Hydroponic Salt → EC Dosing Calculator

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.

7. Seedling and Vegetative EC/PPM Targets

The presets above solve recipes in element PPM, but what you check at the tank is EC. The ranges below are the working bands most growers use as a verification step: run the recipe, read the meter in mS/cm, and confirm the plant is being fed inside its stage band rather than merely inside the sheet. Hydroponic seedlings sit low on this scale on purpose: the seed carries its own endosperm for the first days, and a strong reservoir at that point burns the first root tips before the plant can use the ions.

Three calibration habits keep that table honest. Read mS/cm, not the meter's PPM mode: the same solution prints 600, 768 or 840 depending on whether the manufacturer used the 500, 640 or 700 constant (section 5), and none of those constants know your salts. Correct for temperature: EC drifts roughly +2 % per °C above 25 °C, so a warm reservoir reads high against the same recipe at a cool bench. And measure run-off, not just feed: a feed reading of 1.4 mS/cm with run-off at 2.4 means the block is concentrating salts faster than the plant is drinking, and the fix is a flush, not a stronger recipe.

Seedling health is also a media decision before it is an EC decision. The plug or block a cutting starts in decides how fast the root zone warms up and how fast it dries down: the trade-off between a free-draining and a moisture-holding mix is set out in perlite versus vermiculite. And when a seedling's leaves go pale or blotchy in the first fortnight, resist the urge to raise EC: the nutrient deficiency chart separates the look-alike symptoms (nitrogen versus magnesium, iron versus manganese) that low EC, high EC and a bad pH all produce.

8. Coco Coir, Coir-Pith and Substrate EC/PPM Baselines

Coco coir is milled coconut husk, the pith (spongy fraction) plus varying amounts of fibre and, in a few products, chip. It is a soilless medium rather than a soil amendment: no nutrient value of its own, structure and water-holding instead, and a cation exchange capacity that is high enough to hold calcium and potassium but low enough that the solution you pour in is basically what the roots see. Coir-pith is the fines fraction sold for seedling mixes and as a peat replacement; the term coco coir soil usually means a bagged blend of coir, compost and perlite, which behaves like neither of the pure end members.

The baselines that matter before a plant goes in:

• Rinse and charge first. Coir is often washed with saline irrigation water overseas, so a fresh bale or brick can leach potassium, sodium and chloride straight into the first reservoir. Flush until the run-off settles, then pre-charge with a calcium source so the exchange sites hold Ca rather than displacing it later.
• Fresh medium: leachate pH around 5.8–6.5 (most coir arrives limed into that band), run-off EC below about 0.8 mS/cm after flushing.
• Established crop: aim for run-off EC within roughly 0.3–0.5 mS/cm above the feed. A climbing run-off means the medium is concentrating salts: increase drainage fraction or flush. A falling run-off means the plant is drinking faster than you are feeding.
• Watering behaviour: coir holds several times its dry weight in water and still leaves air in the pore space, which is the whole reason it replaces peat. Over-filling it is the common failure, not under-filling.

Measure the block, not the bottle: a probe dipped in the reservoir cannot tell you what the root zone is doing, so take a leachate sample from the pot. And coir is rarely used at 100 % in practice: it is blended to tune drainage and moisture retention, which is precisely the decision the perlite versus vermiculite comparison walks through, since perlite opens a mix out and vermiculite holds water in it. If growth stalls after a media change, run the symptoms past the nutrient deficiency chart before touching the EC: a calcium lockout from a high-sodium bale and a genuine calcium shortage look identical on the leaf and are fixed in completely different places.