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Growing Media

Top dressing

Top dressing means adding compost, worm castings or a dry fertilizer to the surface of a pot or bed. Bags list a rate, but usually not for your pot size. Below are the four kinds of rate you'll see on a bag and how to work out the amount for each.

Updated November 2026 2 min read

Four kinds of rate

Labels list a rate in one of four ways. Check which one yours uses first, because each one uses a different pot measurement.

Per area 2 cups per 10 sq ft Use the area of the top of the pot or bed.
Per soil 1 tbsp per gallon Use how much soil the pot holds. A 5-gallon pot gets five times the rate.
Per plant or pot 1/4 cup per plant Multiply by the number of plants or pots. If the label lists pot sizes, like 1 tbsp per 6-inch pot, use the line for your size.
A layer 1/2 inch deep Multiply the top area by the depth to get the amount to spread.

Worked examples

These all use a 5-gallon pot that's 12 inches across the top.

Four ways a label rate can be written, worked for one 5-gallon pot
Four ways a label rate can be written, worked for one 5-gallon pot

Per area. A 12-inch circle is 0.785 sq ft (3.14 × 0.5 ft × 0.5 ft). At 1 cup per 10 sq ft, that pot gets 1 × 0.785 / 10 = 0.0785 cup, a little under 4 teaspoons.

Per soil. At 1 tablespoon per gallon of soil, the pot gets 5 tablespoons. The top area isn't used.

A layer. Half an inch is 0.042 ft. Area times depth is 0.785 × 0.042 = 0.033 cubic feet, close to 4 cups.

Rate types differ

For the same pot, the per-area rate came to 4 teaspoons and the per-soil rate came to 5 tablespoons, about four times more. Products are labeled differently, so always use the rate on your bag.

Common mistakes

  • Treating a pot like a bed. A per-area rate written for a garden bed works for a pot too. Use the area of the top of the pot, not its volume.
  • Swapping cups for pounds. A cup of compost and a cup of granular fertilizer weigh very different amounts. If the label gives pounds and you only have a scoop, weigh one scoop of that product first.
  • Rounding up. Too much dry fertilizer can burn roots, especially in small pots. Round down if you're not sure.
  • Skipping the water. Water after top dressing so it settles into the soil.
Pesticides follow the label exactly

Pesticide labels are legally binding. Use the rate and method on the label, and only on the plants and sites it lists.

Top Dress Calculator Enter the bag rate and your pots or bed, and get the amount per pot and for the whole job.
Open the calculator

Summary

  • Check which kind of rate the bag gives before doing any math.
  • Per-area rates use the top of the pot. Per-soil rates use how much soil it holds.
  • To convert between cups and pounds, weigh a cup of that product.
  • Round dry fertilizer down, not up, and water it in.
  • Pesticide labels are the law. Follow them exactly.

Sources

The examples are unit arithmetic. The label rule is from the EPA.

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