Water Soluble Fertilizer Calculator

How to use the water-soluble fertilizer calculator

What this calculator does

This calculator estimates the dry mass of a water-soluble fertilizer needed to reach a chosen elemental nutrient concentration in a specified final solution volume. It is useful for greenhouse irrigation, hydroponic reservoirs, laboratory plant trials, and repeatable hobby-growing recipes. The result is a mixing calculation, not a crop-specific feeding recommendation: plant species, growth stage, source-water chemistry, pH, electrical conductivity, and interactions between salts still require separate judgment.

When to use it

Use it when scaling a label recipe to a different tank size, targeting a specific nitrogen or mineral concentration, comparing common soluble fertilizers, or documenting a repeatable nutrient solution for staff or future batches.

How to calculate

1. Select a Fertilizer preset, or choose Custom and enter the guaranteed analysis.
2. Check Nitrogen (N), Phosphate (P₂O₅), Potash (K₂O), Calcium (Ca), Magnesium (Mg), and Sulfur (S). Values are percentages by fertilizer mass.
3. Select the Desired element. Enter the Desired concentration in ppm, equivalent to mg/L for dilute aqueous solutions.
4. Enter the Final solution volume in liters. Read Dry fertilizer required, the mixing rate, and the nutrient table.
5. Use Download Excel to save the current model, or Reset to restore 20-20-20 fertilizer, 200 ppm nitrogen, and 100 L.

Input guide

Fertilizer preset is required and fills the analysis fields with a standard label composition. The six percentage fields accept ordinary decimal notation from 0 to 100; their total may not exceed 100%. A value such as 20 means 20 g of that labeled nutrient per 100 g fertilizer. Do not enter 0.20 for a 20% label. Desired element is required and must have a positive elemental fraction in the selected fertilizer. Desired concentration is required, must be greater than zero, and accepts a value such as 200 ppm. Higher targets increase fertilizer mass proportionally. Final solution volume is required, must be greater than zero, and accepts a value such as 100 L. Doubling the volume doubles the required mass but does not change the g/L mixing rate.

Output guide

Dry fertilizer required is the estimated mass to weigh. Fertilizer concentration is the dose per liter. Target nutrient fraction is the elemental percentage actually used in the calculation; for phosphorus and potassium it is lower than the P₂O₅ or K₂O label number because oxygen contributes to the oxide mass. The phosphorus and potassium cards show resulting elemental ppm. The Resulting nutrient profile table reports each nutrient's label basis, elemental fraction, concentration, and total nutrient mass. A zero concentration means that nutrient is absent from the entered analysis, not that the water contains none.

Worked example

For 20-20-20 fertilizer, a target of 200 ppm nitrogen in 100 L uses a nitrogen fraction of 0.20. Fertilizer concentration equals 200 mg/L ÷ 0.20 = 1,000 mg/L, or 1.000 g/L. Multiplying by 100 L gives exactly 100.00 g dry fertilizer. The same dose supplies 20% P₂O₅ and 20% K₂O; after oxide-to-element conversion, the solution contains about 87.29 ppm elemental phosphorus and 166.03 ppm elemental potassium.

Learn more

The Penn State Extension guide to greenhouse fertilizer calculations explains ppm-based mixing, while the IUPAC periodic table provides the atomic-weight basis behind oxide-to-element conversions.