Selecting a Fertilizer

Here are some tips to make sure that you get the best result when selecting a turf fertilizer, whether it be slow release or otherwise. A fertilizers physical make up has a major impact on the performance of any product applied.

Blended or compound fertilizers

There are so many products to choose from it is very confusing to make a decision. For example, a common question is are compound fertilizers better than blends?

A compound or complex fertilizer contains two or more nutrients in one homogenous granule so chemically all the granules are the same. A blend also known as a “salt and pepper” mix is made up of individual nutrients. The answer is that there is little difference between quality blended and compound fertilizers.

From both an agronomic and handling perspective the physical make up of a fertilizer is important. Issues such as “caking”, amount of dust, and poor granule distribution all need to be taken into account so that the end result is free-flowing, dust free and stable under a range of conditions.

The importance of granule size.

Larger granules are thrown further than smaller granules so you can use a wider spread width. Size also has a role in where you can use a product. The reasons for this are:

  • It impacts ball roll;
  • On low cut turf larger granules are mown off and go to waste; a smaller granule is also easier to water in;
  • With a smaller granule you get a more even coverage; this why pre emergent combination products like Onset 10GR use a small granule;
  • In some cases a large granule size does not work because they take too long to break down. Some fertilizers have a low water solubility so that they need to be applied in as small a size as possible so they break down. For example, blended fertilizers tend to use a mesh size of 4mm and upwards. At this size a sulphur granule takes several weeks to oxidize to SO4, while Granomag even longer.

Another example is agricultural lime as the finer the powder the better the result. This contradicts the belief that coarser lime gives a long-term benefit because it dissolves more slowly.

From the perspective of blending, granule size is also important. When blending you want a high degree of product uniformity and blended granules need to have the same upper and lower size limits as well as be the same size between these limits. This ensures that the end product has an even particle size and so spreads well.

Granule size metrics

The SGN stands for size guide number and the higher the number the higher the larger the size. SGN values are the mean granule size multiplied by 100.

The UI (Uniformity Index) is the ratio of granule sizes representing the smallest 10% and the largest 5% times 100. Values within the range of 40–60 indicate a uniform fertilizer size and the larger the UI value, the more uniform the granule size of a product.


particle size distribution is important when selecting a fertilizer. Here one product has a much more uniform particle distribution than the other

How particle size effects the rate of S oxidization

Particle Size (mesh size/inch)

% S oxidized in 2 weeks

% S oxidized in 4 weeks






















What is fertilizer segregation?

Segregation has a big influence on fertilizer performance and it is an important factor to be aware of when choosing a fertilizer. The greater the variation in granule size, the greater the risk of segregation. Similar sized granules behave differently during handling as those with a similar size will tend to congregate together. With segregation it is the size and not the shape or density has the most influence.

If all the granules have the same chemical make up but differ in size, segregation does not effect the chemical uniformity. This is the case with Nitrophoska®.

This variation in granule size means that you can’t apply this evenly. As a guide granule sizes in a blend should be within 10% of each other. You can avoid this with better granulation or screening to narrower size ranges.

Granule hardness in selecting a fertilizer.

Granules should be hard enough to not break or form dust when handled and good granulation requires a degree of sophistication, engineering, and temperature control that only a few producers have.

The use of fillers in selecting a fertilizer

Fillers are often used to bulk out a fertilizer and are anything from limestone chip and organic materials to bentonite. 

The Critical Relative Humdity (CRH).

Have you ever opened a bag and it was wet or became wet in hot humid weather? This is due to hygroscopicity and is a result of what is called the critical relative humidity.

The CRH is the temperature above which moisture from the air begins to be absorbed by a fertilizer. The CRH is related to the air temperature and as the air temperature increases, the CRH decreases.

The more hygroscopic a material, the more moisture it will absorb and so the lower the CRH. Blending fertilizers together tends to result in a lower CRH than that of the individual fertilizers and this can result in fertilizer caking.


Table Showing Critical Relative Humidity (CRH) of commonly used fertilizers

Fertilizer salt

Critical Relative Humidity

Calcium nitrate


Ammonium nitrate




Ammonium sulphate


Potassium chloride


Potassium nitrate


Monoammonium phosphate (MAP)


Potassium sulphate


©, 2022, Gilba Solutions, All rights reserved.

Senior Turf Agronomist at Gilba Solutions Pty Ltd | Website | + posts

After Graduating from Newcastle University with an Hons Degree in Soil Science in 1988, Jerry then worked for the Sports Turf Research Institute (STRI) before emigrating to Australia in 1993.

He followed this by gaining a Grad Dip in Business Managment from UTS and has worked in a number of managment roles for companies as diverse as Samsung Australia, Arthur Yates and Paton Fertilizers.

He has always had a strong affinity with the Australian sports turf industry and as a result he established Gilba Solutions in 1993. Jerry has written over 100 articles and two books on a wide range of topics such as Turf Pesticides and Nutrition which have been published in Australia and overseas.