Rötrester och rötrestprodukter som gödselmedel till vete och havre: fältförsök i Umeå inom projekt Boost Nordic Biogas

Anaerobic digestion is an efficient way to reuse energy and plant nutrients in organic waste. Digestate is a good fertilizer but it is often very diluted and needs to be concentrated to facilitate transports longer than some 10 kilometres. In two experiments at Röbäcksdalen field station in Umeå, Västerbotten, we have investigated the value of different digestate products as fertilizers to spring wheat and oat. The first experiment was comparing the fertilizer effect of increasing amounts, 40–140 kg/ha digestate pellets of swine manure and offal with NPK-fertilizer. The second experiment, the large experiment, was investigating combinations of different nitrogen and phosphorous fertilizers. However, the soil in the experimental field was too rich in phosphorous to reveal any significant differences with or without phosphorous fertilization. Spring wheat is not commonly grown in Västerbotten, so one aim was to investigate the feasibility of spring wheat cropping in this northern climate. We used the variety Quarna which is maturing early and has high protein content, but not very high yield. The oat variety Cilla was used 2024. The early summer 2023 was very dry and warm. Then both yield (3500–4500 kg/ha with full fertilization) and quality of wheat was enough for milling. In 2025 the wheat yield was similar but a long rain period in the autumn made the falling numbers, a measure of starch quality, very low and all N-fertilized wheat was downgraded to fodder quality. Unfertilized wheat had higher falling numbers than N-fertilized both years. Oat variety Cilla gave very good yield (3300–3900 kg/ha) even without fertilization and it increased with increasing N-fertilization to 5500–6500 kg/ha. The digestate pellets we tested gave results very much like NPK-fertilizer. The yield increased with increasing N-fertilization all three experimental years but further yield increase with fertilization higher than 70 kg N per ha was significant only the first year. Raw protein concentrations were high in all treatments 2023, but 2024 and 2025 the protein concentrations were higher after fertilization with 70 kg N/ha or more. The fat concentration in oats also increased with increasing N-fertilization and this was seen also in the large experiment. Recycled ammonium sulphate was compared with ammonium nitrate in the large experiment There were no significant differences in yield or quality between these fertilizers. Both fertilizers were acidifying for the soil and ammonium sulphate was somewhat more acidifying than ammonium nitrate. This caused higher concentrations of cadmium in both wheat and oat grains. However, the concentrations were lower than the limit for wheat. Biochar and dewatered digestate were only applied to the plots once in the beginning of the experiment and compared to mineral phosphorous. There were only minor quality differences between these fertilizers and mineral phosphorous. Sludge biochar gave lower thousand grain weight and hectolitre weight than mineral phosphorous for wheat 2023 and dewatered digestate gave lower thousand grain weight. One minor positive effect was lower proportion of lodging when 100 kg N/ha was combined with sludge biochar. Sludge biochar also prevented lodging somewhat 2025 when combined with 100 kg N/ha. Dewatered digestate had a nitrogen effect on yield and protein content in wheat 2023 that was proportional to its ammonium content. The effect of liquid digestate was measured on plots from a band across the large experiment. Thus, we could not make a proper statistical comparison. However, we could conclude that yield, grain quality and lodging after fertilization with liquid digestate were very similar to after fertilization with 100 kg ammonium nitrate N/ha. An exception was falling number 2023 that was lower after fertilization with liquid digestate.