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Only part of manure nitrogen mineralises in the year of application — typically 25-35% for farmyard manure, more for poultry litter and slurry. The rate needed is therefore N requirement divided by (manure N content x first-year availability). The remaining nitrogen becomes available in later seasons and should be credited then.
Manure rate
Rate (kg/ha) = N required / (manure N fraction x first-year availability)
Manure nitrogen and phosphate can pollute groundwater and surface water. Many jurisdictions cap organic nitrogen loading (commonly 170 kg N/ha/year) and restrict spreading dates and distances from watercourses.
Most manure nitrogen is in organic form and mineralises slowly. Sizing on the available fraction means a large total N loading, which builds soil organic matter but must be tracked against nutrient limits.
Yes. Farmyard manure at 20 t/ha typically supplies 40-60 kg P2O5 and 60-100 kg K2O, so deduct that from the mineral fertiliser plan or phosphate will be badly over-applied.