How to Calculate Fertilizer Concentration for Fertigation Systems

Fertigation delivers dissolved fertilizer through an irrigation system, allowing nutrients to be applied with irrigation water. Calculating the right fertilizer concentration is essential for delivering the required amount of nutrients while avoiding under- or over-application.

The calculation starts with the crop's nutrient requirement and the fertilizer's nutrient content. Irrigation volume, stock solution concentration, and injector dilution ratio then determine how much fertilizer is delivered through the irrigation system.

This guide explains how to calculate fertilizer concentration for fertigation, prepare a stock solution, and avoid common calculation errors.

Quick Fertigation Fertilizer Formula

For a basic fertigation calculation, use these formulas:

Required fertilizer = Target nutrient amount ÷ Fertilizer nutrient percentage

Fertilizer concentration = Required fertilizer ÷ Irrigation water volume

For systems using a concentrated stock solution:

Approximate final fertilizer concentration = Stock solution concentration ÷ Injector dilution ratio

For example, if a fertilizer contains 15% nitrogen and the target application is 20 kg of nitrogen:

20 kg ÷ 0.15 = 133.3 kg fertilizer

If this fertilizer is applied with 20,000 L of irrigation water:

133.3 kg ÷ 20,000 L = 6.67 g/L fertilizer

These formulas provide a starting point for a fertigation calculation. Actual application should also consider crop nutrient requirements, water quality, fertilizer solubility and compatibility, and injector performance.

What Is Fertilizer Concentration in Fertigation?

Fertilizer concentration describes the amount of fertilizer contained in a specific volume of solution. It is commonly expressed in g/L or kg/L.

The basic formula is:

Fertilizer concentration = Amount of fertilizer ÷ Volume of solution

For example, dissolving 10 kg of fertilizer in 100 L of solution gives:

10 kg ÷ 100 L = 100 g/L

This is the concentration of the prepared solution. In a fertigation system, however, the concentration delivered to the crop may be lower because the stock solution is diluted through an injector.

It is therefore important to distinguish between fertilizer application rate, stock solution concentration, and final irrigation-water concentration.

What Information Do You Need for Fertigation Calculations?

Before calculating fertilizer concentration, identify these key values:

The most important point is to calculate from the nutrient requirement, not from fertilizer weight alone.

For example, 100 kg of a fertilizer containing 15% nitrogen supplies 15 kg of nitrogen.

How to Calculate Fertilizer Concentration for Fertigation

A practical calculation can be completed in three steps.

Step 1: Determine the Required Nutrient Amount

Start with the nutrient requirement for the application area.

For example:

Target nitrogen application = 20 kg N/ha

Step 2: Calculate the Required Fertilizer Amount

Assume the selected fertilizer contains 15% nitrogen.

Use:

Required fertilizer = Required nutrient ÷ Nutrient percentage

Therefore:

20 kg N ÷ 0.15 = 133.3 kg fertilizer/ha

Approximately 133.3 kg of fertilizer is required to supply 20 kg of nitrogen.

Step 3: Calculate the Fertilizer Concentration

Suppose the fertilizer is applied with 20,000 L of irrigation water per hectare.

Use:

Fertilizer concentration = Fertilizer amount ÷ Irrigation volume

Therefore:

133.3 kg ÷ 20,000 L = 0.00667 kg/L or: 6.67 g/L

The resulting irrigation water contains approximately 6.67 g of fertilizer per liter.

The nitrogen concentration can also be calculated:

20 kg N ÷ 20,000 L = 1 g N/L

This illustrates an important distinction:

• Fertilizer concentration: 6.67 g fertilizer/L

• Nitrogen concentration: 1 g N/L

The difference comes from the fertilizer's 15% nitrogen content.

How to Calculate Fertilizer Stock Solution Concentration

Many fertigation systems use a concentrated stock solution that is injected into the irrigation line.

The stock solution formula is:

Stock solution concentration = Fertilizer amount ÷ Stock solution volume

For example, if you dissolve 50 kg of fertilizer in a 500 L stock tank:

50 kg ÷ 500 L = 0.1 kg/L or: 100 g/L

The stock solution therefore contains 100 g of fertilizer per liter.

The actual stock solution concentration should be selected according to the target final concentration, injector dilution ratio, stock tank capacity, and fertilizer solubility. The maximum concentration is limited by how much fertilizer can dissolve under the actual mixing conditions.

How Does the Injector Dilution Ratio Affect Concentration?

The injector dilution ratio describes how the stock solution is diluted into the final irrigation solution.

For example, a 1:100 dilution ratio means approximately one unit of stock solution is diluted into 100 units of final solution.

If the stock solution concentration is 100 g/L:

100 g/L ÷ 100 = 1 g/L

The approximate final fertilizer concentration is therefore 1 g/L.

With a 1:200 dilution ratio:

100 g/L ÷ 200 = 0.5 g/L

Therefore, the same stock solution can produce different final concentrations depending on the injector setting.

For accurate fertigation, the injector should be calibrated and its actual output verified rather than relying only on the nominal dilution ratio.

How Does Irrigation Volume Affect Fertilizer Concentration?

When the fertilizer amount remains constant, increasing the irrigation volume decreases the fertilizer concentration.

For example, applying 10 kg of fertilizer results in:

However, fertilizer concentration should not be adjusted based on water volume alone. The overall calculation should start with the required nutrient application rate.

The objective is to deliver the correct amount of nutrients while maintaining a suitable concentration for the irrigation and fertigation equipment.

Common Fertigation Calculation Mistakes and How to Avoid Them

Even simple formulas can produce incorrect results when key variables are overlooked.

1. Confusing Fertilizer Weight With Nutrient Weight

A 15% nitrogen fertilizer does not contain 15 kg of nitrogen per kilogram. Always use the actual nutrient percentage when calculating fertilizer requirements.

2. Ignoring Irrigation Volume

The same amount of fertilizer produces different concentrations in different volumes of irrigation water. Use the actual or properly estimated irrigation volume.

3. Confusing Stock Solution With Final Concentration

A stock solution of 100 g/L does not mean the irrigation water contains 100 g/L. The injector dilutes the stock solution before application.

4. Relying Only on the Nominal Injector Ratio

Actual injection performance can vary. Calibration and periodic measurement can help confirm that the system is delivering the expected amount of fertilizer.

5. Mixing Incompatible Fertilizers

Water solubility does not guarantee compatibility. Some fertilizer combinations can form precipitates when mixed in concentrated solutions.

Always check compatibility before preparing a mixed stock solution.

Choosing Water-Soluble Fertilizers for Fertigation

Accurate calculations are only useful when the fertilizer is suitable for the fertigation system.

When selecting a water-soluble fertilizer, consider:

• Water solubility under actual application conditions

• Nutrient content and consistency

• Low insoluble matter

• Product quality and batch consistency

• Compatibility with other fertilizers

• Technical specifications and quality documentation

Nitrate-based fertilizers can be suitable for fertigation when their nutrient profile, solubility, and compatibility meet the requirements of the application.

KNLAN Chemical supplies water-soluble nitrate fertilizers for agricultural applications, including calcium nitrate, potassium nitrate, and magnesium nitrate. Product selection should be based on the crop's nutrient requirements, fertigation system, water conditions, and product specifications.

Frequently Asked Questions

What is a good fertilizer concentration for fertigation?

There is no single fertilizer concentration that works for every fertigation system. The appropriate concentration depends on the crop's nutrient requirements, fertilizer formulation, irrigation volume, injector ratio, and growing conditions.

How often should I check or calibrate a fertilizer injector?

Fertilizer injectors should be checked and calibrated regularly, especially after installation, maintenance, or changes in operating conditions. Measuring actual injection volume helps confirm that the system is delivering the intended fertilizer concentration.

Does fertilizer solubility affect fertigation concentration?

Yes. Fertilizer solubility limits how concentrated a stock solution can be. If too much fertilizer is added, undissolved material can remain in the tank and potentially clog filters or emitters.

Should fertilizer concentration be calculated from nitrogen or total fertilizer?

It depends on the nutrient being managed. If the fertigation program targets nitrogen, calculate the required fertilizer amount from the fertilizer's nitrogen content. The same principle applies when calculating for potassium, calcium, magnesium, or other nutrients.

Conclusion

Calculating fertilizer concentration for fertigation starts with the required nutrient amount and the fertilizer's nutrient content. Irrigation volume, stock solution concentration, and injector dilution ratio then determine how nutrients are delivered through the irrigation system.

The calculation process can be summarized as:

Nutrient requirement → Fertilizer amount → Stock solution → Injector dilution → Final concentration

By following this process and selecting a suitable water-soluble fertilizer, growers can improve nutrient delivery, reduce calculation errors, and achieve more consistent fertigation.