Guideline Index

Chapter 11. Introducing Fertilisers

11.4 Nutrient analysis of fertiliser products

When applying fertilisers it is important to be clear about the terms used. There are many colloquial and imperial terms lingering which can add to the confusion and create errors; for example, bags/acre or so many units of a particular nutrient.

As suggestions, when making fertiliser recommendations:

  • Use the amount of fertiliser product per hectare; e.g. urea at 100 kg/ha.
  • Do not mix imperial and metric measurements; e.g. kg/acre
  • Keep in metric if possible
  • Keep numbers rounded to nearest “0” or “5”; e.g. 10 kg/ha or 15kg/ha not 13.2 kg/ha.

Appendix A contains conversion tables to help you convert fertiliser application rates and weights from the imperial system to the metric system. Appendix B contains explanatory information about the metric system.

11.4.1 NPKS system

The NPKS system describes the nutrient analysis of a fertiliser in terms of the percentage of nitrogen (N), phosphorus (P), potassium (K) and sulphur (S) that it contains. By law, the NPKS nutrient analysis must be displayed on the fertiliser product label and on the fertiliser company’s product list – See Appendix F for examples of product lists. This system allows you to quickly and accurately see what nutrients the product contains and calculate the amount (weight) of each nutrient in the product.

NPKS nutrient analyses are often written with a colon (:) between each percentage. For example, the nutrient analysis for single superphosphate is 0:8.8:0:11, which means 0% nitrogen, 8.8% phosphorus, 0% potassium, and 11% sulphur. When the nutrient analysis is written this way, it always gives the nutrient percentages in the same order: N, P, K and S. The analysis of commercial fertilisers can be easily obtained from a product list for each fertiliser company (See Appendix F ).For an analysis of common fertilisers please refer to Appendix G

The amount (weight) of N, P, K and S nutrients in a fertiliser product can be calculated from the percentages given in the nutrient analysis. ‘Per cent’ means ‘in every hundred’, so, 8.8% means 8.8 in every hundred, and 11% means 11 in every hundred.

For example, 100 kg of single superphosphate with a nutrient analysis of 0:8.8:0:11 will contain:

Exercise 11.1

Therefore, the amount of nutrients in 1 tonne (1000 kg) of a fertiliser will be ten times the amount of nutrients in 100 kg. So, 1 tonne of single superphosphate with a nutrient analysis of 0:8.8:0:11 will contain :

Exercise 11.2

Fertilisers used in Australia express the percentage of the individual nutrient in its elemental form e.g. 8.8% P for single superphosphate.

As you can see, 1 tonne of this single superphosphate contains a total of 198 kg of NPKS nutrients. The remaining 802 kg in the tonne contains the other elements that formed the less plant available forms.

The nutrient analyses of a typical single superphosphate and a typical triple superphosphate are shown as a diagram in Figure 11.2.

Practice Exercise :

Exercise 11.3


Figure 11.2
Figure 11.2 Diagrammatic representation of typical single superphosphate and triple superphosphate nutrient analyses

11.4.2 Available or elemental versus total nutrient

Fertiliser nutrients are shown on fertiliser labels and in product charts in their elemental form (e.g. %N, % P, %K) however this is not how nutrients exist in their natural state in the soil or in fertilisers. For example, phosphorus exists in combination with other elements including calcium, oxygen, hydrogen, iron and aluminium. Collectively all these forms would add up to the total P, however only a relatively small fraction of this is plant available. Plants take up phosphorus in the phosphate or orthophosphate form (H2PO4-, HPO42-) which is water soluble and plant available. It is therefore important to know the water soluble percentage of the total P nutrient analysis in order to match the nutrient availability to pasture demand. These water soluble forms are changeable and may combine with other nutrients (calcium, iron or aluminium) over time to become less water soluble and less plant available.

Currently fertiliser companies are required to label fertilisers according to State legislation which stipulates that fertiliser labels provide the following detail:

  • Total elemental form of a nutrient, e.g. %N.
  • The forms in which the nutrients are presents, e.g. N as urea.
  • The percentages of each form present (e.g. nitrate-N at 7.8% and ammonium-N at 4.8%).

There is an Australian National Code of Practice that will unify the state legislations, however it has not yet been formally recognised across state and territory borders.

11.4.3 Using the nutrient analyses to find comparable products

Once you know what nutrients you need to apply to your paddocks, you may want to compare products from different companies to see which nutrient analysis and price combination is the most cost-effective. Although the product names may not be similar, you can use the nutrient analyses given in the product lists to find similar products.

For example, you may want to apply a product containing phosphorus, potassium, and sulphur, such as Incitec Pivot SuPerfect Potash 2 & 1, which has a nutrient analysis of 0:5.9:16.6:7.3. The other product lists in Appendix G contain similar products, although the product names may not include the term ‘2 & 1’ or ‘super potash’.