Manure Nutrient Value Calculator

How it works

Nutrient applied (kg/ha) = analysis (kg/t) × rate (t/ha); available N = total N × availability factor for method and timing; P₂O₅ available ≈ 60%, K₂O ≈ 90% in year 1; value = Σ available nutrient × fertiliser price

Manure is fertiliser with water and organic matter attached, and valuing it properly changes how it is spread. Total nitrogen in slurry is roughly half ammonium, which crops can use immediately but which is also lost as ammonia gas within hours of surface spreading; injecting or incorporating quickly keeps up to 55–60% of total N available in the first year, autumn broadcasting as little as 10–15%. Phosphate and potash are mostly available and carry forward in the soil, which is why manure fields build P indices over time. The calculator converts an analysis (or typical book values — a lab test is always better, since dry matter varies enormously) into the kilograms of N, P₂O₅ and K₂O applied, their first-year availability, the bagged fertiliser they replace and its cash value, and flags the 170 kg organic-N per hectare limit that many regulations impose.

Worked example

Dairy slurry at 30 t/ha, injected: 78 kg total N of which about 43 kg is available this year, plus 22 kg P₂O₅ and 86 kg K₂O — worth roughly $166 per hectare, or $1,990 across 12 ha (360 t, $5.50 a tonne). That replaces about 93 kg of urea, 47 kg of TSP and 144 kg of MOP per hectare.

Frequently asked questions