Total Nitrogen (1.5 – 3.5% of TS) (TN)
Also known as: total N · Kjeldahl nitrogen
The sum of all nitrogen forms — organic, ammonium, nitrate, and nitrite — in a material, expressed in g/kg or % of dry matter. Determines the total nitrogen fertiliser value of digestate.
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What is Total Nitrogen (1.5 – 3.5% of TS)?
Total Nitrogen (TN) is the sum of all nitrogen forms present in a sample — organic nitrogen, ammonium (NH₄⁺), nitrate (NO₃⁻), and nitrite (NO₂⁻) combined — expressed in grams per kilogram or as a percentage of dry matter. In biogas digestate, the typical TN range is 1.5–3.5% of total solids, equivalent to 15–35 g/kg DM or 1.5–15 g/kg fresh weight depending on moisture content.
TN measurement uses Kjeldahl digestion as the reference method: sample is digested in concentrated sulfuric acid with catalyst, which converts all organic nitrogen to ammonium, the ammonium is distilled into receiver solution, and the receiver is titrated. The result is reported as Kjeldahl Nitrogen (TKN), which captures organic N plus ammonium N but not nitrate or nitrite — typically 95%+ of TN in digestate. The Fertiliser Control Order, 1985 references Kjeldahl N as the specification basis for organic fertilisers, with FOM (Fermented Organic Manure) requiring minimum 0.8% N on dry basis and PROM (Phosphate-Rich Organic Manure) requiring minimum 1.0% N on dry basis.
Within total nitrogen, the split between forms is what matters for crop performance. Ammoniacal nitrogen (NH₄-N) is plant-available immediately upon application — crops take it up directly through roots. Organic nitrogen is bound in proteins and amino acids and must be mineralised by soil microbes before becoming plant-available, typically over 4–12 weeks. Nitrate-N is rare in fresh digestate (anaerobic conditions favour ammonium) but appears after soil application as nitrification proceeds. In biogas digestate, typically 50–80% of TN is ammoniacal nitrogen — significantly higher than in raw manure (30–50%) or compost (10–30%) — making digestate a fast-acting nitrogen source. The trade-off is that ammoniacal nitrogen is also the most volatile form: surface-applied digestate can lose 20–40% of its ammoniacal-N to atmosphere within 24 hours unless incorporated into soil within 2–4 hours, water-irrigated immediately, or applied via injection. This volatilisation loss is the single largest source of agronomic value erosion in liquid digestate fertiliser programmes and is why best-practice application includes either trailing-hose injectors, drip-fertigation, or pre-rainfall timing.
Common questions about Total Nitrogen (1.5 – 3.5% of TS)
Plain-English answers to what people most often ask.
What is total nitrogen in digestate and why does it matter?
What fraction of total nitrogen in digestate is available to plants?
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