Dose by nitrogen concentration, not by scoop. One ounce of a fully soluble material in 100 gallons of water delivers 75 ppm. So ppm N equals ounces per 100 gallons times the percent nitrogen times 0.75. Run that on a 20-20-20 and 13.5 ounces per 100 gallons lands at 200 ppm N. Everything below follows from that one line of arithmetic, which Virginia Tech and UMass Extension both publish.
What does a teaspoon per gallon actually deliver?
Nobody can tell you, because a teaspoon measures volume and fertilizer sells by weight. Powder density varies with the blend, the humidity in the bag and how hard you pack the spoon. The proof sits on two labels for the same analysis. Jack's Classic All Purpose 20-20-20 directs outdoor growers to 1 tablespoon per gallon. Peters Professional 20-20-20 directs 1 teaspoon per gallon. Same three numbers on the bag, a threefold difference in the dose.
Two rules follow. Use the label in front of you rather than a spoon rate you remember from another product. And once you care about precision, buy a $15 kitchen scale and work in grams per gallon.
Grams per gallon for a target concentration
These figures come from the extension formula above, converted to grams for a single gallon. Check them against your own label, since some products carry inert filler that shifts the math.
| Analysis | 100 ppm N | 150 ppm N | 200 ppm N | Use |
| 20-20-20 (20% N) | 1.9 g/gal | 2.8 g/gal | 3.8 g/gal | General feed, seedlings at the low end. |
| 24-8-16 (24% N) | 1.6 g/gal | 2.4 g/gal | 3.2 g/gal | Common consumer all-purpose powder. |
| 30-10-10 (30% N) | 1.3 g/gal | 1.9 g/gal | 2.5 g/gal | Early vegetative growth only. |
| 15-30-15 (15% N) | 2.5 g/gal | 3.8 g/gal | 5.0 g/gal | Transition feed where a test shows low phosphorus. |
| Calcium nitrate 15.5-0-0 | 2.4 g/gal | 3.7 g/gal | 4.9 g/gal | Soilless mixes with no calcium of their own. |
| 5-10-30 (5% N) | 7.6 g/gal | 11.3 g/gal | 15.1 g/gal | Fruit fill. Target potassium here, not nitrogen. |
That last row shows why a nitrogen target misleads you on a finisher. At 5 percent N, hitting 150 ppm N means 11 grams of salt in a gallon and a potassium load far past anything the plant wants at once. Feed a high-potassium product at its label rate and let the nitrogen fall where it falls.
How much fertilizer do pepper seedlings need?
Less than most people give them. UGA Extension tells commercial transplant growers who feed with every watering to start at 30 to 50 ppm N, using the higher end for pepper, tomato and cole crops. UGA adds the warning that matters: above 50 ppm N applied daily, the transplants are probably getting too much, and excessive growth lowers transplant quality. Weekly feeding supports a stronger dose, in the 100 to 150 ppm range, because the tray dries between feeds.
The old advice to feed seedlings at quarter strength lands in roughly the same place when the label rate sits near 400 ppm N. It fails when the label is already weak. Convert once and you stop guessing. A soft, stretched, dark green seedling is usually an overfed seedling, not an underlit one, though check the light too.
Through the season
Feed nitrogen while the plant builds its frame, then back off before flowering. Nitrogen applied during flowering delays fruit set and buys you a large green plant, which NMSU says plainly in its garden guide for chile. Move to a potassium-forward feed once pods size up, and keep calcium and magnesium in the program rather than reaching for them after damage appears, a pattern the fruiting problems guide traces.
Skip the high-phosphorus bloom booster. NDSU Extension states that high phosphorus rates do not increase flower number, and a plant with adequate phosphorus flowers as well as it can. Our fertilizer chart lists the stage-by-stage analyses, and the fertilizer guide covers product choice.
Organic liquids dose by area, not by plant
Read a fish emulsion label closely and the unit changes. Alaska Fish Fertilizer 5-1-1 directs 2 tablespoons per gallon for vegetables, bedding plants, bulbs and annuals, and that gallon covers 25 square feet of soil. Root vegetables take 1 tablespoon. Outdoor container plants take 1 tablespoon per gallon of water per cubic foot of soil. The label sets the frequency at every three weeks during the growing season.
So the dilution is only half the instruction. Pouring the correct 2-tablespoon mix over a single plant that occupies 2 square feet delivers more than ten times the intended rate. Measure the bed, then mix enough solution to cover it.
Dry fertilizer and side-dressing
Granular products skip dilution entirely and get measured by area. Texas A&M works 2 to 3 pounds of 10-10-10 into each 100 square feet before planting, or about 2 level tablespoons mixed into the soil where a single plant will go. After the first fruit starts to enlarge, the same guide places 2 tablespoons around each plant, 6 inches out from the stem, then waters it in. Watering after a side-dress moves the salt off the crown and into the root zone where it belongs.
Salt is the failure mode
Peppers rank as moderately sensitive to salinity. FAO's crop tolerance tables put the threshold at 1.5 dS/m of soil saturation extract, with yield falling 14 percent for every additional dS/m above it. Tomato tolerates 2.5 dS/m before it loses anything. That narrow margin explains why container peppers crash after a month of heavy feeding while the same program works in open ground.
Three habits keep you clear of it. Measure the EC of your solution with a cheap meter and compare it against the chart on the bag, which is how growers catch an injector or a mis-scooped batch before the plants show it. Flush containers with plain water every three to four weeks. And water to a small amount of runoff each time rather than stopping at damp, since the runoff carries accumulated salt out of the pot. The nutrient lockout guide covers what happens when you skip that step, and soil testing covers the in-ground version.
Mixing notes
Dissolve powders fully in warm water before they meet the reservoir. Never mix calcium nitrate into a concentrated tank with a phosphate or a sulfate feed, because calcium phosphate and calcium sulfate drop out of solution and plug emitters. Halve the rate for foliar work and spray at dawn or dusk. Chelated forms hold iron and manganese where the water runs alkaline, which the micronutrient guide covers. A C. chinense such as a Habanero runs a long season and punishes a late nitrogen push more than an C. annuum does, so time the switch by species, not by calendar.
Sources & Further Reading
- Virginia Cooperative Extension. The Basics of Fertilizer Calculations for Greenhouse Crops, 430-100, for the 75 constant and injector math.
- UMass Extension Greenhouse & Floriculture. Fertilizer calculations for greenhouse crops.
- UGA Cooperative Extension. Commercial Production of Vegetable Transplants, B1144, for transplant feeding rates.
- Pennington. Alaska Fish Fertilizer 5-1-1 product label, application rates by plant group and area.
- Texas A&M AgriLife Extension. Easy Gardening: Peppers, E-515.
- McGinnis, E. The myth of high phosphorus fertilizers for more flowers. NDSU Extension, 2023.
- FAO. Crop salt tolerance data, Annex 1, pepper threshold and slope.
- Priest, C.T., and D.J. Austin. The Chile Pepper Almanac. Harambe Publishing, 2026. Amazon