How to calculate fertiliser application rate

From nutrient dose to product weight

Agronomists plan in nutrients, and spreaders apply products. The bridge is the grade printed on the bag, three numbers for N, P2O5 and K2O. Manitoba Agriculture's rule is to multiply the desired nutrient rate by 100 and divide by the nutrient content of the fertiliser.

Product rate, kg/ha = nutrient dose, kg/ha × 100 ÷ nutrient content of the product. The nutrient content is the number of kilograms of the nutrient in every 100 kg of product.

Three examples

These are the sums Manitoba works in pounds per acre, repeated here in kilograms per hectare. The arithmetic does not depend on the unit.

Where the nutrient dose comes from

FAO states that the amounts of nutrients required can be derived from soil testing and from the nutrient removal by grain and straw. That is the balance approach: estimate what the target yield will remove, subtract what the soil is expected to supply, and divide by how much of the fertiliser nutrient the crop actually takes up.

• Removal. FAO gives about 25 kg N per tonne of wheat grain in temperate regions, for the grain alone, at 15 percent protein. Other tables include straw or use a different basis, so use the one for your region.
• Soil supply. FAO notes that fertile soils generally provide about a third of the nitrogen a high-yielding wheat crop takes up. A soil test replaces that guess with a measurement; see soil testing for farms.
• Efficiency. Not all applied nutrient reaches the crop, and the share depends on soil, moisture and timing. National tables give coefficients.

One dose, several passes

FAO illustrates the point for high-yielding winter wheat in temperate regions, where a total of 200 to 250 kg N/ha is split into several dressings. The example calls for only about 30 kg N/ha in autumn, or none where enough nitrogen remains from the previous crop, then roughly 120 kg N/ha in early spring minus the mineral nitrogen already in the soil, and further portions of about 30 to 50 kg N/ha at the beginning of tillering and 40 to 60 kg N/ha at ear emergence. Plan the product weight for each pass separately.

Common mistakes

• Mixing nutrient and product. A dose of 80 kg N is not 80 kg of urea.
• Forgetting the oxide basis. Phosphate and potash grades are quoted as P2O5 and K2O, not as elemental P and K.
• Ignoring what a compound product carries. A phosphate product such as 11-52-0 also supplies nitrogen. Size it for phosphate first, then subtract its nitrogen from the nitrogen you still need.
• Reusing last year's numbers. The yield target and the soil test change, so the dose changes.

How this works in Planva

• Fertiliser catalogue. Each product holds its N, P, K, Ca, Mg and Se content and its density.
• Application profile. An agroplan holds fertiliser by rate or quantity, plan and actual.
• Field history. Each field shows its operations with the planned and applied rates side by side.

Planva records the dose and the product you choose. It does not set the dose. For the checking side see plan versus actual.

Sources

• Manitoba Agriculture, Soil Fertility Guide: calculating fertilizer rates
• FAO, Plant nutrition for food security: a guide for integrated nutrient management, chapter 8, nutrient management guidelines for some major field crops