sulphur and polymer coatings degrade very slowly and are effectively inert in the soil after the nutrients have been released. They are designed to be as thin as possible (as thin as a human hair) and permeable, while still allowing the coating to fulfil its function: to control the release of nutrients over a predefined time. In the next decade, upcoming legislation, such as the new European rules around the biodegradability of CRF coatings, will challenge fertiliser manufacturers to create new coating technologies that degrade quicker.