Jacks 321 (the maker spells it Jack’s 321) is a two-part hydroponic nutrient program from JR Peters: 3.6 grams of Part A (5-12-26), 2.4 grams of Part B (calcium nitrate) and 1.1 grams of Epsom salt per US gallon of water. Mix in this order: dissolve Part A completely, then the Epsom salt, then Part B. Never combine the dry powders.
| Water volume | Part A (5-12-26) | Epsom salt | Part B (calcium nitrate) |
|---|---|---|---|
| 1 liter | 0.95 g | 0.29 g | 0.63 g |
| 1 gallon | 3.6 g | 1.1 g | 2.4 g |
| 5 gallons | 18 g | 5.5 g | 12 g |
| 10 gallons | 36 g | 11 g | 24 g |
| 20 gallons | 72 g | 22 g | 48 g |
| 55 gallons | 198 g | 60.5 g | 132 g |
Scaled from Jack’s published per-gallon rates. Liter values are rounded.
What Is Jacks 321?
Jack’s Nutrients calls the program its “tried and true two-part fertilizer program.” Its technical bulletin lists Jack’s Nutrients Part A 5-12-26, Part B 15.5-0-0 and Epsom salts, and explains the name: “321” is the shortened amount of each fertilizer in grams per gallon, from 3.6, 2.4 and 1.1.
The maker says this blend works for “almost any controlled environment crop, in any growing system.” Growers use it for lettuce, herbs, tomatoes, peppers and cannabis in deep water culture, NFT, drip and coco. It’s popular because it’s cheap per gallon and simple: three ingredients, one ratio.
Jacks 321 Mixing Order (Step by Step)
Jack’s technical bulletin says the order of additions “is VERY important.” Here’s the process for a reservoir:
- Fill with water first. Start with most of your final volume of clean water.
- Add Part A (5-12-26). Weigh 3.6 g per gallon and stir until it’s completely dissolved.
- Add Epsom salt. Weigh 1.1 g per gallon and dissolve completely.
- Add Part B (calcium nitrate). Weigh 2.4 g per gallon and dissolve.
- Top up to final volume and stir again.
- Check EC and pH. Adjust pH last, after all nutrients are in.
Why the order matters: calcium from Part B can react with sulfate and phosphate if they meet at high concentration, forming insoluble solids. Fully diluting each part first prevents that. Our guide to nutrient mixing order explains the chemistry.
What Does Jacks 321 Deliver in PPM?
Using the label grades and standard conversions, here’s what full-strength Jack’s 321 adds to 1 gallon (3.785 liters) of pure water. These are our calculations, not numbers published by the maker.
| Nutrient | From | Approximate ppm |
|---|---|---|
| Nitrogen (N) | Part A (5% N) + Part B (15.5% N) | About 146 ppm |
| Phosphorus (P) | Part A (12% P2O5 × 0.436) | About 50 ppm |
| Potassium (K) | Part A (26% K2O × 0.830) | About 205 ppm |
| Magnesium (Mg) | Epsom salt only (about 9.9% Mg) | About 29 ppm, plus any Mg in Part A |
| Calcium (Ca) | Part B | Depends on the Ca % printed on your bag |
Want to check the math or build your own variations? Our hydroponic nutrient calculator guide shows the formula step by step.
How to Weigh Jacks 321 Accurately
- Use a 0.01 g scale for small batches. A kitchen scale that reads in whole grams is too rough for 1 gallon.
- Mix bigger batches when you can. Weighing for 5 or 10 gallons is far more accurate than for 1.
- Avoid teaspoon measures. Powder density varies, so spoons drift.
- Keep the bags sealed and dry. Calcium nitrate pulls moisture from the air and clumps.
- Label separate scoops for Part A and Part B so you never cross-contaminate.
Adjusting Strength by Crop and Stage
The 3.6/2.4/1.1 ratio is a full-strength recipe. Many growers feed seedlings and clones at partial strength and raise it as plants grow. The simplest way is to scale all three parts by the same percentage, so the ratio stays intact, and let an EC meter guide you.
- Seedlings and clones: start at reduced strength and watch for tip burn.
- Leafy greens: many lettuce growers run lighter than fruiting crops. See our leafy vs fruiting recipes.
- Fruiting crops: tomatoes and peppers generally use the full rate in active growth.
- Hard tap water: your water may already contain calcium and magnesium. Test it first; our hard water guide helps.
Jack’s also publishes a separate chart of grams needed to hit EC targets, and it offers phone and email support for custom questions.
Jacks 321 vs Other Popular Nutrients
| Program | Parts | Form | Best for |
|---|---|---|---|
| Jack’s 321 | Part A + Part B + Epsom | Dry powders | Low cost per gallon, any system |
| Masterblend 4-18-38 | Masterblend + calcium nitrate + Epsom | Dry powders | Similar 3-ingredient approach; see our Masterblend recipe |
| GH Flora Series | 3 liquids | Liquid | Easy measuring in ml; see our Flora Series chart |
Troubleshooting a Jacks 321 Reservoir
| Problem | Likely cause | Fix |
|---|---|---|
| Cloudy water or white sediment | Part B met Part A or Epsom before they dissolved | Remix fresh, following the A, Epsom, B order |
| EC much higher than expected | Hard source water or a weighing error | Test plain water EC; re-check your scale |
| Leaf tip burn | Solution too strong for the crop or stage | Scale all three parts down evenly |
| Yellow new growth | pH drift locking out iron | Recheck pH and adjust into range |
Common Jack’s 321 Mistakes
- Mixing the dry powders together in one container.
- Adding Part B before Part A or before the Epsom salt is dissolved.
- Making concentrated A+B stock in one jug. Keep stocks separate.
- Skipping pH adjustment after mixing. Most hydroponic crops do well around pH 5.5–6.5; see our pH up and down guide.
- Guessing weights with spoons instead of a scale.
FAQs: Jacks 321
What is the Jacks 321 ratio per gallon?
3.6 g Part A (5-12-26), 2.4 g Part B (calcium nitrate) and 1.1 g Epsom salt per US gallon.
What order do you mix Jacks 321?
Part A first, then Epsom salt, then Part B, dissolving each completely before adding the next.
Can I use Jacks 321 in soil?
It’s designed for controlled environment crops in soilless systems. In soil, feed less often and watch for salt buildup; see our guide to hydroponic nutrients in soil.
Is Jacks 321 good for lettuce?
Yes, many growers use it for lettuce, often at reduced strength compared with fruiting crops.
Related HydroponicEra Guides
- Best hydroponic nutrients
- Epsom salt in hydroponics
- EC and TDS in hydroponics
- Mixing hydroponic nutrient solution
Sources
- Jack’s Nutrients — How do I mix Jack’s 321?
- Jack’s Nutrients — Technical Bulletin: Jack’s 321 (PDF)
- Jack’s Nutrients — Jack’s 321: Grams to Hit EC Targets (PDF)
- Penn State Extension — Hydroponics Systems: Calculating Nutrient Solution Concentrations Using the Two Basic Equations
- Photo credits (Wikimedia Commons, edited with titles): Medals weighed in digital scale by Kpubkalanami (CC BY-SA 4.0); PH- und Leitfähigkeitsmessgeräte by Rotkaeppchen68 (CC0); Hydroponic lettuce @ Plaine des Cafres 03 by Rémih (CC BY-SA 4.0); Greenhouse filled with tomato plants and a bucket on the path by Shixart1985 (CC BY 2.0)
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