Firstly, not all slow-release fertilizers are the same and each has its advantages and disadvantages. For example the picture above is of a low-quality prill (granule) of a sulphur coated urea (PCSCU) granule that is supposed to last up to 8 weeks. It has released everything (dumped) within 4 days of being applied.

If you want to read up on slow release turf fertilisers in more detail slow release fertilizer technology.

Slow release fertilisers are just one class of turf fertilisers available to the turf manager. There is a massive amount of confusion as to what slow release fertilizers are and this blog will clarify what they are, how they work, and what is invloves in choosing the right fertiliser. 

The main advantages of slow-release fertilizers are:

  • Firstly, an increased turf safety as these are less likely to burn turf;
  • Secondly, a long-term growth response without surge growth. This results in less need to mow;
  • Thirdly, a decrease in lush, disease prone turf and lastly
  • They have less environmental impact as there is less waste.

 

Alzon Neo N dual inhibited slow release fertiliser

In the home garden market, it unfortunately seems to be anything that says it is “slow-release” on the packaging. This makes things very unfair for companies selling a true slow-release fertilizer. Product A could include 5% slow release and be marketed directly against product B containing 80% slow release. Based on bag price Product A is cheaper although it contains a significantly lower % of slow-release compared to Product B. This is despite both products claiming to be slow release! Consequently, always carefully check the label.

Slow-release nitrogen fertilizers can be divided into organic and inorganic products. Products classified as organic in Australia include those based on poultry manure such as Dynamic Lifter®, Neutrog®, Terrafirma®, and Queensland Organics®. All of these are just basically pelletised manure, which is either heat-treated or composted to remove excess moisture. Subsequently, end moisture levels can vary from 8–16% depending on the brand and manufacturing process used.

Burn potential of commonly used slow release fertilizers

Above. Commonly available slow release fertilizers on the Australian marketplace. The slow release percentage relates to the nitrogen content only.

 

 

Synthetic Slow release Fertilizers.

Synthetic slow-release fertilizers overcome the disadvantages of organic products, like unpredictable release. In the table above to keep things simple, UF38 is the same as MU in the table below. All of the synthetic fertilizers listed above are used as the base for home garden slow-release fertilizers. All have slightly different release characteristics and all contain differing percentages of slow-release N. So be aware one that any one product isn’t always the best in every situation.

When costing products always make sure you are comparing apples with apples. Granule size can make products with a similar analysis vary dramatically in price. As a point of note products marketed as containing IBDU are now only available in the country as Floranid® in a greens grade prill.

Things to be aware of:

 

  • Firstly, huge claims of longevities. Always ask for the data to support these claims. Another point is that be aware that the Australian fertilizer is pretty much self regulated so your relying on the honesty of the supplier;
  • Secondly, little information about the actual slow-release content.
Comparison between commonly available slow release lawn fertilizers

Some examples of variable costing, percentage slow-release N, and the technologies available.

How they work.

UF and MU are release by microbial activity. The warmer the temperature the quicker the release. Some of these are true compounds which means every prill is the same and the product is not a blended fertilizer. The other key product characteristic is that if the granule becomes broken or crushed it doesn’t affect how the nitrogen releases. Regardless of the particle size it behaves the same. One furthe point. If you ever buy a box of grass seed containing blue fertilizer granules, thats ureaformaldehyde. The reason this is used is that it is inert. So if the packaging gets wet or the product is stored in far from ideal conditions it won’t release.

This is in contrast to the PCSCU-based products. These are usually the yellow or orange granules that you see in fertilizer blends. With these, if the granule becomes cracked or damaged it loses all its slow-release properties. They release by basically dissolving in water. Subsequent variations in temperature cause the granules to swell, crack and release their contents. The higher the temperature the greater the swelling and the greater the rate of release.

In the home garden market a lot of PCSCU products make some pretty big claims regarding how long they last for. Making a claim that a slow release fertilizer based on PCSCU will last for 16 weeks is dependent upon the temperature. For example, if it is over 30°C it will release a lot quicker than say at 15°C. Subsequently, the quality of the PCSCU has a major factor influencing release times.

Other slow release fertilizer options

Slow release fertilizers are mainly focused on nitrogen. But there are also products that can be categorised as slow release as a result of the slowness of their physical breakdown. Examples of these include:

  • Magnesium – Granomag (magnesium oxide); a high analysis magnesium fertilizer containing 57% Mg. This is used a lot in fertilizer blends to increase the Mg content up but it is basically useless as its insoluble;
  • Potassium – polyhalite. This contains 16% S as sulphate, 11.6% as from sulphate of potash, 3.6% Mg as from magnesium sulphate and 12% Ca as from calcium sulphate;
  • Phosphorus – Pearl technology. This is magnesium ammonium phosphate hexahydrate and has an analysis of 5-28-0 with 10% Mg. More info here.

 

 

 

Below: Cross section of a PCSCU slow release fertilizer granule

 

 

Cross section of a PCSCU turf fertilizer prill

 

 

The Image below shows how temperature effects the release of three different PCSCU products. Products A and B release significantly faster than Product C as the temperature increases. PCU refers to polymer coated urea which is the premium coated option and offers a stronger prill coupled with more consistent release.

coated slow release fertilizer nitrogen release over time with variable temperature