How dry fertilizer dosing is calculated
1 ppm means 1 mg per liter. To raise a nutrient by a target amount you need target ppm × liters milligrams of that nutrient. Each salt contains only a fraction of the nutrient by weight, so:
grams of salt = target ppm × liters ÷ 1000 ÷ mass fraction
The mass fractions come from the molar masses of each compound:
- KNO3: 61.3% nitrate (NO3), 38.7% potassium
- KH2PO4: 69.8% phosphate (PO4), 28.7% potassium
- K2SO4: 44.9% potassium
- MgSO4·7H2O (Epsom salt): 9.9% magnesium
Example
To raise nitrate by 5 ppm in 20 US gallons (75.7 L) of water you need 5 × 75.7 ÷ 1000 ÷ 0.613 ≈ 0.62 g of KNO3. It also adds about 3.2 ppm of potassium.
Dosing tips
- Use a scale that reads to 0.01 g for small tanks, or make a stock solution and dose it in ml.
- Check whether your test kit reports nitrate as NO3 or as nitrogen (N); the numbers differ by a factor of 4.4.
- Make sure you have pure, unscented Epsom salt (magnesium sulfate heptahydrate) without additives.
Frequently asked questions
How much KNO3 per gallon for 1 ppm of nitrate?
About 6.2 mg (0.0062 g) per US gallon raises nitrate by 1 ppm, so roughly 0.62 g for 5 ppm in 20 gallons.
How much KH2PO4 for 1 ppm phosphate?
About 1.43 mg per liter (5.4 mg per US gallon) raises phosphate (PO4) by 1 ppm.
Does dry fertilizer change pH or KH?
These salts have little direct effect on KH. Plants using nutrients and CO2 changes have a bigger effect on pH.