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Fertilizer Per Acre (N-P-K) Calculator

Turn a target N-P-K rate per acre into pounds of blended fertilizer product, and see which nutrient is the limiting one.

Target N

Target P₂O₅

Target K₂O

Fertilizer grade — N

Fertilizer grade — P₂O₅

Fertilizer grade — K₂O

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How it works

Product for a nutrient = target lbs ÷ (grade % ÷ 100) · Product to apply = largest of the three (the limiting nutrient)

A fertilizer grade like 19-19-19 tells you the percent by weight of nitrogen, available phosphate (P₂O₅) and soluble potash (K₂O). To supply a nutrient you divide the target pounds per acre by that nutrient’s fraction of the bag. With a single blended product the nutrient needing the most product sets the rate, so the others come along in excess — which is why matching the blend to your soil test, or using separate straight fertilizers, avoids over-applying phosphorus or potassium.

Worked example

Targeting 120 lbs N/acre with a 19-19-19 blend needs 120 ÷ 0.19 ≈ 632 lbs of product. That same 632 lbs also delivers about 120 lbs of P₂O₅ and K₂O — well over a 60 lb target — so nitrogen is the limiting nutrient and phosphate and potash are over-applied.

Sources: UGA Extension — Fertilizer calculations

Frequently asked questions

What do the three fertilizer numbers mean?

They are the guaranteed analysis: percent nitrogen (N), percent available phosphate (P₂O₅) and percent soluble potash (K₂O) by weight. A 50 lb bag of 10-20-10 contains 5 lbs N, 10 lbs P₂O₅ and 5 lbs K₂O; the rest is carrier and filler.

Why is one nutrient over-applied?

A single blend has a fixed ratio, but your soil test rarely needs that exact ratio. The tool sizes product to meet the most-demanding nutrient, so the others are supplied in surplus. To apply each nutrient precisely, blend or band separate straight sources like urea, DAP and potash.

Is P₂O₅ the same as elemental phosphorus?

No. Fertilizer labels use the oxide forms P₂O₅ and K₂O by convention. To convert to elemental P multiply P₂O₅ by 0.437, and for elemental K multiply K₂O by 0.830. Soil-test recommendations in the US are normally already given as P₂O₅ and K₂O.

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This calculator is for educational purposes. Double-check important results before acting on them.