Nitrogen Fertilizer Calculator
How many pounds of a nitrogen fertilizer to apply to hit your target nitrogen rate for an area.
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Phosphorus and Potassium Fertilizer Calculator
Pounds and bags of phosphate or potash product to meet a P2O5 or K2O recommendation, with elemental nutrient shown.
Phosphorus and Potassium Fertilizer Calculator
Chlorova · updated for 2026
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Soil test recommendations are usually written as P2O5 or K2O rather than elemental nutrient, and this calculator assumes that oxide convention throughout, so read your report carefully before entering a number. Conversion factors are the standard chemical ones, phosphorus being 0.44 times P2O5 and potassium 0.83 times K2O, while product analyses shown are typical and your bag label governs. Nothing here addresses placement, pH availability or soil test levels, and phosphorus applied to already-rich soils is both wasted money and a water-quality liability, so let the soil test class, not the biggest number, drive the decision.
Soil test reports speak in P2O5 and K2O, the oxide forms extension services have used for a century, while the fertilizer aisle sells products by analysis percentage, and the two vocabularies rarely meet in a gardener's head. This phosphorus and potassium calculator closes that gap, turning a recommended pounds-of-oxide figure into the product weight to spread, the elemental nutrient actually delivered, the rate per thousand square feet and the bags to buy. Pick superphosphate, triple super, DAP or a potash source, and the label decoding happens instantly.
Converting a Soil Test in Steps
- Choose the phosphate or potash product you intend to buy.
- Enter the pounds of P2O5 or K2O the recommendation asks for.
- Set the area being fed so rates read per thousand square feet.
- Type the price per pound shown on the bag or the quote.
- Enter the bag size so the rounding matches the real shelf.
- Read product pounds first, then the elemental nutrient inside.
- Note the oxide-versus-element line before repeating the dose.
- Confirm your report states oxides before trusting any of the outputs.
How Oxide Recommendations Become Product
A guaranteed analysis of 46 percent means 46 pounds of oxide in every hundred pounds of product, so the calculator stores the reciprocal weight, about 2.17 pounds of triple super for each pound of P2O5 wanted, and multiplies that straight into your oxide target. Weak products like twenty-percent superphosphate demand five pounds to carry the same single pound of oxide, which is filler rather than fertility.
The element line applies the standard conversion factors, multiplying P2O5 by 0.44 to report actual phosphorus and K2O by 0.83 for actual potassium, since plant tissue and lab reports sometimes speak elemental rather than oxide. Rates divide the target by area and multiply by one thousand to match extension units, bags round the product weight up to whole sacks, and cost prices the exact product pounds at your entered rate.
Worked Example: Triple Super on a New Bed
- The soil test recommends 2 pounds of P2O5 per 1,000.
- You pick triple superphosphate at 46 percent oxide.
- Each pound of oxide takes about 2.17 lb of product.
- Product weight is 2 times 2.174, so 4.35 pounds.
- Actual phosphorus is 2 times 0.44, or 0.88 lb.
- The area is 1,000 sq ft, so rates match the test exactly.
- One 25 lb sack covers it, leaving plenty for other beds.
- At three dollars a pound this feeding costs 13 dollars.
- That 0.88 lb figure is the phosphorus the crop can actually take up.
- A high soil-test rating would cancel or slash this dose before spreading.
Product Analyses and Conversion Weights
Typical guaranteed analyses for common phosphate and potash products, with the pounds of each needed to carry one pound of oxide and the elemental factor applied. Match your bag to the nearest row, then substitute the label figures where they differ.
| Product | Analysis | lb product per lb oxide |
|---|---|---|
| Superphosphate | 0-20-0 | 5.0 |
| Triple super | 0-46-0 | 2.17 |
| DAP | 18-46-0 | 2.17 |
| Muriate of potash | 0-0-60 | 1.67 |
| Sulfate of potash | 0-0-50 | 2.0 |
| Langbeinite | 0-0-22 | 4.55 |
Key Terms
- P2O5
- Phosphorus pentoxide, the oxide form in which soil tests state phosphorus.
- K2O
- Potassium oxide, the reporting form for potassium on every potash label.
- Guaranteed analysis
- The legally required N-P-P2O5-K2O percentages printed on a fertilizer bag.
- Elemental factor
- The 0.44 and 0.83 multipliers converting oxides to actual phosphorus and potassium.
- Filler
- The inert carrier balance of a low-analysis fertilizer, paid for by weight yet supplying no nutrient at all.
Reading a Soil Test Like an Agronomist
- Oxide conventions are historical rather than botanical, since no one spreads pentoxide, but every extension recommendation keeps the habit, so the first job is confirming which form your report uses.
- High-analysis products win on handling cost, because 46 percent triple super moves half the sack weight of 20 percent super for the same phosphorus, and the labor saved usually outweighs the price gap.
- Phosphorus placement matters more than rate, since banding beside young roots can cut the required amount substantially while the same pounds broadcast on a high-P soil achieve essentially nothing.
- Soil test level outranks the arithmetic, as a build-up recommendation and a maintenance recommendation differ several fold, and applying a building dose to already-rich ground wastes money and runs toward runoff liability.
- DAP hides nitrogen inside a phosphate purchase, so subtract the eighteen percent N against your nitrogen plan or the crop can get a heavier feed than you meant to give it.
Common Mistakes & Fixes
- Entering elemental pounds into an oxide-based calculator and quintupling the intended dose.
- Assuming P and K products are interchangeable between the two nutrient columns on the report.
- Broadcasting phosphate on a high-testing soil instead of banding it near young roots where it works.
- Forgetting the eighteen percent nitrogen that DAP contributes to an otherwise planned feeding.
- Chloride-sensitive crops under repeated muriate of potash use where sulfate of potash was intended.
- Reading the oxide number as if it were product weight and buying double the bags needed.
When to Use This (and When Not To)
Use it when a soil test or crop guide has already named a P2O5 or K2O figure and you need shopping weight rather more than theory. For the nitrogen column of the same report, the nitrogen-calculator runs that inversion, and the fertilizer-npk tool balances all three numbers together; choose organic phosphate and potash sources with the organic-fertilizer calculator instead.
Phosphorus and Potassium Fertilizer Calculator: Frequently Asked Questions
How do you convert P2O5 to actual phosphorus?
Multiply the oxide pounds by 0.44, since phosphorus makes up about forty-four percent of phosphorus pentoxide by weight. Two pounds of P2O5 therefore carries roughly 0.88 pounds of actual phosphorus, the number a tissue-based discussion would quote.
How much muriate of potash equals one pound of K2O?
About 1.67 pounds of product, because 0-0-60 material is 60 percent potassium oxide, so the oxide weighs six-tenths of the bag. Potassium itself is 0.83 times that oxide, meaning one pound of K2O supplies around 0.83 pounds of actual potassium.
Is triple superphosphate better than regular super?
Per pound of phosphorus it is cheaper to buy and handle, at 46 percent versus 20 percent, so a sack carries more than twice the nutrient. Regular super still earns its place where calcium and sulfur are also wanted, since those secondary nutrients come bundled in the extra filler.
Why do soil tests report oxides instead of elements?
Because fertilizer analysis has always been stated that way, and reporting in the same units keeps recommendations and product labels directly comparable with no conversion in the middle. The convention is old but consistent, which is exactly why entering the wrong form doubles or triples a dose.
Should I fertilize phosphorus every year?
Only if the soil test says the level is low or medium, since P builds up and holds in soil for years once applied past need. A high or very high rating generally means no phosphate at all, and skipping it saves money while keeping nutrient runoff risk down.
What is the difference between MOP and SOP potash?
Muriate is 0-0-60 and cheaper, carrying chloride along with the potassium, while sulfate is 0-0-50, costs more per point and suits chloride-sensitive fruit, tobacco and potato ground. For most home vegetable beds the cheaper muriate is fine unless a soil test flags salt buildup.
Sources & Further Reading
- Interpreting Soil Tests and Nutrient Recommendations — University of Minnesota Extension. Basis for oxide conventions, conversion factors and rate units on this page.
- Phosphorus and Potassium Sources for Field Crops — North Carolina State University Extension. Source of the product analyses and placement guidance cited here.
- Chloride and Sulfate Potash Sources in Crop Production — Kansas State University Extension. Underpins the MOP versus SOP comparison in the FAQ section.
Chlorova Gardening Team
Builds, checks and documents every tool we publish
Chlorova editorial review
Checked against extension-service publications
Formula verified against published references · Updated