Free Fertilizer Requirement calculator with clear step-by-step results.
Fertiliser rates are specified as kilograms of nutrient per hectare, but products are sold as bulk material containing a percentage of that nutrient. Subtracting the soil's own supply, dividing by nutrient content and again by spreader efficiency converts an agronomic target into the product rate to actually set on the machine.
Nutrient deficit
Deficit = target rate - soil supply
Product rate
kg product/ha = deficit / nutrient content / application efficiency
Nutrient application is regulated in many regions, with limits on rates, timing and proximity to watercourses. Follow a soil test and any applicable nutrient management plan or legal restriction.
A soil test, plus mineralisation from organic matter and any residues from a previous legume crop. Ignoring it is the most common cause of over-application.
Spreading is never perfectly uniform, and some nutrient is lost to volatilisation or immobilisation before the crop uses it. The efficiency term compensates for both.