The Science of Salt Dosing: Element PPM, EC, and Why Generic Calculators Fail

AquaDose Salt Lab exists to resolve the three most common points of confusion in hydroponic dosing: the difference between element PPM and salt PPM, what EC actually measures, and why a single "PPM × 0.5" multiplier can never be accurate across different fertilizer salts.

1. The Core Confusion: Element PPM vs. Salt PPM

When a recipe says "150 ppm Nitrogen," it means the plant-root zone must contain 150 mg/L of the actual element N. But you cannot buy bottled nitrogen — you buy a salt like calcium nitrate or potassium nitrate. Each salt carries the target element bound to "carrier" atoms (calcium, potassium, oxygen), so the PPM of salt you dissolve is always higher than the PPM of element the plant receives:

Salt PPM = Target Element PPM ÷ (Element % in Salt ÷ 100)

Example: 150 ppm N from calcium nitrate (15.5% N) → 150 ÷ 0.155 = 968 ppm of salt. In a 100-gal (378.5 L) reservoir that is 968 × 378.5 ÷ 1000 = 366 g (0.81 lb) of powder.

2. What Is EC, and Why Do We Use It?

Electrical Conductivity (EC) measures how well water carries current. Pure distilled water reads 0 — no ions to carry charge. Fertilizers dissolve into charged ions (K⁺, NO₃⁻, Ca²⁺, SO₄²⁻) that conduct beautifully. Plants don't "eat EC" — they eat specific ions — but an EC meter is a cheap, instant proxy for total solution strength. AquaDose predicts EC the same way a lab would: by summing each ion's concentration × its molar ionic conductance (Λ) at 25 °C:

Predicted EC (mS/cm) = Σ [ Salt PPM × Σ(count × Λ) ÷ Molecular Weight ÷ 1000 ]

3. Why the "Salt Factor" Is the Secret Sauce

Free calculators that apply one generic multiplier are scientifically inaccurate because every salt has a different molecular weight and a different ion mix. To deliver 100 ppm of potassium:

• Potassium nitrate (38.7% K): 258.6 ppm salt → predicted EC 0.37 mS/cm
• Potassium sulfate (44.9% K): 222.9 ppm salt → predicted EC 0.39 mS/cm

Same element, same target — different powder weight and different meter reading. And extremes matter: urea adds N with almost zero EC (non-ionic), while KCl conducts aggressively. The per-salt factors baked into this tool come from CRC-standard ionic conductance values, so your predicted EC is verifiable at the tank.

4. Real-World Walkthrough (100-Gallon Tomato Tank)

1. Goal: recipe calls for 150 ppm N, sourced from calcium nitrate, in 100 gallons.
2. Enter: N target 150 → source "Calcium Nitrate 15.5-0-0 +19Ca" → volume 100 gal → Calculate.
3. Tool output: weigh 366 g of calcium nitrate; predicted EC ≈ 1.56 mS/cm (it also flags the +184 ppm of co-delivered calcium).
4. Verify: mix, stir, dip a calibrated meter. Reading 1.55–1.65? Perfect. 1.8? You overdosed — dilute with plain water. 1.2? Underdosed — add more salt.

5. The "PPM Scale" Trap on EC Meters

A meter's "PPM" mode never measures PPM — it measures EC and multiplies by an arbitrary manufacturer constant:

• 500 scale (NaCl): PPM₅₀₀ = EC × 500 — common in the US (Bluelab, HM Digital US models).
• 640 scale: PPM₆₄₀ = EC × 640 — common in Europe and Australia.
• 700 scale: PPM₇₀₀ = EC × 700 — some TDS pens.

None of these constants know anything about your actual salts. Professional growers ignore the PPM mode entirely: they compute the expected EC here, then confirm the tank against mS/cm only. This tool shows you all three converted readings so you can translate whichever meter you own.

6. Where This Tool Fits Your Workflow

1. Design — pick a crop recipe in element PPM (use the presets).
2. Calculate — choose your actual salts and tank size; run the solver.
3. Mix — weigh each salt from the Weigh-Out List and dissolve fully.
4. Verify — confirm the meter matches the predicted EC (± 0.1 mS/cm).
5. Feed — deliver via drip, NFT, flood table, or hand-watering.