Nutrient mixing: from grams to elemental ppm

01 / Three numbers that mean different things

A fertilizer grade such as 4-18-38 is a mass percentage label: nitrogen, phosphate expressed as P₂O₅ and potash expressed as K₂O. It is not a ratio of elemental N:P:K in the tank. Elemental phosphorus is approximately P₂O₅ × 0.4364; elemental potassium is K₂O × 0.8301.

For dilute growing solutions, 1 mg/L is approximately 1 ppm by mass. The calculator reports elemental N, P, K, Ca and Mg in mg/L. An EC meter instead measures electrical conductance. Its TDS/ppm display uses a conversion factor and cannot identify these individual nutrient concentrations.

Reading: Hydroponic Nutrient Solutions (G6984) ↗ · Nutrient Solution Formulation for Hydroponic Tomatoes in Florida (HS796) ↗

02 / A fully worked 40-litre batch

Masterblend publishes a 40 L row containing 24 g of its 4-18-38 base, 15 g magnesium sulfate and 24 g calcium nitrate. The calculator scales that exact row; other rounded rows on the manufacturer’s page are not used interchangeably. The example assumes calcium nitrate contains 15.5% N and 19% Ca, magnesium sulfate heptahydrate contains 9.86% Mg, and the base contains 0.5% Mg. Verify your bags before using the calculated analysis.

In 40 L, the base contributes 24 × 0.04 × 1000 ÷ 40 = 24 mg/L N. Calcium nitrate adds 24 × 0.155 × 1000 ÷ 40 = 93 mg/L N. Total fertilizer-added N is 117 mg/L. The same calculation gives P 47.13, K 189.26, Ca 114.00 and Mg 39.98 mg/L under the stated assumptions.

The base supplies micronutrients as well. This calculator reports five major elements; it does not certify micronutrient completeness, sulfur, water chemistry or crop suitability. The manufacturer names this a tomato formula. It should not be treated as the universal nutrient target for lettuce, seedlings or fish systems.

Reading: 4-18-38 Tomato Formula ↗

03 / Mixing order and final volume

Use a scale with enough resolution for the batch. A scale reading whole grams is inadequate for accurately weighing a few tenths of a gram. Use clean dedicated containers and label every ingredient. Start with most of the required water, leaving room to bring the batch to its final volume after dissolving.

For this manufacturer example, dissolve the base completely, then magnesium sulfate, then calcium nitrate last. Dissolve each separately or add it to a well-diluted, moving bulk solution as directed. Never combine calcium with sulfate or phosphate in a shared concentrate: precipitation can remove available nutrients and obstruct emitters.

1. Measure and record source-water EC, pH and available laboratory ion results. Zero source-water inputs in the tool mean an assumption of no contribution, not a test result.
2. Weigh each ingredient separately for the intended final volume and strength. Check the product identity and guaranteed analysis.
3. Dissolve in the prescribed order, with enough dilution and mixing between ingredients. Investigate undissolved residue instead of forcing it through the irrigation system.
4. Bring the mixed batch to the final volume mark. A 40 L recipe means 40 L of finished solution, not 40 L plus stock additions.
5. After mixing and temperature stabilization, measure EC and pH. Make any pH correction using the product label and a small test batch; do not infer an acid dose from pH alone.
6. Record the batch, date, lot, grams, final volume and measurements before feeding the crop.

Reading: 4-18-38 Tomato Formula ↗ · Hydroponic Nutrient Solutions (G6984) ↗

04 / Compare a second recipe correctly

Jack’s published 321 mixing article uses 3.6 g Part A, 1.1 g Epsom salt and 2.4 g Part B per US gallon. Its “321” name is not an instruction to weigh three, two and one grams. One US gallon is 3.785411784 L; an imperial gallon is a different quantity. The calculator retains the article’s 1.1 g Epsom rate.

The example analysis assumes Part A is 5-12-26 with 6.3% Mg and Part B is 15% N with 18% Ca. Product versions and analysis can change, so edit the displayed assumptions to match actual labels. A recipe with a different nitrogen or calcium percentage cannot be substituted gram for gram and still yield the same elemental ppm.

Reading: How do I mix Jack’s 321? ↗

05 / Source water and adjustment arithmetic

If a laboratory reports 40 mg/L elemental Ca in source water, the 40 L Masterblend example has a calculated total of 154 mg/L Ca before uptake or precipitation. If the report instead lists hardness as CaCO₃, do not enter that number in the elemental Ca field.

To raise an element by Δ mg/L in V litres using a product containing p percent of that element, grams = Δ × V ÷ (10 × p). Adding 20 mg/L Mg to 100 L with 9.86% Mg salt requires 20.28 g. That salt also supplies sulfate: calculate every contributed ion when reformulating a complete feed.

Lowering the entire recipe to 50% halves every fertilizer contribution, but does not halve nutrients already in source water. A smaller volume at the same recipe strength changes grams, not ppm. These are useful checks for catching unit errors before mixing.