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Targeted-yield fertiliser recommendation works from a nutrient balance: the crop's total requirement is target yield x uptake per tonne, the soil supplies roughly half of the available pool measured by the test, and the shortfall must be met by fertiliser divided by its use efficiency. Nitrogen efficiency is typically 30-50%, phosphate 15-25% and potash 50-70%.
Targeted yield equation
Fertiliser dose = (target yield x uptake per tonne - 0.5 x soil test value) / fertiliser use efficiency
Roughly 20-25 kg N, 10-12 kg P2O5 and 25-30 kg K2O per tonne of cereal grain including straw. Vegetables and fruit differ substantially, so use a crop-specific table.
The extractant measures a pool that is only partly accessible in a single season. State recommendation systems fit an empirical soil-contribution coefficient, commonly 0.4-0.6, from field trials.