About the water-soluble fertilizer calculation
Water-soluble fertilizers are sold with an N-P-K analysis printed on the label — three numbers giving the percentage by weight of nitrogen (N), phosphate (P2O5), and potash (K2O). To feed plants at a controlled strength you dissolve the product in water and aim for a target concentration measured in parts per million (ppm) of a nutrient, most often nitrogen. This calculator converts a target ppm and the label percentage into the amount of fertilizer to dissolve.
The formula
Concentration in water is a simple mass-per-volume relationship. Dissolving 1 gram of fertilizer in 1 liter of water gives 1000 mg/L, which is 1000 ppm of total product. Because 1 ppm = 1 mg/L, the ppm of a single nutrient is:
- ppm of nutrient = grams of fertilizer per liter × nutrient percent × 10
- Rearranged to find the mixing rate: grams per liter = target ppm ÷ (nutrient percent × 10)
- In US greenhouse units: ounces per 100 gallons = target ppm ÷ (nutrient percent × 0.7489), where 0.7489 comes from 28.3495 g per ounce ÷ 378.541 L per 100 gallons.
For example, a 20-20-20 fertilizer is 20% nitrogen. To reach 200 ppm N you need 200 ÷ (20 × 10) = 1.0 g/L, or 200 ÷ (20 × 0.7489) ≈ 13.35 oz per 100 gallons.
Which N-P-K number to use
Enter the percentage of whichever nutrient you are targeting. For nitrogen — the usual target for leafy growth — use the first N-P-K number. If you instead want a phosphorus or potassium concentration, use the second or third number, but remember those are expressed as P2O5 and K2O, not elemental P and K (elemental P ≈ 0.436 × P2O5; elemental K ≈ 0.830 × K2O).
Common target concentrations
- Constant liquid feed for greenhouse and container crops: roughly 100–250 ppm N, with 150–200 ppm N typical for vigorous growth.
- Seedlings, plugs, and salt-sensitive crops: about 50–100 ppm N.
- Occasional feeding at higher strength (e.g. 250–300 ppm N) is used when plants are watered with clear water in between.