Controlled Release Fertilisers (CRFs) are specialty fertilisers, either compounds or blends, which are designed to release all of their nutrients in a controlled way over a defined period of time (visible in a “release-curve”) in line with plant requirements over time and independent from rainfall quantities. Well known examples of CRF products are Osmocote (ICL), Plantacoat (SQM), Multicoat (Haifa Chemicals) and Basacoat (Compo).
Developments with CRFs are accelerating. New products and technologies have been introduced over the last decade and increasing amounts of CRF products are being used in agriculture. CRFs provide the same nutrition as standard mineral fertilisers, but in a gradual way. Known as controlled release, this has the ultimate aim of matching the release of the nutrients with the nutrient uptake of plants. This provides proven agronomic, economic and environmental benefits. These include:
- Increasing nutrient use efficiency.
- Reduction of nutrient losses to the environment (preventing leaching due to heavy rains).
- Prevention of nutrient-fixation in the soil.
- Maintaining or increasing crop yield at a lower nutrient application rate.
- Improving the quality of plants that need a continuous supply of nutrient at a low rate.
- Reduction of labour.
- Minimising multiple applications of fertiliser.
Note that CRF’s have different characteristics to slow release fertilisers (see Definitions, below), and both are different to inhibitors, whether urease or nitrification inhibitors.
The first description of CRFs was given in the patent literature during the early 1960s (GB 954555). The earlier systems described in the literature usually had a variety of coating layers to control the release of the nutrients from the coated fertilisers. With a majority of standard fertilisers, most nutrients are released during the first 96 hours after application. Shortly after the first description the first sulphur coated formulations were developed (Goertz, 1993). Since then, CRF technology development has accelerated, along with its applications. Current systems enable the metering out of nutrients for up to 1.5 years (18 months). An excellent overview of different kind of nutrient efficient fertilisers including CRFs is given by Trenkel (Trenkel, 1997 and 2010). Due to price constraints, these coated materials were initially used in high value applications such as growing ornamental plants in nursery environments and then, later, applications in turf and amenity followed. During the last decade, continuous technical improvement has resulted in more economical, more efficient products with lower nutrient loss to the environment. While this effect is very well fitting into new policies for reduction of nutrient losses (Farm to Fork Strategy in the EU for example),this has opened up opportunities in the agriculture industry.
The term “controlled release fertiliser” is defined by ISO as: “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). In principle the products contain a fertiliser core on which a coating is applied. See Plate 1.
Often the term “slow release fertiliser” (SRF) is used loosely, although this is not accurate. Most experts consider products like methylene urea or IBDU to be slow release fertilisers. This is also reflected in the definition of slow release fertilisers (SRF) (ISO 8157:2015): Fertiliser, of which, by hydrolysis and/or by biodegradation and/or by limited solubility, the nutrients become available to plants over a period of time, when compared to a ”reference soluble ” product e.g. ammonium sulphate, ammonium nitrate and urea. Clearly the mechanism of release for the two systems is different. The release of nutrients in CRF products is governed by diffusion, while that of SRFs is by hydrolysis or biodegradation. For both slow and controlled release fertilisers the release of nutrients is influenced by temperature and water, but for CRF the release is not influenced by the soil pH and micro-organisms, making its release better predictable
Advantages of Controlled Release Fertilisers
The main advantage of CRFs in agronomic applications is that CRFs are able to match the nutrient supply over time to the nutrient demand of the plants during growth. Another main agronomical advantage of more recent types of CRF, which have a low initial release, is that direct contact of the CRF-fertiliser with plant roots is possible without causing salinity or pH damage at much higher fertiliser-rates than is possible with uncoated fertilisers. If skilfully deployed these advantages can be used to increase the nutrient efficiency of the fertilisers applied and reduce nutrient losses. Moreover labour cost is reduced and the soil structure is less damaged , due to the lower number of application(s) needed, compared to conventional fertilisers.
These advantages have been particularly valuable in two sectors:
Production of ornamental plants. Ornamentals are grown in pots in peat or bark mixtures, which have extremely low levels of available nutrients. If (dolomitic) lime is used to increase the pH of peat-based-media, only the secondary nutrients Mg and Ca are sufficiently available for 6-8 months. The use of CRFs helps to achieve high quality of plants, increased nutrient efficiency, and reduced labour costs.
Turf production, particularly on sports pitches and golf courses, and in landscaping (amenity) settings. In contrast to meadow grass and plant production in general, the main aim of fertiliser application on turf for sports pitches and golf courses is not maximum grass growth and production. The main aim of fertiliser use for turf on sport-fields, golf-courses and landscaping is good grass colour, good sward strength and health, and low grass-growth (minimising mowing frequency). CRFs can help improve turf quality, grass sward density and nutrient use efficiency.
The use of CRFs in agriculture and forestry is challenging, as crops vary widely in a) their length of growing period, b) the part of the plant that is harvested, and c) the degree to which nutrients in the soil are available to the plants, or become fixed. As a consequence, experience with CRFs from ornamentals and turf cannot simply be transferred to agriculture and forestry.
There is scope for CRFs to be used in agriculture or forestry if:
- a nutrient is prone to (high) losses during the growing season,
- a nutrient at high soil-solution concentrations is prone to becoming fixed in the soil during the growing season,
- labour to carry out split applications is a limiting factor,
- crop or plant architecture does not allow split applications to be made.
Another factor that may favour the use of CRF’s is that the placement of nutrients close to roots increases yield (or yield quality), but the risk of salinity damage to the roots prevents the close placement of conventional fertilisers.
There are several specific situations in which the use of CRFs may be cost effective. These include:
- Areas with labour constraints on long-duration crops, where CRFs may be the only option to match nutrient supply to the nutrient demand of the crop, especially if access to the crop is difficult, such as on steep slopes.
- Where crop-architecture limits split applications e.g. sugar cane and endive and other head-forming vegetables.
- With crops that enable increased nutrient efficiency to be achieved by plant-gap placement in direct contact with roots of transplants of vegetables such as leek, celery and cauliflower.
- where regulatory issues limit fertilizer use (e.g. the new Farm to Fork strategy in the EU that forces a 50% reduction on fertilizer use in the near future).
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.
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
469, (2001), Improvement of Fertiliser Efficiency – Product Processing, Positioning and Application Methods, A Shaviv.
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.
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