How to use this calculator
- Enter or choose nutrient to supply.
- Enter or choose target unit.
- Enter or choose nutrient content of the manure.
- Enter or choose content unit.
- Enter or choose available in the first year.
- The rate per hectare (t or m³) updates as you type. Use Copy result or Share to keep it.
Method
Rate = Nutrient target ÷ (Nutrient content × Availability %); Total = Rate × Area
- Nutrient content
- kg per tonne or per m³ (lb/US ton × 0.5 = kg/t; lb/1000 gal × 0.1198 = kg/m³)
- Availability
- first-year plant-available fraction
Worked example
- Available N = 6 kg/t × 0.40 = 2.4 kg per tonne.
- Rate = 100 ÷ 2.4 = 41.67 t/ha (16.86 t or 18.59 US tons per acre).
- For 2 ha: 83.33 t.
To supply 100 kg/ha of available N from manure with 6 kg N/t at 40% availability, spread about 42 t/ha, 83 t on 2 ha.
What the result means
Manure rarely matches crop needs for every nutrient. Check the P and K this rate supplies and top up the shortest nutrient with mineral fertiliser rather than adding more manure.
Assumptions and limits
- Nutrient content is per unit of manure as spread (fresh weight).
- Local rules may cap manure or nitrogen rates (for example, in nitrate-vulnerable zones); check them before spreading.
- Availability figures are estimates that vary with timing and incorporation.
Questions
Why include availability?
Much of the nitrogen in manure is organic and only becomes available over several seasons, so only part counts towards this crop.
Should I base the rate on N or P?
Where phosphorus builds up in soil, advisers often recommend rating manure on P and supplying the extra N from fertiliser.