Difference between elemental P and P2O5 on a fertiliser label?

P = P2O5 x 0.436. Single super labelled as 20% P2O5 contains only 8.7% elemental P. Confirm which basis your label and soil test use — modern AU uses elemental P.

How to calculate a blended application?

Satisfy P first from DAP or MAP. Note how much N that also delivers. Subtract from total N target. Calculate remaining N from urea. Repeat for K from KCl.

When should I apply nitrogen to wheat?

In the WA wheatbelt and SA: at seeding or split between seeding and tillering. In higher-rainfall areas: late application at GS31-39 can be efficient. Follow zone-specific agronomist recommendations.

Fertiliser Rate Calculator — Free Agriculture Calculator

🎯

Soil test results hit the inbox this morning and the agronomist has pencilled in 40 kg N/ha for wheat. The product on hand is urea. You need kg/ha and total tonnes before the truck booking deadline.

Fertiliser Rate Calculator

Cropping

Paddock area (ha)

Application rate (kg/ha)

Imperial: enter lb/ac

Nutrient type

Nutrient content (%)

e.g. Urea=46%N · DAP=18%N · MAP=11%N

Product price ($/tonne) — optional

Common fertilisers (AU):
Urea: 46% N · DAP: 18% N, 20% P · MAP: 11% N, 21.9% P · Muriate of Potash: 50% K · Gypsum: 16% S
Single Super: 8.8% P, 11% S · Ammonium Sulphate: 21% N, 24% S

💡 Always base fertiliser rates on a recent soil test — blanket rates waste money and risk environmental damage.

Reference: GRDC Nutrient Management · International Plant Nutrition Institute

ℹ️ Results are estimates for planning purposes. Verify with current standards and a qualified professional.

1 What this calculator does

Calculates fertiliser product application rate from target nutrient rate and product nutrient content. Converts between nutrient element (N, P, K) and product rates. Calculates total product for the paddock. Handles single and blended applications.

2 Formula & professional reasoning

Product rate (kg/ha) = Target nutrient rate (kg/ha) / (Nutrient% / 100)
Total product (kg) = Product rate x Area (ha)
For blends: calculate each product separately, subtract its nutrient contribution from remaining target

Fertilisers are sold by product weight, not nutrient weight. Urea is 46% N — applying 87 kg/ha delivers only 40 kg/ha of actual N. For blended applications calculate the first product (usually P from DAP), note how much N it also contributes, then calculate remaining N from urea separately.

3 Worked examples

⚠️ Illustrative example only — not clinical or professional instruction.

Basic

Nitrogen top-dressing with urea

Given: Target N: 40 kg/ha | Urea: 46% N | Area: 500 ha

Working: Rate: 40 / 0.46 = 86.96 kg/ha | Total: 86.96 x 500

Answer: 87 kg/ha urea — 43.5 tonnes total

💡 Order 45 tonnes for spreader calibration and paddock irregularities.

Standard

Phosphorus at seeding — single super

Given: Target P: 10 kg/ha | Single super: 8.7% P (elemental) | Area: 320 ha

Working: Rate: 10 / 0.087 = 114.9 kg/ha | Total: 114.9 x 320

Answer: 115 kg/ha single super — 36.8 tonnes

💡 Place in-furrow or close-band. Broadcast P is less efficient for cereals.

Advanced

Blended NPK — DAP + Urea + KCl

Given: Deficiency: 30 kg N, 8 kg P, 15 kg K per ha | DAP 18-20-0, Urea 46-0-0, KCl 0-0-50

Working:

P from DAP: 8/0.20=40 kg/ha, also delivers 40x0.18=7.2 kg N | Remaining N: (30-7.2)/0.46=49.6 kg/ha urea | K: 15/0.50=30 kg/ha KCl

Answer: DAP 40 + Urea 50 + KCl 30 kg/ha

💡 Calculate P first, note the N it provides, then calculate remaining N from urea. Never add extra DAP just for N.

4 Sanity check

Common nutrient contents (% of product)

Urea: 46% N | MAP: 11% N, 22% P | DAP: 18% N, 20% P | Single super: 8.7% P | KCl MOP: 50% K

Check current batch analysis — contents vary slightly.

P2O5 vs elemental P

Elemental P = P2O5 x 0.436 | P2O5 = P x 2.29

Modern AU soil tests and GRDC use elemental P. Old labels may show P2O5 — confirm which basis you're using.

Typical nutrient rates

Wheat seeding P: 10-20 kg/ha | N side-dress cereals: 30-80 kg/ha | Canola N total: 50-120 kg/ha

Always follow soil test recommendations.

Spreader calibration

Calibrate spread width and rate before starting each paddock

A 5% calibration error on 100 ha = 5 ha under or over-applied.

5 Common errors

Error	Cause	Consequence	Fix
Ordering nutrient rate as product rate	Treating kg/ha N as kg/ha urea	Applied only 46% of intended nutrient — severe deficiency and yield loss	Product rate = Nutrient rate / Nutrient%. Never use a nutrient rate as a product rate.
P2O5 vs elemental P confusion	Not checking which basis the label or soil test uses	Applied 2.3x too much or 43% too little phosphorus	Confirm whether label shows elemental P% or P2O5%. Convert: P = P2O5 x 0.436.
Not recalibrating spreader when changing product	Using settings from previous product	Wrong rate across entire paddock	Different products have different density and flow. Always recalibrate.
Not subtracting N from DAP when calculating urea	Treating each product independently	N over-application — yield penalty and environmental risk	Calculate total N from all products. Subtract from target before calculating urea.