In hydroponics, plants get all their nutrition from the water, so mixing the reservoir correctly matters. This calculator doses the nutrient and converts the target strength between EC and ppm.
Dosing the reservoir
Nutrient labels give a dose per gallon (or per liter), often separately for each part of a two- or three-part system (for example Grow, Micro and Bloom, or A and B):
dose per part = reservoir volume × dose × strength
A 20-gallon reservoir at 5 mL per gallon takes 100 mL of each part at full strength.
Always add each part separately to the water and stir before adding the next. Mixing concentrated parts together can lock out calcium and make a precipitate. Fill the reservoir with water first, then add nutrients, then adjust the pH last.
Strength by growth stage
Labels are often written for vigorous, mature plants. Many growers use:
- 25 to 50% strength for seedlings and clones;
- 50 to 75% for lettuce, herbs and leafy greens;
- 100% for fruiting crops like tomatoes, peppers and cucumbers in full growth.
EC and ppm
The electrical conductivity (EC) of the solution, in mS/cm, measures the total dissolved salts. Many meters display ppm instead, but they convert EC with different factors:
- the 500 scale (also called TDS or NaCl): ppm = EC × 500;
- the 700 scale (KCl): ppm = EC × 700.
A target of EC 1.6 reads 800 ppm on a 500-scale meter and 1,120 ppm on a 700-scale meter. Always check which scale your meter uses before following a ppm chart. CF, used in some countries, is EC × 10.
Typical EC ranges are about 0.8 to 1.2 for lettuce and herbs, 1.2 to 1.8 for strawberries and peppers and 2 to 3.5 for tomatoes.
pH
Most hydroponic crops absorb nutrients best at a pH of about 5.5 to 6.5. Adjust with pH up or down after mixing, a few drops at a time, and recheck after 15 minutes.
Reservoir maintenance
- Top up with plain water as plants drink, and watch the EC: if it rises, plants are taking water faster than nutrients.
- Change the solution completely every 1 to 2 weeks.
- Keep the reservoir dark and between about 65 and 72 °F to prevent algae and root rot.