Home·Fertilizer amount

135 fertilizer cases — 10 g/m² of N needs 21.74 g/m² of urea

Every pairing of 15 fertilizers and 9 plot sizes, worked out as the amount to spread. Nutrient and fertilizer are not the same quantity — the analysis on the bag sets the ratio between them, and only dividing the nutrient you need by that content gives the weight to spread. With a compound, matching one nutrient fixes the other two as well.

Divide by the content, never multiply

Urea is only 46% nitrogen, so putting 10 g of N down means spreading 10 ÷ 0.46 = 21.74 g of urea. Multiply instead and you get 4.6 g, a number that looks plausible enough to pass unnoticed. Whenever the content is below 100%, the fertilizer weight is always larger than the nutrient weight — that is the fastest check that you have the division the right way round.

With a compound, one match fixes the other two

Matching 10 g/m² of N with a 21-17-17 puts the fertilizer at 47.62 g/m², and those 47.62 g carry 8.1 g of P₂O₅ and 8.1 g of K₂O with them. You do not get to choose: once the fertilizer weight is fixed, the other two are dragged along in proportion to their content. That is why one compound bag rarely hits all three targets. The order of work is to match one, then top up or accept the surplus with straight fertilizers.

P₂O₅ and P are not the same number

The second and third numbers on the bag are oxide weights, not elements: P₂O₅ and K₂O. Soil reports and some references use the elements P and K instead, so the same field can show figures that differ by more than a factor of two. The factors are fixed molecular-weight ratios — P = P₂O₅ × 0.436 and K = K₂O × 0.830. Nitrogen is already stated as the element, so nothing changes there. Check which notation your target is written in first: with the wrong one, the division is right and the answer is still twice off.

Read the content off the bag, not from a table

The 15 fertilizers here are the ones whose stated analysis is widely settled — urea at 46% is the nitrogen fraction of CO(NH₂)₂ by weight, printed the same way on bags anywhere. Poultry manure, compost and other organics are different: nitrogen swings from about 1% to 4% with drying and maturity, and single superphosphate is labelled 16% in some markets and 20% in others. Those were deliberately left out, because an invented content looks plausible and nobody catches it. Read the three numbers off your bag and put them through the same division; the arithmetic does not change.

At a glance

Fertilizer to spread

UreaContent 46-0-0

1 m²
21.7 gN 46%
5 m²
108.7 gN 46%
10 m²
217.4 gN 46%
20 m²
434.8 gN 46%
50 m²
1.09 kgN 46%
100 m²
2.17 kgN 46%
200 m²
4.35 kgN 46%
500 m²
10.87 kgN 46%
1000 m²
21.74 kgN 46%

Ammonium sulfateContent 21-0-0

1 m²
47.6 gN 21%
5 m²
238.1 gN 21%
10 m²
476.2 gN 21%
20 m²
952.4 gN 21%
50 m²
2.38 kgN 21%
100 m²
4.76 kgN 21%
200 m²
9.52 kgN 21%
500 m²
23.81 kgN 21%
1000 m²
47.62 kgN 21%

Ammonium nitrateContent 34-0-0

1 m²
29.4 gN 34%
5 m²
147.1 gN 34%
10 m²
294.1 gN 34%
20 m²
588.2 gN 34%
50 m²
1.47 kgN 34%
100 m²
2.94 kgN 34%
200 m²
5.88 kgN 34%
500 m²
14.71 kgN 34%
1000 m²
29.41 kgN 34%

Calcium nitrateContent 15.5-0-0

1 m²
64.5 gN 15.5%
5 m²
322.6 gN 15.5%
10 m²
645.2 gN 15.5%
20 m²
1.29 kgN 15.5%
50 m²
3.23 kgN 15.5%
100 m²
6.45 kgN 15.5%
200 m²
12.9 kgN 15.5%
500 m²
32.26 kgN 15.5%
1000 m²
64.52 kgN 15.5%

Diammonium phosphate DAPContent 18-46-0

1 m²
21.7 gP₂O₅ 46%
5 m²
108.7 gP₂O₅ 46%
10 m²
217.4 gP₂O₅ 46%
20 m²
434.8 gP₂O₅ 46%
50 m²
1.09 kgP₂O₅ 46%
100 m²
2.17 kgP₂O₅ 46%
200 m²
4.35 kgP₂O₅ 46%
500 m²
10.87 kgP₂O₅ 46%
1000 m²
21.74 kgP₂O₅ 46%

Monoammonium phosphate MAPContent 11-52-0

1 m²
19.2 gP₂O₅ 52%
5 m²
96.2 gP₂O₅ 52%
10 m²
192.3 gP₂O₅ 52%
20 m²
384.6 gP₂O₅ 52%
50 m²
961.5 gP₂O₅ 52%
100 m²
1.92 kgP₂O₅ 52%
200 m²
3.85 kgP₂O₅ 52%
500 m²
9.62 kgP₂O₅ 52%
1000 m²
19.23 kgP₂O₅ 52%

Triple superphosphate TSPContent 0-46-0

1 m²
21.7 gP₂O₅ 46%
5 m²
108.7 gP₂O₅ 46%
10 m²
217.4 gP₂O₅ 46%
20 m²
434.8 gP₂O₅ 46%
50 m²
1.09 kgP₂O₅ 46%
100 m²
2.17 kgP₂O₅ 46%
200 m²
4.35 kgP₂O₅ 46%
500 m²
10.87 kgP₂O₅ 46%
1000 m²
21.74 kgP₂O₅ 46%

Potassium chloride MOPContent 0-0-60

1 m²
16.7 gK₂O 60%
5 m²
83.3 gK₂O 60%
10 m²
166.7 gK₂O 60%
20 m²
333.3 gK₂O 60%
50 m²
833.3 gK₂O 60%
100 m²
1.67 kgK₂O 60%
200 m²
3.33 kgK₂O 60%
500 m²
8.33 kgK₂O 60%
1000 m²
16.67 kgK₂O 60%

Potassium sulfate SOPContent 0-0-50

1 m²
20 gK₂O 50%
5 m²
100 gK₂O 50%
10 m²
200 gK₂O 50%
20 m²
400 gK₂O 50%
50 m²
1 kgK₂O 50%
100 m²
2 kgK₂O 50%
200 m²
4 kgK₂O 50%
500 m²
10 kgK₂O 50%
1000 m²
20 kgK₂O 50%

Potassium nitrateContent 13-0-46

1 m²
21.7 gK₂O 46%
5 m²
108.7 gK₂O 46%
10 m²
217.4 gK₂O 46%
20 m²
434.8 gK₂O 46%
50 m²
1.09 kgK₂O 46%
100 m²
2.17 kgK₂O 46%
200 m²
4.35 kgK₂O 46%
500 m²
10.87 kgK₂O 46%
1000 m²
21.74 kgK₂O 46%

Monopotassium phosphate MKPContent 0-52-34

1 m²
19.2 gP₂O₅ 52%
5 m²
96.2 gP₂O₅ 52%
10 m²
192.3 gP₂O₅ 52%
20 m²
384.6 gP₂O₅ 52%
50 m²
961.5 gP₂O₅ 52%
100 m²
1.92 kgP₂O₅ 52%
200 m²
3.85 kgP₂O₅ 52%
500 m²
9.62 kgP₂O₅ 52%
1000 m²
19.23 kgP₂O₅ 52%

Compound NPK 21-17-17Content 21-17-17

1 m²
47.6 gN 21%
5 m²
238.1 gN 21%
10 m²
476.2 gN 21%
20 m²
952.4 gN 21%
50 m²
2.38 kgN 21%
100 m²
4.76 kgN 21%
200 m²
9.52 kgN 21%
500 m²
23.81 kgN 21%
1000 m²
47.62 kgN 21%

Compound NPK 17-21-17Content 17-21-17

1 m²
47.6 gP₂O₅ 21%
5 m²
238.1 gP₂O₅ 21%
10 m²
476.2 gP₂O₅ 21%
20 m²
952.4 gP₂O₅ 21%
50 m²
2.38 kgP₂O₅ 21%
100 m²
4.76 kgP₂O₅ 21%
200 m²
9.52 kgP₂O₅ 21%
500 m²
23.81 kgP₂O₅ 21%
1000 m²
47.62 kgP₂O₅ 21%

Compound NPK 20-20-20Content 20-20-20

1 m²
50 gN 20%
5 m²
250 gN 20%
10 m²
500 gN 20%
20 m²
1 kgN 20%
50 m²
2.5 kgN 20%
100 m²
5 kgN 20%
200 m²
10 kgN 20%
500 m²
25 kgN 20%
1000 m²
50 kgN 20%

Compound NPK 10-10-10Content 10-10-10

1 m²
100 gN 10%
5 m²
500 gN 10%
10 m²
1 kgN 10%
20 m²
2 kgN 10%
50 m²
5 kgN 10%
100 m²
10 kgN 10%
200 m²
20 kgN 10%
500 m²
50 kgN 10%
1000 m²
100 kgN 10%

Worth knowing

  • Fertilizer to spread = nutrient needed ÷ (content ÷ 100). It is a division, never a multiplication.
  • Nutrient needed = area × target rate, so twice the area means twice the fertilizer.
  • A fertilizer with half the content takes twice as much for the same nutrient — inverse proportion.
  • With a compound, matching one nutrient drags the other two in at their content ratio.

Common questions

Q. How much urea puts down 10 g of nitrogen?

21.74 g. Urea is 46% nitrogen, so you divide: 10 ÷ 0.46. Spread 10 g of urea and only 4.6 g of nitrogen actually goes in — treating the nutrient weight and the fertilizer weight as one number is the commonest mistake here.

Q. Is the second number on the bag P or P₂O₅?

P₂O₅. The third is K₂O, not K. If your soil report is written in elements, convert with P = P₂O₅ × 0.436 and K = K₂O × 0.830 before comparing. Only nitrogen uses the same notation on both sides.

Q. What target rate should I use?

That is not a number this chart can give you. The crop and the soil test set it, and even the same crop shifts with what the previous one left behind. What this chart does is take that target and divide it by the content on the bag to get the weight to spread.