What is the Difference Between Controlled Release and Slow Release Fertiliser?

Do you know the difference between slow-release and controlled-release fertilisers?

Although these terms are sometimes used interchangeably, the terms 'slow-release fertiliser' (SRF) and 'controlled-release fertiliser' (CRF) strictly do not mean the same thing – even though both do release plant nutrients at a slower rate than when highly-soluble conventional or 'straight' fertilisers are used.

For those who are unclear of these differences, we hope that the following simplified explanation of these distinctions will be both interesting and informative – and explain how each of these two classes of fertiliser releases plant nutrients.

So what are 'Slow' Release Fertilisers?

Slow-release fertilisers (SRFs), unlike controlled-release fertilisers, are not encapsulated in coated prills. The most commonly used slow-release fertilisers supply nitrogen (N) at a slower rate than if a readily-soluble source of nitrogen were applied (e.g. ammonium sulphate, ammonium nitrate or urea). In one of the methods to achieve this, fertiliser manufacturers synthesise what is known as long-chain molecules by chemically combining a nitrogen-source molecule with an aldehyde – for example, urea-formaldehyde or methyl urea. The delayed release of nitrogen is achieved by microbial action in the growing medium – slowly breaking down the long-chain molecules and eventually converting the resulting ammonium nitrogen to nitrate (the form of nitrogen that plant roots can take up).

Other forms of slow-release nitrogen (e.g. IBDU) differ in composition and modes of action. However, the above example explains the concept of a slow-release fertiliser compared with a controlled-release fertiliser (which we will explain below).

It should be emphasised that the duration of release in a slow-release fertiliser cannot be controlled because the effectiveness of the microbial organisms in molecular breakdown is dependent on other factors – including the nature of the growing medium, its moisture level and temperature. Also, a release time extending beyond two or three months cannot be expected.

(As an interesting aside, animal manures, composts and 'green manures' could be deemed slow-release fertilisers, providing many benefits in soil improvement. However, their nutrient chemistry is complex, and microbial activity is still necessary to slowly convert the organic substances into minerals that plants can use. Plant roots don't absorb manure – as such! After all, nitrate (NO ₃^–) derived from manure by microbial action (nitrification) is exactly the same as NO₃^–  in a mineral fertiliser!)

So what are 'Controlled Release' Fertilisers?

Controlled-release fertilisers (CRFs) differ fundamentally from (SRFs) in both technology and mode of nutrient release. Soluble essential plant nutrients, individually or in various homogeneous blends (depending on the application), are encapsulated in an organic resin or polymer coating to form prills. This coating is the secret of the delayed release of nutrients in a CRF. The physical processes by which this is achieved are explained in simplified terms below.

It should be emphasised that the term 'controlled' implies a much greater degree of control in the rate, pattern and duration of nutrient release than can be achieved using SRFs.

The principles behind the success of CRFs were first employed several decades ago. Subsequent technological advances and refinements have led to a range of well-known brands of CRFs – for example, Osmocote, Nutricote, Plantacote, Floracote, Multicote, Basacote and Macracote.

Now let's explain how a CRF works:  The coating on the prills acts as a selectively-permeable or semi-permeable membrane – a barrier to some molecules but allowing specific different molecules to pass through. When a CRF is applied to an adequately-moist growing medium, there is a one-way passage of water through the coating to the inside of the prill. This phenomenon is called 'osmosis'. The absorbed water partially dissolves the mineral nutrients inside the prill to create a highly-concentrated solution. This then increases the hydrostatic pressure within the capsule. No more water will enter when the hydrostatic pressure becomes equal on both sides of the capsule.

How, then, does the fertiliser get out into the growing medium? This is attributable to another phenomenon known as 'diffusion' (the movement of molecules from a liquid of higher concentration into a liquid of lower concentration). Again the key lies in the coating structure, which contains minute micropores. When the plants are watered, the hydrostatic pressures become unequal inside and outside the capsule, and a small amount of dissolved nutrient moves out, by diffusion, through these micropores into the growing medium.

• The rate of nutrient release in a CRF is, in most cases, temperature-related. An increase in temperature causes the micropores to expand in width, allowing more nutrients to diffuse; remember, this is not osmosis but diffusion. (Osmosis is water in, diffusion is nutrient out.) The nutrients are then dispersed within the growing medium (also by diffusion) – coupled with the percolation of dissolved nutrients when the plants are watered. We should regard this correlation between increased nutrient release and increasing temperature as a critical redeeming feature of a CRF: Cooler weather generally means slower plant growth and lower nutrient demand. This lower demand correlates with the reduced rate of nutrient release. Conversely, as temperatures rise, growth increases – demanding more nutrient. This is precisely what happens in the temperature-related release pattern of CRFs!
• The duration of nutrient release is governed in most cases by the thickness of the coating – although a few manufacturers use some other technologies. Products have been developed which offer release times ranging from 2 to 24 months.
• A range of 'patterned' releases can be achieved in a product by blending a mixture of prills with differing release rates or formulae – designed to synchronise with specific nutritional requirements during a growth cycle.

What are the Benefits of using SRFs and CRFs?

The significant benefits of using slow- or controlled-release fertilisers over readily-soluble 'straight' fertilisers include:

• Slower release rates mean longer-term feeding and minimal nutrient wastage through leaching
• A high degree of control over release rates, duration and pattern (CRFs only) means better synchronisation of nutrient release with demand.
• Improved plant growth and health (plants get what they need as they need it)
• Reduced frequency of application, with associated lower labour costs
• Environmental benefits (minimal nutrient in leachate, reducing freshwater/marine contamination
• Minimisation of concentrated nutrient build-up – a risk for high salinity-related root and leaf burn.

There is now a trend to transfer these benefits, long-proven for container-grown stock, to field crop production by changing to slower-release fertilisers – especially where environmental concerns are an issue.