Why use this calculator
Fertiliser is one of the largest variable costs in a cropping budget and the easiest one to get wrong by a factor of two. Soil tests and agronomists speak in kilograms of nutrient per hectare, the shed holds product, and the two are not the same thing: a hundred kilograms of urea is only forty six kilograms of nitrogen.
Australian labels quote phosphorus and potassium as the element, while North American labels quote the oxides, so a figure copied out of an overseas article can look almost twice as strong as it is. Getting this conversion right protects yield from an under-application you will not see until grain fill, protects cash from an over-application that will not be used, and protects waterways and the neighbours from nutrient that moves off the paddock.
It also shows the by-product nutrients, so phosphorus applied as MAP is properly credited with the nitrogen and sulfur that come with it.
How to use it
- Choose the product you have quoted or already have in the shed, or choose custom and type the analysis off the bulk bag if it is a blend.
- Choose the nutrient your target is set for, remembering that Australian labels quote phosphorus and potassium as the element rather than as an oxide.
- Enter the target rate in kilograms of that nutrient per hectare, which comes from your soil test report, a nutrient budget or your agronomist.
- If you chose a custom product, enter the percentage of that nutrient from the delivery docket.
- Enter the area to be spread from your paddock records rather than the title area.
- Enter the delivered price per tonne from your current quote so the cost per hectare includes freight.
- Enter the working capacity of the spreader bin or air cart to see how many loads the job takes and how many hectares you cover between fills.
What each result means
- Product rate
- Kilograms of the actual product to spread per hectare. This is the number you set on the spreader controller.
- Fill for the final load
- How much product to put in the bin for the last run, once every earlier load has gone out full, so you finish the paddock without carting product back to the shed.
- Total product required
- Tonnes to order for the whole paddock, before you round up to whole bulk bags or a tipper load.
- Cost per hectare
- What this application adds to the variable costs of the paddock, the figure a gross margin actually needs.
- Total product cost
- The whole invoice for this paddock at your quoted delivered price, useful for cash flow timing.
- Cost per kilogram of nutrient
- The fair comparison between two products. A cheaper tonne is not cheaper if it carries less nutrient.
- Spreader loads
- Whole bin loads to finish the paddock, which drives how much time the job takes and how the cart is staged.
- Hectares per load
- How far you get between fills. It sets where the cart should sit and how long a run can be.
- Other nutrients supplied
- Nutrients that come along with the product. Credit them in the budget so you do not pay for them twice.
The formula
Product rate in
kg/ha = target nutrient rate in kg/ha x 100, divided by the percentage of that nutrient in the product
Total product in tonnes = product rate x area in hectares, divided by 1,000
Cost per hectare = product rate in kg/ha divided by 1,000, multiplied by the delivered price per tonne
Cost per kilogram of nutrient = price per tonne divided by the kilograms of nutrient in a tonne, which is the analysis percentage multiplied by ten
Hectares per load = bin capacity in tonnes x 1,000, divided by the product rate
How this number is used in practice
A soil test is the starting point, not the answer. A phosphorus figure such as a Colwell P of 25 means something quite different on a grey vertosol in the northern grains region than it does on a sandy earth in the Western Australian wheatbelt, which is why the phosphorus buffering index sits beside it on the report.
Nitrogen decisions are different again: most of the value comes from a deep nitrogen test taken before sowing, the stored soil water in the profile, and a yield target that is honest about the season rather than the best year on record. Rules of thumb such as a fixed rate of urea every year quietly overspend in dry seasons and underspend in wet ones. A test costs a fraction of one hectare of urea.
Timing and placement often matter as much as the rate. Urea spread on a dry surface without incoming rain can lose a meaningful share of its nitrogen to volatilisation, particularly on alkaline or calcareous soils and where there is heavy stubble, so the same kilograms give a different result depending on the forecast.
Splitting nitrogen between sowing and a top up in tillering lets you spend against the season you actually have rather than the one you hoped for in March. Phosphorus generally goes down with the seed because it moves so little in the soil, while potassium and sulfur can be spread.
The number this calculator gives you is a rate, and a rate is only as good as the decisions about when and how it goes on.
Check the spreader before you trust the controller. Spread width and pattern uniformity vary with granule size, density and humidity, and a bout width set for urea will not necessarily be right for a blend or for gypsum. A tray test on a still morning, or a pattern check by a spreader testing service, is a cheap way to find striping that would otherwise show up as green and yellow rows in the crop later in the season.
Worked examples
80 kg of nitrogen per hectare from urea at 46 per cent, over 120 ha, urea at $780 a tonne
173.9 kg/ha of urea, 20.87 t for the paddock and $135.65 a hectare, or $1.70 per kilogram of nitrogen
12 kg of phosphorus per hectare from MAP at 21.9 per cent, over 200 ha, MAP at $1,250 a tonne
54.8 kg/ha of MAP, 10.96 t for the paddock, and 5.5 kg/ha of nitrogen supplied free with it
40 kg of potassium per hectare from muriate of potash at 50 per cent, over 60 ha at $950 a tonne
80.0 kg/ha, 4.80 t for the paddock and $76.00 a hectare, or $1.90 per kilogram of potassium
When people use it
- Converting a soil test recommendation in kilograms of nutrient per hectare into a spread rate
- Comparing urea, sulfate of ammonia and a liquid on cost per kilogram of nitrogen delivered
- Ordering the right tonnage and planning spreader loads and cart placement before the job
- Crediting the nitrogen and sulfur that come free with a starter phosphorus application
- Budgeting the fertiliser line of a gross margin for each paddock rather than for the whole farm
Tips
- Compare products on cost per kilogram of nutrient delivered to the paddock, not on the price per tonne on the quote.
- Australian labels are elemental. Multiply a P2O5 figure by 0.437 and a K2O figure by 0.830 before using an overseas recommendation.
- Credit the nitrogen and sulfur in MAP, DAP and sulfate of ammonia in your budget, or you will apply them twice.
- Spread urea before rain where you can. Volatilisation losses on a dry alkaline surface can undo careful rate arithmetic.
- Do a tray test to check spread pattern when you change products. Granule density changes the effective bout width.
- Split nitrogen between sowing and tillering so the second decision can be made with the season in front of you.
What it assumes, and where it stops being accurate
- Assumes the analysis on the label is what is in the bag. Blends can segregate in transit and in the bin, so the analysis at the spout may vary from the docket.
- Assumes nutrient targets are quoted elemental as Australian labels are. Figures taken from North American sources are usually as P2O5 and K2O and must be converted first.
- Not an agronomic recommendation. Rates should come from a current soil test, a nutrient budget and local advice, not from a default figure in a calculator.
- Ignores nutrient losses after application, including volatilisation of urea on a dry surface, leaching on sands and denitrification in waterlogged soil.
- Assumes an even spread across the whole area. Uneven spread patterns, worn vanes and a bout width set for a different product all move the effective rate.
- Liquid products are treated by weight. To convert a rate for a liquid such as UAN into litres per hectare you also need the product density.
Terms used on this page
- Elemental analysis
- Nutrient content expressed as the element itself, the Australian labelling convention for phosphorus and potassium.
- Colwell P
- A common Australian soil test for plant available phosphorus, interpreted alongside the phosphorus buffering index.
- Volatilisation
- Loss of nitrogen to the air as ammonia gas after urea is applied to a surface without incoming rain.
- Nutrient budget
- A paddock level account of nutrients removed in produce against nutrients applied, used to keep fertility stable.
- Bout width
- The effective spread width of a spreader pass, which changes with granule size and density.
Questions people ask
- Why does Australia quote phosphorus differently from North America?
- Australian labels quote elemental phosphorus and potassium, while North American labels quote them as the oxides P2O5 and K2O. To convert, multiply a P2O5 figure by 0.437 and a K2O figure by 0.830. Skipping that step roughly doubles a phosphorus rate on paper.
- What nitrogen rate should I apply?
- That depends on a deep soil nitrogen test, the stored water in the profile, your yield target and the previous crop, especially if it was a legume. There is no single correct figure, and a fixed annual rate overspends in dry seasons. Use a current test and local advice.
- Can I use this calculator for a blend?
- Yes. Choose the custom product option and enter the percentage of the nutrient you are targeting from the delivery docket. Remember that a blend supplies other nutrients at the same time, so check the full analysis and credit them in your nutrient budget.
- How do I convert a liquid rate into litres per hectare?
- Divide the product rate in kilograms per hectare by the density of the liquid in kilograms per litre. UAN is usually near 1.32 kilograms per litre, so 100 kilograms per hectare is roughly 76 litres. Always confirm density on the product data sheet.
- Does the calculator account for nitrogen losses?
- No. It converts a target nutrient rate into a product rate. Volatilisation from surface applied urea, leaching on sandy soils and denitrification in waterlogged conditions all reduce what the crop gets, and managing those is a timing, placement and product decision.
- Should I fertilise to replace what the crop removed?
- Replacement is a reasonable starting point for phosphorus, potassium and sulfur on soils testing at an adequate level, and it keeps fertility from drifting down over the rotation. It is a poor guide for nitrogen, which is driven by the coming season rather than the last one.
Use this with
Fertiliser Application Calculator answers one part of the question. These pick up where it stops: Soil pH and Lime Rate Calculator, Seed Rate Calculator, Cost Per Hectare Calculator, Spray Dilution Calculator and Crop Yield Estimator.
Where these figures come from
Figures and rules on this page follow Australian law and ATO guidance.
- Grains Research and Development Corporation. Nutrient management, nitrogen decision making and soil testing research
- NSW Department of Primary Industries. Soil testing interpretation and fertiliser guidance for cropping and pastures
- Department of Primary Industries and Regional Development, Western Australia. Soil nutrient management and fertiliser decision support for the wheatbelt
Fertiliser Application Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.