Effect of Initial pH and Retention Time of Cow Dung Anaerobic Digestion on the Digestate Quality and Its Agronomic Performance in Lettuce Cultivation

There is a growing body of research dedicated to developing alternatives to mineral fertilizers, with the objective of enhancing crop yields while minimizing environmental degradation. One alternative is the use of organic fertilizers, like digestates. Anaerobic digestion of cow manure was conducted using initial pH values of 5.5, 6.5, 7.5, and 8.5. and lasted 8 and 20 days. The resulting digestates were used as organic fertilizer in a lettuce cultivation, assessing plant height, leaf count, weight, leaf area, and antioxidant content; these results were compared with that from plants supplied with inorganic fertilizer (control). The digestates from digestions at initial pH of 5.5 for 8 days (5.5/8) and at an initial pH of 6.5 for 20 days (6.5/20) improved the characteristics of lettuce compared with the plants obtained after inorganic fertilization. In the presence of the 5.5/8 treatment, the plants resulted in 9.2% more leaves, 54% greater leaf weight, 76% higher DPPH activity, and 24% more phenolic content compared to the control. However, the plants from 6.5/20 treatment resulted in 35% greater leaf weight, 51% larger leaf area, 78% higher phenolic content, 74% higher DPPH activity, and 133% more flavonoids than the control. It can be concluded that the initial pH and digestion duration had an important positive effect on the fertilizing properties of the digestate, and then it can be used as organic fertilizer and substitute mineral fertilizers in lettuce cultivation.