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Hydroponic Nutrient Calculator

Pick your plant, growth phase and fertilizer — we calculate the exact dose for your tank, the target EC and the pH window. 67 plants, no sign-up.

Plant
Leafy plantGermination: needs lightSowing: Mar – Aug
Growth phase
Fertilizer series
Water volume
Tap water EC (µS/cm)
Strength (0 %)

Dose for 40 L

Masterblend 4-18-38 (Trio)

Masterblend 4-18-3822.8g
Kalcijev nitrat / Calciumnitrat29.6g
Magnezijev sulfat / Bittersalz11.6g
Target EC

1.44 mS/cm

of which from your tap water: 0.40 mS/cm

pH window

5.6 – 6.2

Measure after mixing and correct with pH-Down if needed

The calculated amount is a starting point, not a final answer. Measure the EC after mixing: above target, dilute with water — below it, top up in small steps.
Dissolve each powder separately in the FULL volume of water, calcium nitrate last. Never store as a dry mix or make up a shared concentrate — calcium precipitates there with sulphate and phosphate. In the finished, diluted solution it does not.
Calculated against the guaranteed analysis on masterblend.com (4% N, 18% P₂O₅, 38% K₂O). Several label versions exist under the same name — check your own bag. The calcium nitrate must be greenhouse grade 15.5-0-0 with 19% Ca, and the Epsom salt heptahydrate (MgSO₄·7H₂O, 9.86% Mg) — kieserite holds almost twice as much magnesium per gram. For fruiting crops this ratio does not reach the potassium target; add potassium sulphate to close the gap.
Replace the nutrient solution completely every 2–4 weeks rather than only topping it up. EC does not reveal the drift: calcium, magnesium, sulphate and sodium accumulate while nitrogen and phosphorus run down.

These values are guidance based on common horticultural recommendations and manufacturer data. Always measure EC and pH after mixing and adjust if needed — every water source and location is different.

Frequently asked questions

Which fertilizer suits a tower of lettuce and herbs?

For leafy plants (lettuce, rocket, basil, spinach) the Masterblend trio of powder, calcium nitrate and Epsom salt is a good pick: it keeps for years and costs very little per litre of solution. Worth knowing: 4-18-38 is actually the fruiting-crop formula, and the powder itself contains no calcium — that comes entirely from the second component. For leafy greens we therefore calculate with more calcium nitrate (ratio 1 : 1.3 : 0.5 instead of 1 : 1 : 0.5) so the nitrogen reaches the target profile. Liquid ranges such as TriPart are more convenient but dearer per litre — and you do not need their Bloom component for lettuce at all.

Why does the target EC show how much my water already contributes?

EC measures every dissolved salt, including those from the tap (typically 300–700 µS/cm in Germany and Croatia). The target EC is the value the finished solution should reach — your water supplies part of it, the fertilizer the rest. So measure your own water first, then measure again after mixing. With very hard water the mixture can end up above target; blend with RO or distilled water in that case.

How often does the solution in the tower need changing?

Completely every 2–4 weeks, with plain water topped up to the mark in between. EC alone will not tell you when: plants draw down nitrogen and phosphorus far faster than calcium, magnesium and sulphate, so the ratio shifts without the conductivity having to change. A Purdue University study on head lettuce found markedly lower tissue values after 18 days and 22–36% less fresh weight, even with EC held constant. Measure more often in summer — evaporation raises the concentration on its own.