- Name what each of the three parts adds and why calcium nitrate is dissolved last and alone.
- Read the 40 L row against the calculator’s linear scaling and the assumptions behind N 117, P 47.1, K 189.3, Ca 114 and Mg 40 mg/L.
- Treat the half-rate note and HS796’s stage program as published examples, not targets.
Recipe and elemental ppm calculator
24 g 4-18-38 + 15 g magnesium sulfate + 24 g calcium nitrate per 40 L final solution. Calcium nitrate analysis assumed 15.5% N / 19% Ca; magnesium sulfate assumed 9.86% Mg. Check the actual bags.
Manufacturer’s published mixing example ↗
Check or edit fertilizer label assumptions
Use percent by mass. P and K label fields are P₂O₅ and K₂O. Changing these affects elemental totals, while the recipe’s product weights stay fixed. Blank or invalid analysis produces no result.
Add measured source-water elemental concentrations
Default zero is an assumption of no contribution. Enter laboratory elemental mg/L, never hardness as CaCO₃ or nitrate reported as NO₃ without conversion.
24 g Masterblend 4-18-38, 15 g Magnesium sulfate heptahydrate, 24 g Calcium nitrate; totals N 117, P 47.13, K 189.26, Ca 114, Mg 39.98 mg/L
| Element | Fertilizer added | Source water | Total |
|---|---|---|---|
| N | 117 | 0 | 117 |
| P | 47.13 | 0 | 47.13 |
| K | 189.26 | 0 | 189.26 |
| Ca | 114 | 0 | 114 |
| Mg | 39.98 | 0 | 39.98 |
Five-element accounting only. No prediction of EC, uptake, precipitation or crop suitability. Sulfur and micronutrients are outside this readout. Do not use these mineral-feed recipes in a coupled fish system.
01 / Three parts, and why calcium nitrate stays apart
The manufacturer’s 4-18-38 Tomato Formula page (accessed 17 September 2026) lists the base at 4% N, 18% available phosphate as P₂O₅, 38% soluble potash as K₂O and 0.50% Mg, plus chelated micronutrients, boron and molybdenum. It lists no calcium and no sulfur. Calcium nitrate supplies nitrogen and calcium; magnesium sulfate supplies magnesium and sulfur. UF/IFAS HS796’s Table 2 lists a 4-18-38 pre-mix under the heading “Pre-mix 1” and states that all such formulations need supplementing with calcium and nitrogen.
The parts meet only in the dilute tank. MU G6984 states that calcium from one formulation will immediately react with phosphorus and sulfate from the other, creating insoluble particles that sink to the tank bottom and are unavailable to roots, and that dissolving each formulation separately prevents this.
Reading: 4-18-38 Tomato Formula ↗ · Nutrient Solution Formulation for Hydroponic Tomatoes in Florida (HS796) ↗ · Hydroponic Nutrient Solutions (G6984) ↗
02 / The published rows and how the calculator scales them
The manufacturer prints gallon and litre tables with the columns 4-18-38, magnesium sulfate and calcium nitrate, the last headed “Add LAST!”. The 40 L row is 24 g base, 15 g magnesium sulfate, 24 g calcium nitrate; the calculator on this page takes it as the recipe (0.6, 0.375 and 0.6 g/L) and scales it linearly to any final volume.
The manufacturer rounds its other rows, so the calculator’s grams do not always match the printed table; the 100 L and 8 L rows below show the gap.
| Final volume | Manufacturer row (base / Mg sulfate / Ca nitrate) | Linear scaling of the 40 L row |
|---|---|---|
| 100 L | 60 / 37 / 60 g | 60 / 37.5 / 60 g |
| 40 L | 24 / 15 / 24 g | 24 / 15 / 24 g (reference row) |
| 8 L | 5 / 3 / 5 g | 4.8 / 3 / 4.8 g |
Reading: 4-18-38 Tomato Formula ↗
03 / The analysis assumptions behind the readout
The calculator uses the base label above and two stated assumptions: calcium nitrate at 15.5% N and 19% Ca, magnesium sulfate heptahydrate at 9.86% Mg. Neither product is on the manufacturer’s page, so both are editable fields, not facts about your bag. HS796’s Table 2 lists dry calcium nitrate at 15% N and 19% Ca, and lists Pre-mix 1 without magnesium where today’s label prints 0.50% Mg: labels change, and the calculator follows the current one.
At the 40 L row, g/L × label % × 10 gives mg/L: N = 0.6 × 4 × 10 + 0.6 × 15.5 × 10 = 24 + 93 = 117 mg/L; P = 0.6 × 18 × 10 × 0.4364 = 47.1; K = 0.6 × 38 × 10 × 0.8301 = 189.3; Ca = 0.6 × 19 × 10 = 114; Mg = 0.6 × 0.5 × 10 + 0.375 × 9.86 × 10 = 3.0 + 37.0 = 40.0 mg/L. Change the calcium nitrate grade and only N and Ca move.
FIELD QUESTIONYour calcium nitrate reads 15% N and 19% Ca, the dry grade in HS796’s Table 2, instead of the assumed 15.5% N. What does the 40 L row read at full strength, and at the manufacturer’s half rate for seedlings?
Calcium nitrate N falls from 93 to 0.6 × 15% = 90 mg/L, so total N is 114 instead of 117. Ca stays 114 because its percentage is unchanged; P, K and Mg do not move. At half rate every contribution halves: N 57, P 23.6, K 94.6, Ca 57, Mg 20 mg/L, before anything the source water adds.
Reading: 4-18-38 Tomato Formula ↗ · Nutrient Solution Formulation for Hydroponic Tomatoes in Florida (HS796) ↗
04 / Mixing order and dissolving
The manufacturer’s order is fixed: dissolve the 4-18-38 in water, add the magnesium sulfate and dissolve completely, then add the calcium nitrate last and dissolve completely. The page also says to use the product only after a thorough water quality analysis. Scales, containers and safety data sheets are covered in the general mixing chapter.
- Enter the final volume in the calculator on this page, record the three gram figures and note which row the batch follows.
- Fill the tank with most of the final volume and dissolve the weighed 4-18-38 until no residue remains.
- Add the magnesium sulfate and dissolve completely.
- Add the calcium nitrate last, into the dilute, moving solution. Never pre-dissolve it with either other part.
- Top up to the final mark, let the temperature settle, then log EC and pH beside the grams. A cloudy batch or a settled layer means something has precipitated or not dissolved; find the cause before feeding it to the crop.
Reading: 4-18-38 Tomato Formula ↗ · Hydroponic Nutrient Solutions (G6984) ↗
05 / Strength by stage: published examples, not targets
The manufacturer’s note reads: use half rates during initial growth until seedlings are well established. MU G6984 gives the general reason, that fertilizer amounts and types change because crops require different nutrient levels at different growth stages, but prints no seedling fraction. Half rate is a whole-recipe change: every element halves together.
HS796’s Table 3 adjusts the program the other way. Its 4-18-38 example holds the A stock at 16 lb of pre-mix per 30-gal stock tank, lifts the magnesium sulfate in that stock from 10 lb to 12 lb after the third cluster, and raises the calcium nitrate in the B stock from 7.5 lb at transplant to 20.5 lb after the fifth cluster, so the delivered solution moves from N 70 and Ca 56 to N 148 and Ca 154 mg/L while K stays at 200. That is a Florida greenhouse tomato program at 1:100 dilution, not a target for lettuce or for your water; the fruiting crops chapter reads the stage table behind it.
Reading: 4-18-38 Tomato Formula ↗ · Hydroponic Nutrient Solutions (G6984) ↗ · Nutrient Solution Formulation for Hydroponic Tomatoes in Florida (HS796) ↗
06 / What the readout leaves out
Sulfur and micronutrients. The base lists no sulfur, so all of it comes from the magnesium sulfate; HS796 lists that salt at 10% Mg and 14% S, and at 14% the row’s 0.375 g/L carries about 52.5 mg/L S, which the calculator does not print. Micronutrients come from the base alone and are also outside the readout.
Source water and storage. Penn State’s irrigation-water guidance treats mg/L and ppm as the same in solution and states that calcium below 40 mg/L typically needs fertilizer calcium and magnesium below 25 mg/L needs fertilizer magnesium. Your report’s figures add to the 114 mg/L Ca and 40 mg/L Mg above; the calculator’s zero water fields are an assumption. Storage is original practice, not attributed: keep each dry salt closed and labelled in its own container; a caked or damp bag no longer weighs out at the label’s percentages. The label and safety data sheet govern.
Reading: 4-18-38 Tomato Formula ↗ · Nutrient Solution Formulation for Hydroponic Tomatoes in Florida (HS796) ↗ · Interpreting Irrigation Water Tests ↗
Parts & buying criteria
Build your own parts & cost worksheet →
Showing aerated deep water culture. Quantities describe the teaching model. Specify real working volume, support, fittings and instruments for your installation.
01Nutrient reservoir1 · illustrated quantity+
Stores the measured nutrient solution beneath or beside the growing area.
Inspect: Compare the surface with the diffuser and the lower roots; freeboard is deliberate. There is no pump intake in this single-vessel example.
Maintain: Record level before refilling, keep light out and verify temperature, EC and pH.
Opaque, cleanable, intended-use container with volume marks and service access.
Check size, materials and operating conditions with your chosen supplier before ordering.02Lid & root support1 · illustrated quantity+
Holds the plant above the chamber or reservoir without pinching its crown.
Inspect: Separate the cover and inspect the basket or collar opening.
Maintain: Check fit as plants grow and keep unused openings covered.
Rigid compatible support with removable, correctly fitted baskets or collars.
Check size, materials and operating conditions with your chosen supplier before ordering.03Air pump & diffuser1 · illustrated quantity+
Delivers air below the water surface in this aerated DWC example.
Inspect: Trace the thin line from the dry pump to the diffuser. Bubbles are illustrative.
Maintain: Check actual delivery and dissolved oxygen; preserve backup air and prevent back-siphoning.
Rated output at the installed depth and diffuser resistance; accessible replaceable parts. Power it through ground-fault protection (GFCI/RCD) and keep the pump and its plug above the water line.
Check size, materials and operating conditions with your chosen supplier before ordering.Evidence beside the lesson.
Source checks: September 2026. The geometry, inspection exercises and worksheets are original teaching material. The named organizations have not endorsed or reviewed this site.
Published recipe rates and planning ranges retain their source context. Calculator equations are accounting tools; they do not predict uptake, yield, toxicity or safe stocking. Model dimensions, water speeds, roots and fish counts are illustrative. Verify species, crop, source water and product labels for a real system.
- Masterblend · manufacturer4-18-38 Tomato Formula ↗
Guaranteed base analysis and the 40 L mixing row; a product example, not an endorsement.
- University of Florida IFAS ExtensionNutrient Solution Formulation for Hydroponic Tomatoes in Florida (HS796) ↗
Elemental ppm by fruit-cluster stage in Florida greenhouse tomato production.
- University of Missouri ExtensionHydroponic Nutrient Solutions (G6984) ↗
Solution preparation, elemental nutrition and measurement.
- Penn State ExtensionInterpreting Irrigation Water Tests ↗
Distinguishing alkalinity, pH, salts and source-water problems.
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