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CalcChef

Fertilizer N-P-K Mixer

Blend up to three fertilizers and filler to a target N-P-K: 10-10-10 from urea, triple superphosphate and potash is 21.7%, 21.7%, 16.7% + filler. How it works

Use the Fertilizer N-P-K Mixer

Target N-P-K

Percent of the finished mix by weight, as on a fertilizer bag (P as P₂O₅, K as K₂O).

Unit

Your chosen rate; use the rate on the product label or from your local extension service.

Fertilizers

Enter the N-P-K analysis from the bag. Leave a row empty to skip it; in a used row enter 0 for a nutrient the product does not have.

Quick-add fertilizers

Resulting N-P-K
10-10-10

Target matched exactly

Mixing proportions

Urea(46-0-0)
0.217 kg(21.7%)
Triple superphosphate(0-46-0)
0.217 kg(21.7%)
Muriate of potash(0-0-60)
0.167 kg(16.7%)
Filler(filler, adds no nutrients: sand or similar)
0.399 kg(39.9%)

Nutrients in the batch (kg)

Nitrogen

0.1

Phosphorus (P₂O₅)

0.1

Potassium (K₂O)

0.1

At 1 lb N per 1,000 sq ft, spread 4.54 kg of this mix per 1,000 sq ft. This batch covers about 220 sq ft (20 m²). Check the rate against the product label and your local guidance.

About the Fertilizer N-P-K Mixer

A 10-10-10 blend from urea (46-0-0), triple superphosphate (0-46-0) and muriate of potash (0-0-60) is 21.7% urea, 21.7% superphosphate, 16.7% potash and 39.9% filler (no nutrients) by weight. The fertilizer N-P-K mixer solves the three nutrient equations exactly for up to three products, fills the rest of the batch with filler, and names any nutrient it cannot match. Its Blend mode answers the reverse question: 2 kg of urea, 1 kg of triple superphosphate, 1 kg of potash and 1 kg of filler make 5 kg of 18.4-9.2-12.

Blends from urea, triple superphosphate and muriate of potash

Target N-P-KUrea 46-0-0Triple superphosphate 0-46-0Muriate of potash 0-0-60FillerMix per 1,000 sq ft at 1 lb N
10-10-1021.7%21.7%16.7%39.9%10 lb
5-10-1010.9%21.7%16.7%50.7%20 lb
6-12-1213%26.1%20%40.9%16.67 lb
10-5-521.7%10.9%8.3%59.1%10 lb
12-12-1226.1%26.1%20%27.8%8.33 lb
15-5-1032.6%10.9%16.7%39.9%6.67 lb
16-4-834.8%8.7%13.3%43.2%6.25 lb
20-10-1043.5%21.7%16.7%18.1%5 lb
24-8-852.2%17.4%13.3%17.1%4.17 lb

Percent of the finished mix by weight, with P as P₂O₅ and K as K₂O, as on a bag. The last column is the mix to spread for 1 lb of nitrogen per 1,000 sq ft, the default rate; take your rate from the product label or your local extension service.

Frequently asked questions

How do I mix fertilizer to get 10-10-10?
From urea, triple superphosphate and muriate of potash, a 50 lb batch is 10.87 lb of urea, 10.87 lb of superphosphate, 8.333 lb of potash and 19.928 lb of filler. The three products alone would be stronger than 10-10-10, which is why the filler is there.
Why can’t I make 20-20-20 from urea, superphosphate and potash?
Matching all three nutrients would need 120.3% of the mix to be fertilizer, more than 100%. The mixer then uses the whole batch, with no filler, to come as close as it can: it matches N and K and puts the spare room into triple superphosphate (23.2% of the mix), so P comes out at 10.7, 9.3 points below the target. Lower targets work, for example 15-15-15, which needs 90.2% fertilizer.
What do the three numbers on a fertilizer bag mean?
Percent by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). A 50 lb batch of 10-10-10 holds 5 lb of nitrogen, and the calculator lists the weight of each nutrient in the batch. The targets and the product analyses use the same convention.
How much fertilizer mix do I spread per 1,000 sq ft?
Pounds of mix = pounds of nitrogen you want ÷ (N% ÷ 100). At the default 1 lb N per 1,000 sq ft, a 10-10-10 mix is 10 lb per 1,000 sq ft (4.54 kg), so a 1 kg batch covers about 220 sq ft. The rate is your choice: the calculator does not recommend one.
What N-P-K do I get if I mix fertilizers I already have?
Switch to Blend and type the weight of each product, plus any filler. The analysis of the blend is each nutrient’s weight divided by the total weight: 1 kg each of 46-0-0, 0-46-0 and 0-0-60 is 0.46 kg N, 0.46 kg P₂O₅ and 0.6 kg K₂O in 3 kg, so 15.3-15.3-20. Add 1 kg of urea and 1 kg of filler (2, 1, 1 and 1 kg) and it is 5 kg of 18.4-9.2-12.
What if my products cannot reach the target?
The mixer never uses a negative amount. With only urea and triple superphosphate for 10-10-10 it matches N and P and reports K 10 points below, since no product supplies K. A nutrient that comes out over its target cannot be lowered by adding products: lower that target or choose products with less of it.

Blending from straight fertilizers

The numbers on a bag are percent N, percent phosphate (P₂O₅) and percent potash (K₂O), not elemental phosphorus and potassium. Straight products are much stronger than most targets, so a 10-10-10 from urea, triple superphosphate (0-46-0) and muriate of potash is only 60.1% fertilizer; the other 39.9% is filler that bulks the mix out so it spreads evenly.

Check which superphosphate you have. Single superphosphate is about 0-20-0, and entered as 0-20-0 it takes 50% of a 10-10-10 mix instead of 21.7%, leaving 11.6% filler.

The nitrogen rate is always in lb N per 1,000 sq ft, even with a batch in kg; the coverage line then gives the mix in kg per 1,000 sq ft and the area in sq ft and m².

How the mix is calculated

x₁ × a₁ + x₂ × a₂ + x₃ × a₃ = target, once each for N, P and K

  • x: the share of the mix that is each product, 0 or more, together at most 1

  • a: that product’s percent of the nutrient

  • filler = 1 − (x₁ + x₂ + x₃), any no-nutrient material

The three equations are solved exactly. Example, 10-10-10 from 46-0-0, 0-46-0 and 0-0-60: x₁ = 10 ÷ 46 = 21.7%, x₂ = 21.7%, x₃ = 10 ÷ 60 = 16.7%, filler 39.9%.

When no mix reaches every target, the tool solves for as many nutrients as it can and reports the others in percentage points above or below the target.

Blend mode works the other way: type the weight of each product and any filler, and N% = Σ(weight × N%) ÷ total weight, the same for P and K. 1 kg each of 46-0-0, 0-46-0 and 0-0-60 is 3 kg at 15.3-15.3-20; 2, 1 and 1 kg plus 1 kg of filler is 5 kg at 18.4-9.2-12.

Spread rate: mix per 1,000 sq ft = lb of N wanted ÷ (N% ÷ 100), 10 lb of 10-10-10 for 1 lb of N.

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