"Found a tomato recipe on a forum — using it." But that recipe was formulated for specific water, a specific substrate, and specific lighting. With your GH 10°dH water it will deliver excess Ca and magnesium antagonism. With your rockwool instead of coco — the same recipe at EC 2.5 will provide different actual nutrition due to the substrate's different CEC. A recipe is not a table to copy — it is a logic to understand and adapt to your own conditions.
Quick Glossary
- Nutrient recipe — the list of fertilisers and their concentrations that together achieve target values for N, P, K, Ca, Mg, S, and microelements in solution; always tied to a specific water source and substrate
- EC — total ion concentration in solution; varies by crop and growth phase
- NPK — abbreviation for nitrogen (N), phosphorus (P), and potassium (K)
- Ion balance — the correspondence between the sum of cations and anions in solution, and the absence of critical antagonisms
Why a "Correct Recipe" Does Not Exist Without Context
A recipe defines the final solution composition in mg/L or mmol/L. But the final composition depends not only on fertilisers — it depends on the sum of fertilisers and the source water composition.
With water at GH 8°dH (typical for most regions of Ukraine), the water already contains approximately 80 mg/L Ca and 15 mg/L Mg. A standard recipe targeting Ca 150 mg/L will actually deliver Ca 230 mg/L — and that excess Ca will block Mg and K uptake even if Mg and K in the recipe are "correct."
This is why the first step in building a recipe is not a fertiliser table — it is a source water analysis: EC, pH, KH, GH, Na content. From this data you calculate how much Ca, Mg, and SO₄ is already in the water — and only then determine how much to add via fertilisers.
Building a Recipe from Scratch: Step by Step
Step 1 — Define target values for the crop and growth phase. Reference ranges for common crops:
- Microgreens: EC 0.8–1.2, N 80–120 mg/L, Ca 60–100 mg/L, K 80–120 mg/L
- Leafy greens (lettuce, basil): EC 1.2–2.0, N 150–200 mg/L, Ca 120–160 mg/L, K 150–200 mg/L
- Tomato vegetative: EC 2.0–2.5, N 180–220 mg/L, Ca 150–200 mg/L, K 200–250 mg/L
- Tomato fruiting: EC 2.5–3.5, N 150–180 mg/L, Ca 160–200 mg/L, K 280–350 mg/L
Step 2 — Subtract source water content. If water contains 80 mg/L Ca and the target is 150 mg/L — only 70 mg/L Ca needs to be added via fertiliser.
Step 3 — Select fertilisers. Calcium nitrate Ca(NO₃)₂ supplies Ca and NO₃⁻. Potassium nitrate KNO₃ supplies K and NO₃⁻. Monopotassium phosphate KH₂PO₄ supplies K and P. Magnesium sulphate MgSO₄ supplies Mg and SO₄. Important: every fertiliser contributes multiple elements simultaneously — account for the full contribution, not just the target element.
Step 4 — Check key ratios. Ca:Mg → 3:1–4:1 by mg/L. K:Mg → no more than 3:1 by molar concentration. NO₃⁻:NH₄⁺ → 70–90% of total N as nitrate. Fe:Mn → no more than 5:1.
Step 5 — Verify actual EC. The sum of ions added can be converted to EC: every 10 mmol/L NO₃⁻ contributes approximately 0.16 mS/cm; 10 mmol/L K⁺ contributes approximately 0.15 mS/cm. Or use a recipe calculator (HydroBuddy, Nutrient Calculator).
Adjusting the Recipe When Issues Arise
A recipe is not a fixed table. Three situations require recalculation:
Change of water source. New water batch, seasonal change, or switch to RO — always recalculate background Ca and Mg and adjust additions accordingly.
Change of growth phase. Transition from vegetative to fruiting: raise K by 30–50%, reduce N (especially NH₄⁺), increase Ca. The vegetative recipe and the fruiting recipe are different recipes.
Deficiency or toxicity symptoms. A symptom signals that something in the recipe or conditions has gone out of range. Fix it by analysing ratios (what is in excess and competing) — not by mechanically adding more of the "deficient" element.
Three Mistakes That Cost the Most
Building a recipe without accounting for source water composition. This is the most common error when working with hard water. The result: systematic excess Ca and Mg regardless of how "correct" the recipe looks on paper.
Raising EC when symptoms appear without analysing the cause. Symptoms at EC 2.0 are not necessarily a nutrient deficiency. More often: antagonism from an incorrect ratio, pH-induced micronutrient lockout, or osmotic stress. Raising EC in these cases makes things worse.
Not revisiting the recipe when changing crops or seasons. The recipe for basil and the recipe for tomato are substantially different. A recipe that worked well in winter at 18°C solution temperature and 12-hour photoperiod needs adjustment in summer at 25°C and 16 hours.
How to Know the Recipe Is Right for Your Conditions
- Plants develop without deficiency symptoms at stable EC and pH
- Drain EC consistently runs +0.3–0.5 above feed EC — no accumulation
- Key ratios (Ca:Mg, K:Mg, NO₃⁻:NH₄⁺) are all within normal ranges
- The recipe has been calculated against the actual source water composition and reflects the current growth phase and season