Phosphates recycled from semi-liquid manure and digestate are suitable alternative fertilizers for ornamentals

In several regions in Europe, the amounts of both manure produced by pig husbandry and biogas digestates from anaerobic digestion are too high to be sustainably applied to the surrounding fields. In these regions, nutrient surpluses are therefore often a problem. The research projects GOBi and BioEcoSIM succeeded in developing innovative recycling technologies for the recovery of phosphorus (P) from biogas digestates and manure, converting them into valuable fertilizers. This study tested the suitability of recovered phosphate salts (""P-Salts"") and dried solids as P fertilizers for sunflower, marigold and Chinese cabbage in a greenhouse experiment. Treatments included two recovered P-Salts (from manure and digestate), two dried solids (air-dried and steam-dried), a combination of salt and solid, and triple superphosphate (TSP) as reference, each at two fertilization levels. Measurements included biomass production (ornamentals separated into shoots and flowers), P concentration in the biomass and plant-available P in the growing medium. Both P-Salts had more or less the same effect as TSP on biomass production. The combination of P-Salt and air-dried solids resulted in a synergistic effect on sunflower in terms of biomass yield, P concentration and number of flowers. The P concentration was mostly higher in plants treated at the higher P fertilizer level. A fast P uptake into plants and thus high plant availability is particularly important in the horticultural sector due to the short production periods of potted plants. In general, all the tested recycled products except the air-dried solids could be adapted to the requirements of different ornamentals, met their P demand as efficiently as TSP and thus have high potential as P fertilizers. The P-Salts are more suitable for short-term and the steam-dried solids more for long-term P supply. The combination of both may ensure optimal P supply and guarantee long-term product quality.