Guideline Index

# Chapter 14: Calculating Rates and Costs

With an increasing range of fertiliser products now available on the market, fertiliser recommendations are often given as kilograms of nutrient per hectare (kg nutrient/ha), rather than as an amount of a specific fertiliser product. For example, the unit of measure for phosphorus is kg P/ha. This is called the nutrient application rate. To order your fertiliser, you need to know how to convert the nutrient application rate into the fertiliser application rate.

On the other hand, if your fertiliser recommendation has been given as an amount of fertiliser (for example, 350 kg/ha of superphosphate), then it is useful to know how to convert this fertiliser application rate into a nutrient application rate, so you can record how much N, P, K, S or other nutrients you will be applying to your paddocks.

#### 14.2.1 What is the fertiliser application rate required to apply a particular nutrient application rate?

To calculate the fertiliser application rate required for a particular nutrient application rate, use the formula : Example. How much single superphosphate per hectare is required to provide 25 kilograms per hectare of phosphorus (25 kg P/ha)?

If you decide to apply 25 kg P/ha, you can calculate the fertiliser application rate required when using single superphosphate. This example uses a generic single superphosphate, which contains 8.9% total P (see Appendix G ). Example. How much triple superphosphate per hectare is required to provide 25 kg P/ha?

The fertiliser application rate needed to apply 25 kg of P/ha when using triple superphosphate can also be calculated using the above formula. This example uses a generic triple superphosphate, which contains 20% total P . So, if you use triple superphosphate, you need less fertiliser to apply the same amount of phosphorus/ha because it is a high-analysis fertiliser.

Exercise 14.1 provides additional practice in calculating the fertiliser application rate required to apply a particular nutrient application rate.

#### Exercise 14.1

What is the fertiliser application rate required to apply a particular nutrient application rate?

You have calculated that your farm needs 20 kg P/ha applied per year.

The following formula allows you to calculate the fertiliser application rate that will result in a particular nutrient application rate. You can use the same formula with a different nutrient application rate or with a different nutrient, such as N, K, or S. Just follow the three steps given below.

Step 1. Write the required nutrient application rate in the boxes in column 1 of the table below. (In this exercise, we have done this for you, using a rate of 20 kg P/ha.)

Step 2. Using the generic fertiliser product links (Appendix G) ; write the percentage of phosphorus for each product in the boxes in column 2. Remember to use total P, not available P.

Step 3. Calculate the rate of fertiliser to be applied, using the formula below, and fill in the boxes in column 3.  What is the fertiliser application rate required to apply a particular nutrient application rate?

The following formula allows you to calculate the fertiliser application rate that will result in a particular nutrient application rate. You can use the same formula with a different nutrient application rate or with a different nutrient, such as N, K, or S. Just follow the three steps given below.

Step 1. Write the required nutrient application rate in the boxes in column 1 of the table below. (In this exercise, we have done this for you, using a rate of 20 kg P/ha.)

Step 2. Using the fertiliser product links (Appendix G ), write the percentage of phosphorus for each product in the boxes in column 2. Remember to use total P, not available P.

Step 3. Calculate the rate of fertiliser to be applied, using the formula below, and fill in the boxes in column 3.  #### 14.2.2 How many kg/ha of nutrient are applied by a particular fertiliser application rate?

To convert the fertiliser application rate (kg/ha) to the nutrient application rate (kg nutrient/ha), use the formula: Example. What is the nutrient application rate of P/ha when 450 kg/ha of single superphosphate is applied? This example uses generic superphosphate, which contains 8.9% total P. Example. What is the nutrient application rate of S/ha when 450 kg/ha of single superphosphate is applied? This example uses a generic single superphosphate, which contains 11% total S. Exercise 14.2 provides practice in calculating the amount of nutrient applied when you know the fertiliser application rate.

#### Exercise 14.2

How many kg/ha of nutrient are applied by a particular fertiliser application rate?

You have just bought more land, and the previous farmer tells you he has used 400 kg/ha of single superphosphate every year. The two farmers next door have been using the same fertiliser application rate but different fertiliser products. You want to know what amount of nutrients they have been applying.

The following formula allows you to calculate the nutrient application rate that will result from a particular fertiliser application rate. You can use the same formula with different fertiliser application rates or with a different nutrient, such as N, K, or S. Just follow the three steps given below.

Step 1. Write the fertiliser application rate in column 1. (For this exercise, we have done this for you, using a rate of 400 kg/ha for each product.)

Step 2. Using the fertiliser product links (Appendix G ), write the percentage of phosphorus for each product in the boxes in column 2. Remember to use total P, not available P.

Step 3. Calculate the nutrient application rate, using the formula below, and fill in the boxes in column 3.  How many kg/ha of nutrient are applied by a particular fertiliser application rate?

The following formula allows you to calculate the nutrient application rate that will result from a particular fertiliser application rate. You can use the same formula with different fertiliser application rates or with a different nutrient, such as N, K, or S. Just follow the three steps given below.

Step 1. Write the fertiliser application rate in column 1. (For this exercise, we have done this for you, using a rate of 400 kg/ha for each product.)

Step 2. Using the fertiliser product links (Appendix G ), write the percentage of phosphorus for each product in the boxes in column 2. Remember to use total P, not available P.

Step 3. Calculate the nutrient application rate, using the formula below, and fill in the boxes in column 3.  