Skip to main content
Calcimator

Fertilizer Blend Calculator

Custom blend proportions for target N-P-K analysis.

About this calculator

This calculator works out how much of up to three fertilizer sources to combine so the blend hits your target pounds-per-acre of N, P₂O₅, and K₂O, solving the sources in a specific order rather than simultaneously. It first sizes Source B (typically a phosphate carrier like DAP, 18-46-0) purely to meet the P₂O₅ target, since Source B is treated as the only phosphate source in the model. Whatever nitrogen that dose of Source B happens to contribute is then credited against the nitrogen target, and Source A (typically straight urea, 46-0-0) is sized to make up only the *remaining* nitrogen need — this ordering matters because it means Source A's rate depends entirely on Source B's N content, not the other way around.

Any potash contributed incidentally by Sources A and B is likewise credited before Source C (typically potash/KCl, 0-0-60) is sized to fill the remaining K₂O gap. The calculator reports each source's rate per acre and as a percentage of the total blend by weight, scales everything up to total-field quantities for purchasing, and recomputes the actual delivered N-P-K from the rounded rates so you can verify the blend really meets your targets. The core assumption worth watching is that this is a two-nutrient-then-one sequential solve, not a true three-way optimization — if your Source A or B analysis changes, or if Source B supplies more nitrogen than your target calls for, the model still floors "remaining N" at zero rather than warning you that you're now over-applying nitrogen just to hit the phosphate rate, so always cross-check the delivered totals against your soil test recommendation before ordering product.

Progress0%

Step 1 of 4

How to Use This Calculator
  1. Enter Field Size (acres), Target N (lbs/acre), Target P₂O₅ (lbs/acre), and Target K₂O (lbs/acre) from your soil test recommendation.
  2. Enter the nutrient analysis (N%, P₂O₅%, K₂O%) for up to three fertilizer sources (e.g., urea 46-0-0, DAP 18-46-0, potash 0-0-60).
  3. Review Source A, B, and C (lbs/acre) — these are the rates to apply of each product.
  4. Check Total Product (lbs/acre) and Total Field Quantities (lbs) for each source to size purchases.
  5. Verify the blend meets your targets by comparing Actual Delivered Nutrients to your soil test recommendations.

How the result changes with Target N (lbs/acre)

Target N (lbs/acre)Source A (lbs/acre)Source B (lbs/acre)Source C (lbs/acre)
6079.4130.4133.3
90144.6130.4133.3
180340.3130.4133.3
300601.1130.4133.3

What each input means

Field size (acres)
Total field area to fertilize.
Target N (lbs/acre)
Desired nitrogen application rate per acre.
Target P₂O₅ (lbs/acre)
Desired phosphate application rate per acre.
Target K₂O (lbs/acre)
Desired potash application rate per acre.
Source A: N%
Nitrogen analysis of Source A (e.g., urea = 46%).
Source A: P₂O₅%
Phosphate analysis of Source A.
Source A: K₂O%
Potash analysis of Source A.
Source B: N%
Nitrogen analysis of Source B (e.g., DAP = 18%).
Source B: P₂O₅%
Phosphate analysis of Source B (e.g., DAP = 46%).
Source B: K₂O%
Potash analysis of Source B.
Source C: K₂O%
Potash analysis of Source C (e.g., potash/KCl = 60%).

What each result means

Source A (lbs/acre)
Pounds of Source A per acre.
Source B (lbs/acre)
Pounds of Source B per acre.
Source C (lbs/acre)
Pounds of Source C per acre.
Total product (lbs/acre)
Combined product weight per acre.
Blend % Source A
Source A fraction of the blend by weight.
Blend % Source B
Source B fraction of the blend by weight.
Blend % Source C
Source C fraction of the blend by weight.
Total Source A (lbs)
Total Source A for the whole field.
Total Source B (lbs)
Total Source B for the whole field.
Total Source C (lbs)
Total Source C for the whole field.
Total all sources (lbs)
Grand total product for the whole field.
Delivered N (lbs/acre)
Actual nitrogen delivered per acre.
Delivered P₂O₅ (lbs/acre)
Actual phosphate delivered per acre.
Delivered K₂O (lbs/acre)
Actual potash delivered per acre.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Field size (acres) = 10, Target N (lbs/acre) = 120, Target P₂O₅ (lbs/acre) = 60, Target K₂O (lbs/acre) = 80 = 11 input(s) provided
  2. Calculate Source A
    209.8 = 209.8
  3. Calculate Source B
    130.4 = 130.4
  4. Calculate Source C
    133.3 = 133.3
  5. Calculate Total product
    Total product = sourceALbsPerAcre + sourceBLbsPerAcre + sourceCLbsPerAcre
    473.6 = 473.6
  6. Calculate Blend % Source A
    44.3 = 44.3%

Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

Why does the calculator solve for Source B before Source A instead of both at once?

It sizes Source B first purely to hit the P₂O₅ target, since Source B is the model's only phosphate source. Whatever nitrogen that dose happens to contribute is then subtracted from the N target, and Source A is sized to cover only what's left. This sequential order means Source A's rate is entirely dependent on Source B's nitrogen content — change Source B's analysis and Source A's rate changes too, even though your N target didn't.

What happens if Source B alone already supplies more nitrogen than my target?

The 'remaining N' figure is floored at zero (Math.max(0, targetN - nFromB)), so Source A's rate drops to zero rather than going negative. The calculator doesn't flag this as an over-application warning, though — it just silently stops recommending Source A, so you should check the delivered N figure against your target whenever Source B's N% is high relative to your N target.

How is Source C's rate determined?

After sizing Source A and B, the calculator credits any potash they contributed incidentally (kFromA plus kFromB) against your K₂O target, and sizes Source C only to fill whatever K₂O gap remains. If Sources A and B already meet or exceed the K₂O target on their own, Source C's rate comes out to zero.

Why does the calculator show both a rate per acre and a blend percentage for each source?

The per-acre rate (lbs/acre) tells you how much of each product to apply in the field. The blend percentage is that source's share of the total combined product weight, calculated as its rate divided by the sum of all three sources' rates — useful if you're physically pre-mixing the three products into one blend before spreading rather than applying them separately.

Does the Source A: P₂O₅% input affect the results?

No — it's collected as part of Source A's nutrient analysis, but the calculator treats Source B as the model's only phosphate source, so this field is not currently read by any calculation. Source A's N% and K₂O% figures are used normally.

Related Calculators

The questions that sit next to this one — chosen by subject, including calculators filed under a different category.

Garden Planning

Fertilizer Ratio Calculator

Calculate fertilizer needs based on NPK ratio, garden area, and application rate. Find out how many bags you need and the cost.

Fertilizer

Fertilizer Cost Comparison Calculator

Per-unit nutrient cost across fertilizer sources.

Fertilizer

Manure Nutrient Value Calculator

N-P-K value from manure type and application rate.

Fertilizer

Liquid Fertilizer Dilution Calculator

Gallons per acre from product analysis and target rate.

Landscaping

Lawn Fertilizer Calculator

Calculate the amount of fertilizer needed based on lawn size, NPK ratio, and desired nitrogen application rate.

More in Agriculture & Farming.