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) 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 this in more detail slow release fertilizer technology.

There is a massive amount of confusion regarding what s fertilizers are. This should help clarify what constitutes a slow-release Fertilizer, how they work, and when is the best time to use them.

From a home garden perspective, the main advantages of slow-release fertilizers are:

First, increased turf safety as these are less likely to burn turf.

Second, a long-term growth response without surge growth results in a decreased increased need to mow.

Third, a decrease in disease due to less lush turf.

Lastly, reduced environmental impacts are there is less waste.

 

Alzon Neo N dual inhibited slow release fertiliser

Sadly in the home garden market, it 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 contains a significantly lower % of slow-release compared to Product B. Consequently, always check the label!

Slow-release nitrogen fertilizers can be divided into organic and inorganic. Organics available in Australia include products 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 with the slow release component relating to the nitrogen content only.

 

 

Synthetic Slow release Fertilizers.

Synthetic slow-release fertilizers have been introduced to overcome the disadvantages that organic forms possess, especially 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 in the above table are used as the base for home garden slow-release fertilizers. All have slightly different release characteristics and all contain different percentages of slow-release N. Be aware one product isn’t always the best in every situation.

In relation to the cost of these always make sure you are comparing apples with apples. Prill (granule) size can make products with a similar analysis vary dramatically in price. With the synthetic slow-release nitrogen fertilizers as far as I am aware those marketed as containing IBDU are now only available in the country as Floranid® in a greens grade prill.

Things to be aware of:

 

  • Huge claims of excessively long release times of 16 weeks with no information to support this and
  • Secondly, limited information regarding the actual percentage of 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 basically release by microbial activity so the warmer the temperature the quicker the release. These can be true compounds which means every pill is the same and the product is not a blended fertilizer. The other key characteristic of these is if the granule becomes broken or crushed it doesn’t affect how the nitrogen releases.

This is in contrast to the PCSCU-based products, which are those yellow pills you see in fertilizer. If the granule becomes cracked or damaged it loses all its slow-release characteristics. These release by basically dissolving with water and temperature variations cause the prill to swell, crack and release. The higher the temperature the greater the swelling and the greater the release. Claiming that a product lasts for example 16 weeks is totally dependent upon the temperature.  For example if it is say 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.

 

 

Below: Cross section of a PCSCU prill

 

 

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