- Convert a three-number grade into elemental percentages and grams per kilogram.
- Read the analysis panel, nitrogen breakdown and derived-from list for what a recirculating solution can use.
- Decide from the label whether a product is one bag or two parts, and dose it by mass.
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 numbers by mass, not three elements
UF/IFAS SL-3/SS170 defines the grade as the percentages of total nitrogen expressed as N, available phosphate expressed as P₂O₅ and soluble potassium expressed as K₂O, in whole numbers in that order. Only the first figure is an element. SS170 puts elemental phosphorus at 44% of the available phosphate figure and elemental potassium at 83% of the potash figure; the site’s calculator uses the four-decimal factors 0.4364 and 0.8301.
So a 4-18-38 bag carries 4% N, 18 × 0.4364 = 7.86% P and 38 × 0.8301 = 31.54% K by mass: 40 g N, 78.6 g P and 315.4 g K per kilogram. Enter label figures into the calculator as printed; it converts the oxides itself.
02 / The analysis panel below the grade
SS170 describes the analysis section as total nitrogen, the sum of all nitrogen forms present, then available phosphate, soluble potassium and each secondary nutrient. Secondary nutrients are stated in elemental form; magnesium, iron, zinc, copper and manganese are expressed as “total” and/or “water-soluble”, and a chelated element is stated separately when a chelating agent appears in the derived-from list.
The Masterblend 4-18-38 page, accessed 17 September 2026, is tabulated below. It has no calcium line and no sulfur line, although magnesium sulfate is a listed source: an ingredient absent from the panel has no stated percentage, and the site’s calculator, which reports only N, P, K, Ca and Mg, does not fill that gap: the sulfate load of this recipe is unstated.
| Panel line (4-18-38 example) | Printed value | Per kilogram of product |
|---|---|---|
| Total nitrogen (N) | 4% (3.5% nitrate, 0.5% ammoniacal) | 40 g N; 87.5% nitrate by arithmetic |
| Available phosphate (P₂O₅) | 18% | 180 g P₂O₅ = 78.6 g P |
| Soluble potash (K₂O) | 38% | 380 g K₂O = 315.4 g K |
| Iron (Fe) | 0.40% chelated (iron DTPA in derived-from) | 4 g Fe held in chelate |
| Derived from | potassium nitrate, potassium phosphate, ammonium phosphate, potassium chloride, magnesium sulfate, boric acid, copper EDTA, iron DTPA, manganese EDTA, sodium molybdate, zinc EDTA | No calcium source; sulfur and chloride unstated |
Reading: The Florida Fertilizer Label (SL-3/SS170) ↗ · 4-18-38 Tomato Formula ↗
03 / Nitrate, ammoniacal and urea nitrogen
SS170 lists the nitrogen forms a label may declare, each with its percentage: nitrate, ammoniacal, water-soluble, urea and water-insoluble; urea may be declared as water-soluble or as urea nitrogen. The UMass greenhouse nutrient-management page explains why the split matters in soilless culture: fertilizers with more urea and ammoniacal nitrogen are acidic in reaction, those with primarily nitrate are basic, and ideally no more than 50% of the total nitrogen supplied in soilless media should be ammonium, because ammonium toxicity can occur from high levels of ammonium or urea fertilizer.
UMass expresses potential acidity or basicity as pounds of calcium carbonate per ton of fertilizer, the larger the number the greater the effect on pH; the Masterblend page prints 209 lb potential basicity in that unit. MU G6984 lists fertilizers with a high nitrate concentration (less than 25% of total nitrogen from ammonia or urea) among the bases that raise pH. For 4-18-38, 0.5 ÷ 4 = 12.5% of the base’s nitrogen is ammoniacal; the whole-recipe share also depends on the calcium nitrate label.
Reading: The Florida Fertilizer Label (SL-3/SS170) ↗ · Nutrient Management (Greenhouse Best Management Practices Manual) ↗ · Hydroponic Nutrient Solutions (G6984) ↗ · 4-18-38 Tomato Formula ↗
04 / Water-soluble, slow-release and soil fertilizers
A soil fertilizer can print an honest grade and still be wrong for a reservoir. SS170 explains that available phosphate is water-soluble plus citrate-soluble phosphorus, with the soil solution acting as the weak acid that makes the citrate-soluble part available; that water-insoluble nitrogen covers sources such as urea-formaldehyde; and that slow- or controlled-release nutrients are stated in a footnote after the source materials. A recirculating solution has no soil to work on those fractions, so anything declared water-insoluble, citrate-soluble or controlled-release cannot be counted on to dissolve.
The reading rule is MU G6984’s: verify that the fertilizer is water-soluble, because some formulations have limited solubility. Prefer a label that states a maximum solubility, as the Masterblend page does (3.5 lb per gallon), so a stock concentration can be checked against it; G6984 notes that some labels also state the expected EC.
Reading: The Florida Fertilizer Label (SL-3/SS170) ↗ · Hydroponic Nutrient Solutions (G6984) ↗ · 4-18-38 Tomato Formula ↗
05 / The derived-from list, calcium and one bag versus two
SS170 describes the derived-from section as the actual source materials for the primary and secondary nutrients. It tells you what the grade cannot: whether calcium is present, whether chloride rides in with potassium chloride, and which chelating agent holds each micronutrient. MU G6984 explains why calcium decides the bag count: recipes may need two formulations because calcium from one immediately reacts with phosphorus and sulfate from the other, forming insoluble particles that sink and become unavailable; dissolving each separately prevents it, and such products are often sold as Part A and Part B.
Both site recipes follow that pattern: the Masterblend derived-from list has no calcium source and its mixing table adds calcium nitrate last; Jack’s article keeps calcium nitrate as Part B, dissolved after Part A (5-12-26) and Epsom salt.
- Write the grade as percentages by mass; convert P₂O₅ and K₂O with 0.4364 and 0.8301, or leave them for the calculator.
- Read the nitrogen breakdown and calculate the nitrate share; note any urea or water-insoluble line.
- Read each secondary and micronutrient line, separating “total”, “water-soluble” and chelated figures.
- Read the derived-from list for calcium, chloride and chelating agents; calcium beside phosphate or sulfate means separate stocks.
- Reject any slow-release footnote, coated product or citrate-soluble claim for a recirculating tank.
- Record net weight, maximum solubility and expected EC, then enter the panel figures into the calculator as printed.
Reading: The Florida Fertilizer Label (SL-3/SS170) ↗ · Hydroponic Nutrient Solutions (G6984) ↗ · 4-18-38 Tomato Formula ↗ · How do I mix Jack’s 321? ↗
06 / Mass, volume and a worked conversion
Every panel percentage is a fraction of the product’s mass, so a dry salt is dosed with a scale. A liquid concentrate is measured by volume, and a litre of it does not weigh a kilogram; unless the label gives a density, weigh a measured volume before trusting a millilitres-per-litre instruction.
Masterblend’s 40 L row uses 24 g of 4-18-38, or 0.6 g/L. The panel lines the calculator does not report convert the same way: iron, 0.6 g/L × 1000 mg/g × 0.40 ÷ 100 = 2.4 mg/L Fe; magnesium from the base alone, 0.6 × 1000 × 0.50 ÷ 100 = 3.0 mg/L Mg, before the 15 g of magnesium sulfate. The N, P and K figures for this row are worked in the mixing chapter.
FIELD QUESTIONA bag reads 5-12-26 (the analysis the site assumes for Jack’s Part A). What are its elemental P and K percentages, and how many grams per litre deliver 50 mg/L P?
P: 12 × 0.4364 = 5.24%. K: 26 × 0.8301 = 21.58%. Grams = Δ × V ÷ (10 × p) = 50 × 1 ÷ (10 × 5.24) = 0.95 g/L, or 0.95 × 3.785 = 3.6 g per US gallon, the Part A rate in Jack’s article. The same dose carries 205 mg/L K and 48 mg/L N.
Reading: 4-18-38 Tomato Formula ↗ · How do I mix Jack’s 321? ↗
Parts & buying criteria
Build your own parts & cost worksheet →
Showing nutrient film technique. Quantities describe the teaching model. Specify real working volume, support, fittings and instruments for your installation.
01Reservoir1 · illustrated quantity+
Holds the recirculating nutrient solution. This is the common sampling and refill point for both channels.
Inspect: In cutaway view, compare the water surface with the pump intake. The drawing leaves room above the water; it does not specify a working volume.
Maintain: Record the level before adding water. Inspect the cover, clean accessible deposits and keep the intake submerged during use.
Opaque, cleanable container rated by its manufacturer for the intended use; accessible lid and drain.
Check size, materials and operating conditions with your chosen supplier before ordering.02Pump & supply1 · illustrated quantity+
Lifts solution from the reservoir to the channel inlets. Cyan packets indicate the direction of delivery.
Inspect: Follow the rising supply pipe and its branch to each channel. Both inlets need an observable supply.
Maintain: Disconnect power before opening the pump. Inspect its inlet screen and confirm both branches flow after reassembly.
Compare the pump curve at the installed lift and pipe resistance; include service access and replacement availability. Supply the pump, timer and any air pump through ground-fault protection (GFCI/RCD), keep plugs and cord connections above the water line with a drip loop, and switch off and unplug before reaching into the solution.
Check size, materials and operating conditions with your chosen supplier before ordering.03Growing channels1 · illustrated quantity+
Contain the shallow stream beneath the plants and guide it toward the return.
Inspect: Open the cutaway to find the channel floor beneath the roots. The slope is exaggerated for reading.
Maintain: Look along each channel for ponding, debris or an obstructing root mat; inspect the outlet before changing the pump.
Removable covers, supported slope, smooth cleanable surfaces and outlets accessible with plants in place.
Check size, materials and operating conditions with your chosen supplier before ordering.04Plant baskets6 illustrated · illustrated quantity+
Bridge the opening in the channel cover while allowing roots to pass through.
Inspect: In exploded view, the baskets lift above the channels. The open slats reveal the relationship between support and roots.
Maintain: Check that each rim sits securely and that roots are not pinched at transplanting or inspection.
Match the basket rim and hole diameter; allow removal without damaging adjacent plants.
Check size, materials and operating conditions with your chosen supplier before ordering.05Support mediumFor 6 baskets · illustrated quantity+
The illustrated expanded clay supports the seedling in its basket. The channels are not filled with aggregate.
Inspect: Select the brown pebbles and separate the basket in exploded view. Media texture is simplified.
Maintain: Remove loose particles before installation. Inspect whether the starter plug stays appropriately moist.
Select a stable grade that stays in the basket; compare water retention, dust and cleaning requirements.
Check size, materials and operating conditions with your chosen supplier before ordering.06Gravity return1 · illustrated quantity+
Collects channel outflow and brings it back to the reservoir without a second pump.
Inspect: Trace the low ends of both channels into the common return. Inspect the full path before assuming the circuit is clear.
Maintain: Remove obstructions and check for leaks. Test drainage with the pump off during commissioning.
Removable unions, supported pipe and sufficient drain capacity under the actual installation conditions.
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.
- University of Florida IFAS ExtensionThe Florida Fertilizer Label (SL-3/SS170) ↗
Definition of the grade as total nitrogen as N, available phosphate as P₂O₅ and soluble potassium as K₂O in whole numbers; elemental phosphorus as 44% of available phosphoric acid and elemental potassium as 83% of soluble potash; the analysis section as total nitrogen (the sum of all forms), available phosphate, soluble potassium and each secondary nutrient; nitrogen declared as nitrate, ammoniacal, water-soluble, urea and water-insoluble forms, with urea declarable as water-soluble or urea nitrogen; available phosphate as water-soluble plus citrate-soluble phosphorus made available by the weak acid of the soil solution; water-insoluble nitrogen sources such as urea-formaldehyde, isobutylidene diurea and magnesium ammonium phosphate; slow- or controlled-release nutrients stated in a footnote after the source materials; secondary nutrients in elemental form with total and water-soluble expressions and chelated elements stated separately when a chelating agent is named; the derived-from section as the actual source materials; and net weight.
- Masterblend · manufacturer4-18-38 Tomato Formula ↗
Guaranteed base analysis and the 40 L mixing row; a product example, not an endorsement.
- UMass Amherst Center for Agriculture, Food, and the Environment · Greenhouse & FloricultureNutrient Management (Greenhouse Best Management Practices Manual) ↗
Web page without a publication number. Fertilizers containing more urea and ammoniacal nitrogen being acidic in reaction and those primarily nitrate being basic; potential acidity and basicity expressed as pounds of calcium carbonate needed to neutralize, or equivalent in reaction to, one ton of fertilizer, with the larger number the greater the effect on pH; ideally no more than 50 percent of total nitrogen supplied to plants in soilless media in the ammonium form; ammonium toxicity in soilless media from high levels of ammonium or urea fertilizer; and choosing a fertilizer by the ratio of ammonium to nitrate-N, trace element charge, calcium and magnesium content and potential acidity or basicity.
- University of Missouri ExtensionHydroponic Nutrient Solutions (G6984) ↗
Solution preparation, elemental nutrition and measurement.
- Jack’s Nutrients · manufacturerHow do I mix Jack’s 321? ↗
Published 3.6 / 1.1 / 2.4 gram per US gallon mixing example.
The next useful connections.
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Scale two documented fertilizer examples, read the bag analysis and calculate what actually enters the final solution.
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Read a laboratory report, distinguish equivalent units from elemental nutrients and prepare water deliberately.
6 min + guided practice →Nutrients & mixingMasterblend 4-18-38: the three-part recipe and its elemental ppm
Three bags against one published row: what each part adds, why calcium nitrate goes in last and alone.
Interactive tool · 7 min + guided practice →Nutrients & mixingJack’s 321 explained: grams per gallon, mixing order and elemental ppm
Jack’s 321 is one published mixing example for a two-part fertilizer plus Epsom salt; this guide turns its three gram figures into elemental ppm and shows where the readout stops.
Interactive tool · 7 min + guided practice →