How Much Fertilizer to Use in a Vegetable Garden
By Paul Smith, pemdasgarden.com
Quick Answer: The N–P–K numbers on a fertilizer bag are the percentages of nitrogen, phosphorus, and potassium by weight. To figure out how much to apply, divide the nitrogen your garden needs by the first number on the bag: for a 10-10-10 fertilizer and a target of 1 pound of nitrogen per 1,000 square feet, you need 10 pounds of fertilizer. An NPK calculator does this math for any bag and any bed size.
Try our free Fertilizer Calculator — enter the N–P–K from the label, your garden size, and your nitrogen target, and it estimates exactly how much fertilizer to apply.
Reading the N–P–K Label
Every bag shows three numbers, for example 10-10-10 or 5-10-5. They stand for nitrogen (N), phosphate (P2O5), and potash (K2O), expressed as percentages of the total bag weight. So a 10-10-10 bag is 10 percent nitrogen: 100 pounds of it carries 10 pounds of actual nitrogen.
How to Use the Tool
- Read your bag’s N–P–K. Copy the three numbers from the fertilizer label.
- Enter your garden size. Measure the bed or use the total growing area in square feet.
- Set your nitrogen target. A common baseline for vegetable gardens is about 1 pound of nitrogen per 1,000 square feet per application — adjust for heavy feeders like corn or light feeders like beans.
- Get your amount. The calculator divides your target by the bag’s nitrogen percentage and scales it to your area.
- Apply and water in. Spread evenly around plants (not against stems), then water so nutrients reach the root zone.
Practical Tips
- More is not better. Over-fertilizing burns roots and pushes leafy growth at the expense of fruit. If in doubt, apply the lower end of the range and side-dress later.
- Feed the crop, not the calendar. Heavy feeders (corn, tomatoes, squash) want more nitrogen; legumes fix their own and need little.
- A soil test pays for itself. Your extension office can test your soil cheaply, so you fertilize only what is actually missing instead of guessing.
Frequently Asked Questions
What does 10-10-10 mean on a fertilizer bag?
It means 10 percent nitrogen, 10 percent phosphate, and 10 percent potash by weight. A 50-pound bag of 10-10-10 therefore contains 5 pounds of nitrogen.
How much fertilizer does one tomato plant need?
Work per area rather than per plant: for a row or bed of tomatoes, about 1 pound of nitrogen per 1,000 square feet per feeding is a common baseline. Enter your bed size in the calculator to get the exact bag weight for your space.
Can I burn my plants with fertilizer?
Yes — especially with concentrated synthetic fertilizer applied too heavily or too close to stems. Measure carefully, spread evenly, and water it in.
Should I fertilize at planting time?
A light starter dose at planting helps transplants establish. Most vegetables then benefit from a side-dressing when they start flowering or fruiting.
Stop guessing — try the free Fertilizer Calculator and apply exactly the right amount for your garden.
Stock Up on Seeds
Healthy feeding starts with strong varieties: browse our vegetable seed collection or tomato seeds.
