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Home gardening tool

How to Dose Fertilizer for Plants

A free calculator for dosing plants in pots and gardens: mix the right amount of water-soluble fertilizer per litre of water, based on university extension feeding rates.

Dose for

Mix fertilizer into water: grams per litre, total for your can, and per-pot dose.

Product form

Feeding presets are published for nitrogen. For phosphate or potash, enter your own target.

The three numbers printed on the bag, e.g. 20-20-20.

Target nitrogen concentration in the watering solution. Presets use the conservative end of published extension ranges.

Source: MSU Extension E2186 — 150–200 ppm N supplemental feed for container plants (preset uses the low end)

Size of your watering can or tank.

Approximate soil volume of a standard nursery pot.

How it works

Dosing by ppm, the way growers do it

Commercial growers don't guess spoonfuls — they mix fertilizer to a target nutrient concentration in parts per million (ppm). This calculator brings the same method to home gardeners.

  1. Pick your fertilizer grade

    Read the N-P-K numbers off the bag, e.g. 20-20-20, or enter a custom grade.

  2. Choose a feeding strength

    Houseplants and slow growers thrive on gentle rates (~100 ppm N); most container plants on regular rates (~150 ppm N); vigorous heavy feeders can take ~200 ppm N.

  3. Mix and water normally

    Dissolve the calculated grams in your watering can and water as usual, letting excess drain. Feed every watering at these rates, or half as often at double strength — never both.

Sources

Where the feeding rates come from

  • UMass Extension — "Fertilizing Bedding Plant Seedlings" (D. Cox, 2011): bedding plants 200–250 ppm N constant feed; slow-growing types 100–150 ppm N until established.
  • Michigan State University Extension (E2186): container-grown plants — slow-release fertilizer plus supplemental liquid feed at 150–200 ppm N.
  • Virginia Tech SPES-744 — "The Basics of Fertilizer Calculations for Greenhouse Crops": the g/L = ppm ÷ (%N × 10) formula and worked 200 ppm N example.

Presets use the low end of each published range, the conservative choice for home light levels. Always follow your product's label and your local extension service's advice.

Plant fertilizer dosing — FAQs

Most potted plants do well with a watering solution of 100–150 ppm nitrogen — roughly 0.5–1 g of 20-20-20 per litre of water. Heavy feeders like vigorous flowering annuals can take up to 200 ppm. Start gentle; you can always increase next time.
A level teaspoon of granular fertilizer is roughly 5–6 g, but density varies by product, so spoons are only a rough guide. For small pots and seedlings, a cheap gram scale is far more reliable — a 50% spoon error is easy and enough to burn roots.
At the gentle and regular rates above, yes — that is how commercial growers do it ("fertigate"). The alternative is feeding half as often at double strength. Don't do both: pick one schedule and watch the plants.
Brown, crispy leaf edges, wilting despite wet soil, and a white crust on the medium surface. If you see these, flush the pot with plain water until it runs freely from the drainage holes, and skip the next feeding.

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