Comparison of Struvite as a P Source with Chemical Fertilizers and Evaluation of Additional Contribution to Growth and Mineral Nutrition of Lettuce Grown on Acidic and Calcareous Soils

This study investigated the effects of two different struvites (Struvite1 (STR1) and Struvite2 (STR2)) with different P amounts on the P nutrition of lettuce grown in acid and calcareous soils and fertilizer efficiency against di-ammonium phosphate (DAP), monoammonium phosphate (MAP), triple superphosphate (TSP), and 20–20-20 (NPK fertilizer). Two distinct forms of STR1 and STR2 obtained from the biogas liquid digestate (LD, liquid fraction of a centrifuge decanter) at different molar ratios (NH4+/Mg2+/PO43−), STR1 (1.0/1.3/1.0), and STR2 (1.0/1.3/1.3) were used. The results showed that struvites were the most effective treatments for plant growth. STR1 and STR2 treatments increased the plant fresh weights by 2.7-fold in acidic soil and 2.1-fold in alkaline soil. As for the plant dry weights, STR1 and STR2 treatments increased the plant dry weights by 2.4–2.2-fold and 2.3–2.2-fold in acid and alkaline soils, respectively. Struvites were more effective on the total plant weights than other fertilizers. Struvite-applied plants yielded higher P concentrations, uptakes, and recovery than MAP, DAP, TSP, and 20–20-20 fertilizers for both soils. Due to its constituent Mg, STR1 and STR2 were found to be the most effective materials for the Mg nutrition of the plants. It was also determined that they contributed to the N and K nutrition of the plants more than most other fertilizers. In conclusion, it was seen that struvites are not only a good source of P but also a fertilizer material with a high agronomic value. Therefore, they can be evaluated as an alternative fertilizer source.