Seventeen elements are essential for growth. Carbon, hydrogen and oxygen come from air and water — those are free. The remaining 14 must arrive in the nutrient solution, and in the right ratio to each other. An excess is just as damaging as a shortage, because ions compete at the root surface: lots of potassium suppresses calcium and magnesium uptake, lots of ammonium suppresses calcium.
What each element does
- Nitrogen (N) — builds protein and chlorophyll, the engine of leaf mass. Too much gives soft, watery leaves that attract aphids.
- Phosphorus (P) — energy transfer, root development, flowering. Poorly absorbed when the water is below 15 °C.
- Potassium (K) — controls stomata, cell firmness, sugars and fruit flavour.
- Calcium (Ca) — the cement of cell walls. Moves upward only, travelling with the water.
- Magnesium (Mg) — the central atom of the chlorophyll molecule. No Mg, no green.
- Sulphur (S) — amino acids; arrives automatically with sulphate-based fertilizers and rarely runs short.
- Trace elements — Fe, Mn, Zn, B, Cu, Mo. Needed in milligrams, yet a shortage halts growth as fast as missing nitrogen.
How much of what — actual numbers
| Element | Leafy greens (mg/l) | Fruiting crops (mg/l) |
|---|---|---|
| Nitrogen (N) | 100–130 | 150–190 |
| Phosphorus (P) | 30–45 | 40–55 |
| Potassium (K) | 120–160 | 250–320 |
| Calcium (Ca) | 100–140 | 150–190 |
| Magnesium (Mg) | 30–45 | 45–60 |
| Iron (Fe) | 1.5–2.5 | 2.0–3.0 |
| Mn / Zn / B | 0.5 / 0.1 / 0.3 | 0.6 / 0.15 / 0.4 |
If your water is hard — 15–25 °dH is normal across much of Central Europe — the tap already delivers roughly 100–180 mg/l of calcium plus some magnesium. That covers almost all of a lettuce crop's demand. This is why the same dose of fertilizer gives different results in two towns; see water quality for the detail.
Where the symptom appears is half the diagnosis
Nitrogen, phosphorus, potassium and magnesium are mobile: when supply runs short the plant moves them out of old leaves into new ones. Their deficiency therefore always starts at the bottom. Calcium, iron, boron and manganese are immobile — once built in, they stay put, so their deficiency shows at the top, on the youngest leaves.
| Symptom | Where | Cause | What to do |
|---|---|---|---|
| Older leaves fade to yellow evenly | bottom | Nitrogen (N) | Replace the solution, check EC |
| Yellow between green veins on the youngest leaves | top | Iron (Fe) — in 9 of 10 cases pH above 6.5 | Drop pH to 5.8 first, chelate second |
| Same pattern but on old leaves | bottom | Magnesium (Mg) | Epsom salt 0.3–0.5 g/l |
| Brown, papery edges on young heart leaves | top, inside | Calcium (Ca) — tipburn | Air movement, humidity under 75 %, lower EC |
| Black sunken patch at the base of the fruit | fruit | Calcium transport, not a shortage in water | Steady watering, lower EC and K |
| Deformed growing tip, hollow brittle stems | top | Boron (B) | Dose very sparingly, 0.3 mg/l is enough |
| Small dark green leaves, purple stems | bottom | Phosphorus (P), often water below 15 °C | Measure reservoir temperature |
| Necrotic margins on old leaves, plant limp with wet roots | bottom | Potassium (K) or root rot | Check root colour and aeration |
With iron, the chelate form matters more than the amount: Fe-EDTA holds iron in solution up to pH 6.5, Fe-DTPA up to 7.0, and Fe-EDDHA even above 8 (though it dyes the water red). If your pH keeps drifting upward, switching chelate does more than adding iron. See pH value.
Calcium: a transport problem, not a dosing problem
Calcium travels through the xylem only with water, and water goes where evaporation is highest — into large, open leaves. The young heart of a lettuce or the tip of a tomato transpires almost nothing. So calcium can be perfectly adequate in the tank while the inner leaf tips still turn brown. That is tipburn, the most common complaint with indoor lettuce.
- Humidity permanently above 80 % — the leaf does not transpire, so calcium does not travel. A small fan helps more than any additive.
- Growth that is too fast under high nitrogen — new tissue forms quicker than calcium can reach it.
- EC too high — the root pulls water with more effort and transport slows down.
- Plenty of potassium and magnesium — they suppress calcium right at the root.
- In tomatoes the same mechanism produces blossom end rot: a black, sunken patch at the base of the fruit. Spraying calcium onto the fruit barely helps, because fruit have almost no stomata.
One-, two- or three-part fertilizer
Calcium nitrate must never meet sulphates and phosphates in a concentrate — you get gypsum and calcium phosphate, a white precipitate that is lost to the plant. That is why serious fertilizers come in at least two parts: A (calcium) and B (everything else). The rule: never pour A and B together undiluted; add each part to the water separately.
- One-part — simplest, but the ratio is locked and you cannot switch a plant from leaf phase to fruit phase.
- Two-part (A + B) — the hydroponic standard and a good compromise for a home tower.
- Three-part (Grow / Micro / Bloom) — maximum freedom, and maximum room for mistakes.
The Masterblend principle
Masterblend 4-18-38 is the best-known “pro” recipe and works with three powders: Masterblend itself, calcium nitrate and Epsom salt (magnesium sulphate), in a 2 : 2 : 1 ratio.
| Component | Per 10 l — fruiting | Per 10 l — leafy |
|---|---|---|
| Masterblend 4-18-38 | 6.3 g | 3.2 g |
| Calcium nitrate | 6.3 g | 3.2 g |
| Magnesium sulphate | 3.2 g | 1.6 g |
| Expected EC | around 2.2 mS/cm | around 1.2 mS/cm |
Dissolve each powder separately in a little lukewarm water, then pour it into the reservoir, calcium nitrate last. For a 40-litre tower that is roughly 13 + 13 + 6 g for fruiting crops, or half of that for lettuce. The scale is your starting point, but EC is the truth — measure after mixing.
NPK: leaf is not fruit
Lettuce and herbs want an N : K ratio around 1 : 1.2. Tomato, pepper and strawberry want 1 : 2 — potassium builds sugar and fruit firmness, while too much nitrogen during fruiting gives a lush plant with little yield. In a vertical tower all 32 slots share one reservoir, so you cannot run two recipes at once. In practice: either run the leafy recipe and accept a slightly smaller but perfectly good tomato harvest, or dedicate the tower to fruiting crops for one season.