NPK Fertilizer Calculator: Crop Nutrient Balance and Plan

NPK fertiliser balance: product rate per hectare and cost

The tool uses the balance method: crop requirement − soil supply − organic input = amount to cover with mineral fertiliser, then product rate = balance ÷ fertiliser content. For wheat at 8 t/ha (240 kg N) with 40 kg of residual soil nitrogen, 200 kg N remain, i.e. 597 kg of 33.5% ammonium nitrate per hectare.

How it is calculated

• Default nitrogen: total requirement = b × yield, with the b values of the Île-de-France regional reference (France, 2024): soft wheat 8 t × 30 = 240 kg N/ha (3 kg N per 100 kg), grain maize 10 t × 22 = 220, oilseed rape 3.5 t × 70 = 245, sunflower 3 t × 45 = 135, and a flat 220 for sugar beet (90 t). Potato 45 t: 180, an indicative value of the tool (the same reference gives 60 to 275 kg N/ha depending on planting and haulm-killing dates). Cut grassland 8 t DM: 160, an indicative value of the tool.
• Default P₂O₅ and K₂O: nutrients removed by the harvest at the preset yield, from the COMIFER contents (kg/ha): wheat 52/40 for grain only (straw removed from the field adds 1.7 kg P₂O₅ and 12.3 kg K₂O per tonne), maize 60/55, oilseed rape 44/30, potato 43/176, sugar beet 45/162, sunflower 36/32, grassland 46/212 ("temporary grassland, hay" line). The soil P and K supply is 0 by default: in the COMIFER method, the soil test, the crop's demand and recent applications decide whether to skip, apply a maintenance rate or build up relative to removal. All these fields can be edited.
• Organic input per tonne or m³: cattle manure, 2 kg available N (an indicative value of the tool, for total nitrogen of 5.8 kg/t in strawy manure and 5.3 in well-rotted manure); pig slurry, 3.5 kg total N per m³ × 0.65 = 2.3 kg available (0.65 is the fertiliser-equivalence coefficient over the whole cycle for mixed pig slurry spread in late summer before oilseed rape; over the balance period alone it falls to 0.05); poultry manure, 15 kg available (an indicative value of the tool, for 24 to 40 kg total N per tonne of dry droppings and 25 with litter). Manure P₂O₅ and K₂O (cattle 2.5/6, slurry 2/2.5, poultry 18/14) are indicative values of the tool.
• Balance = max(0, requirement − soil − organic input) for each nutrient.
• Rate = balance ÷ content. The chosen fertiliser is dosed on its most limiting nutrient; the tool then tops up each remaining nutrient with a straight fertiliser: 33.5% ammonium nitrate, 46% urea or N30 solution for nitrogen (your choice, ammonium nitrate by default), 45% triple superphosphate for P₂O₅ and 60% muriate of potash for K₂O. The programme table gives, for each product, the rate per hectare, the tonnage for the field and the cost; the total budget is the sum of the rows.
• Other fertiliser: choose "Other fertiliser", then enter its N, P₂O₅ and K₂O contents (0 to 100%) and its price per tonne. It goes through the same calculation, limiting nutrient first and then top-ups; without a price, its row is left out of the budget.
• Export: the CSV button downloads the programme table (product, kg/ha, tonnes, cost per hectare and for the field), with the area and the N, P₂O₅ and K₂O balances.
• Suggested split: 1 dressing up to 70 kg N, 2 up to 150 kg, 3 above, at the crop's key growth stages.
• Fertiliser prices: indicative French prices in euros, set by the tool, to be replaced by your own, including those of the top-up fertilisers.

Worked example

20 ha of wheat, 40 kg residual N, soil P and K supply at 0, no manure: balances 200 N, 52 P₂O₅, 40 K₂O. Ammonium nitrate 33.5%: 200 ÷ 0.335 = 597 kg/ha, 11.9 t for the field, €251/ha at €420/t, in 3 dressings. The programme adds 52 ÷ 0.45 = 116 kg/ha of triple superphosphate (€62/ha at €540/t) and 40 ÷ 0.60 = 67 kg/ha of muriate of potash (€35/ha at €520/t): €348/ha, or €6,956 for the field. With DAP 18-46-0 as the first fertiliser, phosphorus is limiting: 52 ÷ 0.46 = 113 kg/ha, which also supply 20.3 kg of nitrogen; 179.7 kg N remain, i.e. 536 kg/ha of ammonium nitrate, then 67 kg/ha of muriate of potash, for €337/ha and €6,735 in total. With 30 t/ha of cattle manure, the nitrogen balance drops to 140 kg and the rate to 418 kg/ha.

Limits

A compound fertiliser (DAP, NPK 15-15-15) is dosed on its most limiting nutrient, so that none exceeds its balance; the tool tops up the rest with one straight fertiliser per nutrient, without looking for the cheapest combination. The full forecast balance reads X = (Pf + Rf) − (Pi + Ri + Mh + Mhp + Mr + MrCi + Xa + Nirr); the tool only keeps residual soil nitrogen Ri (40 kg N/ha by default, an indicative value of the tool) and a flat organic input. It ignores nitrogen already taken up when the balance opens, mineralisation of humus and residues, soil nitrogen left at closing and nitrogen in irrigation water: the references set by French regional orders prevail. The slurry pit calculator estimates what a herd produces; this one starts from what the crop needs.

Sources and dates: in French nitrate vulnerable zones, nitrogen from spread livestock manure is capped at 170 kg per hectare of farmland (order of 19 December 2011, national nitrates action programme, version in force since 10/02/2023, checked 14/10/2026); b values, flat rates, balance equation and total manure nitrogen: DRIAAF Île-de-France order no. IDF-2024-12-02-00014, GREN regional reference, December 2024; P₂O₅ and K₂O removal: COMIFER, P, K and Mg contents of harvested plant parts, 2007; slurry coefficient: COMIFER, Calcul de la fertilisation azotée, 2013; prices: the Insee index of fertiliser and soil improver prices stood at 180.6 in August 2026 (2020 = 100), up 15% over one year (Insee, Informations rapides no. 244, 30/09/2026) (checked 28/10/2026).

Frequently Asked Questions

How do I calculate how much fertiliser to spread per hectare?

Divide the balance to supply by the fertiliser's content: for 200 kg of nitrogen with 33.5% ammonium nitrate, 200 ÷ 0.335 = 597 kg of product per hectare. The balance is the crop requirement minus residual soil nitrogen and organic inputs.

How much nitrogen does an 8 t/ha wheat crop need?

The tool suggests 240 kg N/ha, i.e. 3 kg of nitrogen per 100 kg of grain (Île-de-France regional reference, France, 2024), before deducting residual soil nitrogen and manure. With 40 kg of residual nitrogen, 200 kg N remain, split into 3 dressings according to the tool.

How much nitrogen does one tonne of cattle manure supply?

The Île-de-France reference (France, 2024) gives 5.8 kg of total nitrogen per tonne of strawy cattle manure and 5.3 kg for well-rotted manure. Only part of it reaches the crop: the tool counts 2 kg of available nitrogen per tonne, an indicative value to adjust. Spreading 30 t/ha therefore cuts the mineral nitrogen requirement by 60 kg/ha.

Why split nitrogen applications?

Applying nitrogen in several passes limits losses and follows the plant's uptake. The tool suggests 1 dressing up to 70 kg N, 2 up to 150 kg and 3 above, for example at tillering, at the 1 cm ear stage and at flag leaf for wheat.

Is this tool really free?

Yes, all tools on Kristools are 100% free and accessible without registration.

Is my data protected?

Yes. All processing is performed directly and securely inside your browser. No personal data is sent to our servers.