Enrichment · Cultural Studies · Gardening

Three numbers on a bag,
turned into pounds
for your beds.

Type the grade, the rate you want and the area. The calculator tells you how much product to spread, how many bags to buy and what the other two nutrients add, so the garden gets what it needs and no more.

Work out the amountWhat you get

The fertilizer calculator

The worked example is made up: a 10-10-10 fertilizer on 600 sq ft of vegetable beds. Nothing you type leaves this browser.

Loading the calculator. If nothing appears, turn on JavaScript.

What you get

Enrichment · Cultural StudiesFree

What you finish with
Pounds of product per 1,000 sq ft and in all, bags and cost, the N:P:K ratio and the pounds of each nutrient applied, including elemental P and K. It prints as a summary and downloads as a spreadsheet.
Who it is for
Home and school gardeners, families with a lawn, and students in agriculture and environmental science.
Format
Browser calculator for a phone, tablet or computer, by touch or keyboard. Prints a summary; downloads a spreadsheet (.csv); export and import a file.
Access
Free. No account, no sign-in and nothing to install. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The Cultural Studies space in Caveat Enrichment.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Type the three numbers on the bag, its weight and price.
  2. Choose the nutrient you are aiming for and the rate from your soil test or crop guide.
  3. Type the area.
  4. Read how much to spread and what else comes with it.
  5. Print the summary to take to the shed.

Work out the amount

How the amounts are worked out

Percent by weight, then area.

The worked example: 10-10-10 on vegetable bedsTarget1 lb Nper 1,000 sq ftBag is 10% N÷ 0.1010 lb product per 1,000Area 600 sq ft6.0 lb10 × 600 ÷ 1,000Also applied0.6 lb P₂O₅and 0.6 lb K₂O in totalThe worked example: 10-10-10 on vegetable bedsTarget1 lb Nper 1,000 sq ftBag is 10% N÷ 0.1010 lb product per 1,000Area 600 sq ft6.0 lb10 × 600 ÷ 1,000Also applied0.6 lb P₂O₅and 0.6 lb K₂O in total
The grade is percent by weight. Divide the nutrient you want by the percent in the bag to get pounds of product; the other two nutrients come along in proportion.

Reading the grade

The grade, such as 10-10-10 or 24-8-16, is the percent by weight of nitrogen, phosphate (P₂O₅) and potash (K₂O). A 40 lb bag of 10-10-10 holds 4 lb of each; the rest is the other parts of the compounds and filler that spreads evenly. The ratio, 1:1:1 or 3:1:2, says how the three compare. Labels report phosphate and potash, so elemental phosphorus is 43.64% of the phosphate figure and potassium 83.01% of the potash.

Choosing a rate

A soil test is the best guide: it reports what the soil lacks and how much to add, often in pounds per 1,000 sq ft. Without one, extension services commonly suggest no more than about 1 lb of nitrogen per 1,000 sq ft in a single application for lawns and gardens. Fertilizer that plants do not take up can wash into streams and lakes, so measure the area and the product.

Sources

  1. Association of American Plant Food Control Officials (AAPFCO). Official Publication: uniform fertilizer labeling terms, grade as total N, available phosphate and soluble potash (aapfco.org).
  2. University of Minnesota Extension. Fertilizing lawns and Fertilizing vegetable gardens (extension.umn.edu).
  3. Penn State Extension. Calculating fertilizer rates for small areas (extension.psu.edu).
  4. IUPAC Commission on Isotopic Abundances and Atomic Weights. Standard atomic weights of P, K and O (ciaaw.org).

The worked example is made up. Read the product label and follow its directions and any local rules. Your figures stay in this browser. Support: team@caveatacademic.com.