The bag says "Tomato & Vegetable Fertilizer", which is exactly why people growing greens walk past it. They should not. Run the popular recipe through the arithmetic and it lands on the Cornell CEA reference solution for lettuce almost exactly: nitrogen 124 against 125, potassium 200 against 215. The tomato formula turns out to be a lettuce formula, and it got there by accident.
Quick glossary: Masterblend 4-18-38 - dry water soluble blend: N 4%, P₂O₅ 18%, K₂O 38%, Mg 0.50%, plus micronutrients. It contains no calcium at all, which is why calcium nitrate is mandatory rather than optional. Epsom salt - magnesium sulfate MgSO₄·7H₂O, 9.86% magnesium. The forum recipe - 12 g Masterblend, 12 g calcium nitrate, 6 g Epsom salt per 5 gallons, which works out to 2.4 / 2.4 / 1.2 g per gallon.
There Are Two Recipes and They Are Not the Same
The one circulating on forums: 2.4 g Masterblend + 2.4 g calcium nitrate + 1.2 g Epsom salt per gallon (3.785 L).
The one the manufacturer publishes: 8 oz of 4-18-38 per 100 gallons of water along with 5 oz of magnesium sulfate, then 8 oz of calcium nitrate. Converted, that is 2.27 / 1.42 / 2.27 g per gallon.
The gap sits in magnesium: 1.42 against 1.2, so the manufacturer runs about 15% more of it. On the Masterblend and the calcium nitrate the forum recipe is marginally higher instead. Both work. Calling the forum version "the official recipe" does not, and it happens constantly.
For transplants the manufacturer gives a separate, weaker set: the same 8 oz of 4-18-38, but 4 oz of magnesium sulfate and 6 oz of calcium nitrate per 100 gallons.
Why a Tomato Formula Lands on Lettuce
The forum recipe against the Cornell CEA lettuce solution, in mg per liter, comes out nearly identical on nitrogen and potassium, with one real overshoot - phosphorus, running 1.6x high because the blend's 18% P₂O₅ is built for tomato fruit set, though the surplus simply leaves with the drain water.
- Nitrogen: 124 against 125
- Potassium: 200 against 215
- Sulfur: 41 against 35
- Magnesium: 34 against 24
- Calcium: 120 against 84
- Phosphorus: 50 against 31, the one real overshoot at 1.6x
Nitrogen and potassium are dead on. Phosphorus runs high because the blend carries 18% P₂O₅, and that is the tomato half of the design: fruiting crops want phosphorus at fruit set. Lettuce does not need it in that quantity, though it does no harm either. The surplus simply leaves with the drain water.
Compare that with Jack's 321, which solves the same problem from the opposite direction. Jack's Part A carries 6.3% magnesium, so the magnesium is already inside the blend, and the recipe adds Epsom salt on top anyway: the element gets counted twice and ends up 3.7x over. Masterblend carries 0.50%, close to nothing, and every bit of magnesium arrives deliberately in a separate bag. Two blends for one job, built back to front from each other.
Mixing Order and Where the Precipitate Comes From
The three components go into the reservoir in sequence, each fully dissolved before the next. Never combine the dry powders and never build a single concentrate out of them.
The chemistry is the same one covered in stock solutions A and B. Calcium from the nitrate plus sulfate from the Epsom salt gives you gypsum. Calcium plus phosphate from the Masterblend gives you calcium phosphate. Both are insoluble, and both form more readily the higher the local concentration. So Masterblend and Epsom salt can share a step without trouble, while calcium nitrate always goes in separately and last, into already diluted solution.
The sign you got it wrong: a cloudy reservoir, or a white film on the bottom after a day. That solution now holds less calcium and phosphorus than your calculation says, and the leaf will report it in about two weeks, by which point you will be looking at pH for the cause.
The Micronutrients Nobody Checks
The three numbers on the front cover nitrogen, phosphorus and potassium, but Masterblend diverges most on the micronutrients: at 2.4 g per gallon it runs iron, manganese, and boron several times above the Cornell reference, with zinc closer to it.
- Iron: 2.54 against 0.94
- Manganese: 1.27 against 0.14
- Boron: 1.27 against 0.16
- Zinc: 0.32 against 0.13
Read this as direction rather than verdict: Cornell runs a deliberately lean micronutrient program, and plenty of working recipes sit above it on iron. The one worth watching is boron, which has the narrowest gap between deficiency and excess of any micronutrient. If you run Masterblend at full tomato strength on lettuce under low light, boron is the first thing to reconsider, not the nitrogen.
Three Mistakes That Cost the Most
Dropping the calcium nitrate as a "redundant third bag". Masterblend contains zero calcium. Without the nitrate you are growing lettuce with no calcium at all, which is a direct route to tipburn. The nitrate is not an additive here, it is half the recipe.
Running the tomato dose on lettuce and never measuring EC. The forum recipe was built around greenhouse tomatoes. On lettuce under weak light the same solution runs strong: the crop does not draw it down, EC climbs as water evaporates, and the root starts paying an osmotic price. Cornell's figure is 1150 to 1250 µS/cm above the source water, and it has to be read exactly that way, as covered in EC in hydroponics.
Premixing all three powders into one jar "for convenience". The dry mix behaves until it picks up humidity. After that the reaction runs inside the jar, and what you spoon into the reservoir is calcium that is already partly bound.
How to Tell It Is Mixed Right
The solution is clear a day after mixing, with no sediment and no film. EC matches your calculation for your own water rather than an absolute number copied from someone else. pH holds between 5.6 and 6.0 instead of drifting daily.
On the plant: even color, steady growth without surges, no marginal burn on young leaves. Burn while reservoir calcium reads normal points at delivery rather than dose: humidity, air movement, growth rate.
Which of the common systems is worth buying, and what each actually costs per hundred liters, is compared separately in Masterblend vs Jack's 321 vs Flora Series.
If you want to go deeper: Nutrient solution recipes: the logic of building one, not copying one covers how to calculate a formula for your own water and crop instead of borrowing someone else's grams.