Formulating efficient P-rich biobased starter fertilizers: Effects of acidification and pelletizing on fertilizer properties

Animal-derived biowastes can be alternatives to mineral phosphorus fertilizers. However, they typically have lower efficiency and higher transport costs because of their low bulk density. Pelleting can reduce their volume and facilitate their use as placement fertilizers but may also decrease phosphorus availability. This study examined how acidification and pelletizing affect phosphorus availability in biowastes. Digestate solid fraction and meat and bone meal were treated in four ways: (1) untreated (U), (2) acidified (A), (3) untreated pelletized (UP), and (4) acidified pelletized (AP). These treatments were tested in soil incubation and pea growth experiments, with fertilizers placed 5 cm beneath the seeds to evaluate their effectiveness as placement fertilizers. Acidification significantly enhanced the phosphorus solubility of DSF and MBM by approximately 5 and 7 times respectively, while pelletizing acidified materials reduced it. In the incubation experiment, acidified materials in the powdery form showed the highest soil water-extractable phosphorus, with no significant differences among U, UP, and AP ways. In the rhizobox experiment, pelletizing untreated digestate significantly reduced plant dry matter compared to the untreated fibrous form (from 2.0 g to 1.35 g). Acidified and acidified pelletized digestate treatments resulted in the highest shoot dry matter (2.8 g and 2.95 g, respectively), surpassing even triple the amount of superphosphate (2.53 g). For meat and bone meal, the acidified powder led to the highest plant growth (2.0 g), while untreated powder resulted in the lowest amount of plant growth (0.4 g), which was lower than that of the negative control (0.6 g). No significant differences were noted between untreated and acidified pellets. These findings indicate that acidification enhances phosphorus availability in biowastes, while pelletizing reduces it. The acidified pelletized digestate solid fraction has lower volume and higher P use efficiency than its untreated material, showing higher plant growth when compared to mineral P fertilizer.