Slow versus controlled release fertilizers
Controlled release fertilizers (CRF) as well as slow release fertilizers (SRF) supply nutrients to the plant gradually. Often the term CRF and SRF are mixed. Although the gradual way of releasing is comparable, there are some key differences that need to be taken into account when talking about slow release fertilizers versus controlled release fertilizers. Think of the technology they use, the way the release is controlled and the release mechanism, to name a few.
Controlled Release Fertilizers
Fertilizers such as Ekote apply a polymer coating. The coating process and technology - which in case of Ekote is the newest on the market - applies a vegetable oil as main component. The nutrients are released into the soil by means of diffusion. In general, longevities of controlled release fertilizers are longer than slow release fertilizers. Typically 2 to 18 months, if we talk about Ekote CRF. The Ekote coating only responds to soil temperature. This determines the speed with which nutrients are released. Products can be 100% coated, although less is also possible.
Slow Release Fertilizers
Slow release fertilizers release the nutrients at a slower pace than conventional fertilizers do. The limitation is that the release (and rate and pattern) is not well controlled and influenced by some additional factors like microbial activity, pH, organic matter and moisture.
Sulphur coated Urea (SCU) and Polymer Sulphur Coated Urea (PSCU)
One of the most common types of coated mineral fertilisers is sulphur-coated urea (SCU). During the production process the sulphur needs to solidify as crystalline and not as amorphous material so that a difference in density occurs and hair-thin cracks are formed in the sulphur. These cracks ensure that diffusion takes place, while the sulphur itself does act as a membrane. In case sulphur is solidified to an amorphous form no cracks can be formed and no release of minerals can take place unless the coating is damaged. The higher the longevity of the product, the higher the content of amorphous material.
In order to improve the longevity of the product the sulphur-coated urea (SCU) is coated with a polymer wax and is known under the name PSCU (Polymer Sulpher Coated Urea). Unfortunately it still has certain ‘burst’ and ‘lock-off’ characteristics. That is why officially these products do not fall under the classification Controlled Release Fertilisers, but are Slow Release Fertilisers.
IBDU and MU
These provide only slow-working Nitrogen while the rest of the nutrients, i.e. the P and K are provided under conventional form and are immediately soluble. The IBDU and MU have a longer molecules which ‘locks’ the Nitrogen for a longer period of time, bacteria attack and degrade the molecules so that nitrogen is made available in the form of urea. Moisture, soil temperature and soil microorganisms determine the longevity, which varies between 1 and 4 months. These multiple factors of influence make the release uncontrolled in comparison to controlled release fertilzers.