Placement of acidified digestate solid fraction as an efficient starter phosphorus fertilizer for horticulture crops

This study proposes employing acidified digestate solid fraction (DSF) as an effective P starter fertilizer for horticulture crops. Our investigation comprised two experiments to assess the effects of acidification and different application methods (mixed vs. acidified) of DSF on P use efficiency and root growth in horticulture crops. Acidification (7.5 % of H2SO4, ww(-1)) reduced the DSF pH from 9.6 to 1.6 and increased the P solubility from 14 % to 59 % of the total P. In the first experiment, we observed crop-specific responses to placement, with all ten crops exhibiting significantly higher P uptake when for acidified DSF, compared to untreated DSF mixed with the soil. Notably, root crops (carrot, onion, and fennel) and beans demonstrated the most favorable response to acidified DSF placement. The second experiment focused on six selected crops, comparing the effects of untreated DSF versus acidified DSF on root growth in rhizoboxes. Bean, pea, broccoli, carrot, and onion exhibited increased total root growth and in the soil surrounding the placement zone, due to higher P availability that boosted early-stage crop growth. Furthermore, the placement of acidified DSF also increased its sulfur fertilizer value. In conclusion, the placement of acidified DSF can make it an efficient phosphorus starter fertilizer. However, it is worth noting that the DSF has low density, and processing such as pelletization may be needed to facilitate its handling and allocation as a biobased fertilizer. Moreover, further studies are required to assess the effects on commercial yield, vegetable nutraceutical properties, and optimal application and acidification rates.