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Fertilizer calculator from a soil test

How much fertilizer do I need for the rate my soil test calls for?

Showing for United States

Your numbers

Weight unit
lb
lb
lb
Your soil test states P and K as
The recommendation is per
Area measured in
Work out the result from the numbers you entered.

Result

See the formula and a worked example.

Fertilizer for your area

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Nutrient that limits the amount
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Rate from
—
Fertilizer per the recommendation's area
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Nitrogen it gives, per the recommendation's area
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Phosphate it gives (P2O5, or P if you chose elemental), per the recommendation's area
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Potash it gives (K2O, or K if you chose elemental), per the recommendation's area
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This calculator never recommends a rate. Enter the recommendation from your own soil test; a lab or your extension office can test your soil. Follow the product label, and any rules on nutrient use where you farm.

The amount is the most of this product that keeps every nutrient you entered at or under its recommendation. A nutrient that falls short needs a second product.

Fertilizer bags state phosphate as P2O5 and potash as K2O, not as elemental phosphorus and potassium. If your soil test gives elemental P or K, choose that option and the shares are converted with Iowa State University Extension's factors, 0.44 and 0.83.

A nutrient the product contains that you entered no recommendation for is still applied. If your soil test calls for none of it, choose a product without it.

Enter your numbers to see the result.

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Platform help and more options

Use this calculator in an AI chat

You can ask Claude, ChatGPT or another AI app to work this out for you, and this calculator opens right inside the chat. You connect TallyTill to your AI app once. It is free, and there is no account and no sign-in.

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  2. Press + Add, then Add custom connector.
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  5. In a new chat, press + at the bottom left, choose Connectors and switch TallyTill calculators on.

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3. Ask your question

Say "use TallyTill", so the AI uses the calculator instead of doing the sum itself. For example:

"Use TallyTill: How much fertilizer do I need for the rate my soil test calls for?"

What you will see

  • The calculator opens in the chat with the numbers you gave already filled in. Change any number and the answer updates.
  • If a number is missing, the box is highlighted, or the AI asks you for it in plain words.
  • The answer links back to this page, so you can check the working.
  • The first time, your AI app asks if it may use TallyTill. Choose Allow.

The calculators take numbers only. They never ask for your name, your email or a password.

How this is worked out

The product that meets a nutrient is the soil test's recommendation divided by that nutrient's share of the product (the number on the bag divided by 100). With more than one recommendation, the amount is the smallest of those, so no nutrient you entered goes over its recommendation; that nutrient limits the amount, and the others fall short by the difference. The amount for your area is that times how many of the recommendation's areas your area holds. The bag states phosphate as P2O5 and potash as K2O; if your soil test states elemental P or K, the shares are multiplied by 0.44 and 0.83, as Iowa State University Extension does.

A worked example

Example numbers only; use your own soil test. Iowa State University Extension's example lawn of 5,625 square feet, with a recommendation of 1 lb of nitrogen per 1,000 square feet and a 32-0-4 fertilizer: 1 ÷ 0.32 = 3.125 lb per 1,000 square feet, so 17.58 lb in all. With 10-10-10 and a recommendation of 100 lb of nitrogen, 50 of phosphate and 50 of potash an acre, phosphate limits the amount to 500 lb an acre, which leaves nitrogen 50 lb short.

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