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Fertilizer Calculator

Most people do not need to know N-P-K by heart. Choose what you are growing. The calculator says which of the three bag numbers should be highest — or if they should be about equal. Amounts, when you already have a bag, are conservative starting scoops. Too much fertilizer can burn plants. Compost-rich raised beds often need less than a bag chart implies.

Fertilizer bags print three numbers, in order: nitrogen (N), phosphorus (P), potassium (K). You do not need to memorize that. Pick the crop; the result tells you which number should stand out — or if they should be about equal.

A mixed bed cannot follow four different bags. An all-purpose vegetable fertilizer, plus compost, is the practical default.

Skip: Lawn fertilizer with a huge first number, such as 20-5-5 or 29-0-4.

I already have a bag — kitchen spoons

Optional. Read the three numbers on the bag. Rates here are intentionally low so you are less likely to burn plants. You can always feed again. If the bag lists a lower garden rate, use the bag. Keep granules off stems and water them in.

How hard should this feeding be?

Bag to look for

Mixed vegetables (whole bed)

Balanced5-5-5, 4-4-4, 10-10-10

You are growing
Mixed vegetables (whole bed)
Look for
Balanced (N, P, and K similar)5-5-5, 4-4-4, 10-10-10
How to read the bag
Three numbers close together

Matching product type

Placeholder only until a real bag is researched. When affiliate links go live, this slot is the one that will match the crop you picked.

Fertilizer

All-purpose vegetable fertilizer (research pending)

Placeholder

Placeholder for a balanced N-P-K vegetable fertilizer. Add a specific product after independent research.

Best for
A mixed raised bed, or when you do not want a crop-specific bag.
Price range
To be researched

Pros

  • Fits mixed beds where one bag has to cover several crops
  • Easy to match: three numbers close together

Cons

  • Not the best match if you only grow leafy greens or only fruiting crops

Retailer not set

Retailer link disabled until a real product URL is added. Swap externalUrl in the product data to enable it.

How this calculation works

Bags print three numbers in order: nitrogen (N, leaves), phosphorus (P, early roots and flowers), potassium (K, fruit and overall toughness). Optional amount math uses Light = 0.15, Typical = 0.375, Stronger = 0.6 lb actual N per 1,000 sq ft — below many bag charts so you are less likely to burn plants, but not the lowest possible scoop. Product (lb) = nitrogen needed ÷ (N% ÷ 100). Kitchen spoons assume 1 oz of typical granular fertilizer ≈ 2 Tbsp. Pellet size varies. The product label still wins, especially if it lists a lower rate.

Example: tomatoes in a mixed raised bed

Pick tomatoes. The tool points you to a higher-potassium bag — third number biggest, or at least matching nitrogen — such as a 4-6-8 or 5-5-10 style vegetable food. A 20-5-5 lawn fertilizer is the wrong aisle. If you already own a bag, open “I already have a bag,” enter the three numbers, and choose Light / Typical / Stronger. The amount comes back in Tbsp or tsp, not ounces. Follow the bag if it lists its own garden rate.

Frequently asked questions

What do the three numbers on the bag mean?

They are always in the same order: nitrogen, phosphorus, potassium. You do not enter them to get a recommendation. Pick your crop, then look for which number the result says should stand out.

Why teaspoons instead of ounces?

Most people measure a side-dress with a kitchen spoon, not a scale. One ounce of typical granular fertilizer is about 2 tablespoons. That is a scooping estimate — pellets and prills do not all weigh the same. If the bag prints cups or ounces, use the bag.

Do I need to know target nitrogen?

No. Open “I already have a bag,” type the three numbers off the label, and pick Light, Typical, or Stronger from how the plants look. If the bag prints its own rate for vegetable gardens, use that instead.

Is a 10-10-10 always the safe choice?

It is a reasonable mixed-bed default. Leafy crops often want a higher first number. Fruiting crops often want the third number to keep up. Compost already in the bed can cover a light feeding.

Why is high phosphorus not the default for tomatoes?

Bloom-booster bags are easy to overuse. Many compost-filled raised beds already have enough phosphorus. This tool only points to a high-P bag for new transplants, or when you have a reason such as a soil test.

Will you recommend a specific brand?

Not until a real product is researched and a retailer URL is added. The product card is a placeholder for later affiliate links. It is not a review.

Can this tell me if I will burn plants?

No calculator can promise that. These rates are deliberately low, and the default is Light. Burn still depends on the product, wet vs dry granules, and whether fertilizer sits on stems. Keep it off the plant, water it in, and stop if leaf edges scorch. The label and a soil test override this tool.

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