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Effects of corn steep liquor (organic fertilizer, OF) and conventional chemical fertilizer (CF) on the growth and yield of bok choy (Brassica rapa var. chinensis) in summer and autumn hydroponic growing systems were compared. When OF and CF were applied with the same amount of total nitrogen in summer cultivation, there was no significant difference between yields; however, the growth rate in OF was slower than in CF. When OF was applied with twice the amount of nitrogen in CF (OF2), bok choy growth and yield were significantly inhibited in summer cultivation, likely owing to dissolved oxygen deficiency and different rates of nitrification and nitrogen absorbance by the plant root. Although the contents of potassium, calcium, and magnesium in bok choy showed no difference among the three treatments in both cultivation seasons, the carbon/nitrogen ratio tended to be higher in OF and OF2 than in CF. Lower nitric acid and higher ascorbic acid content was found in OF and OF2 than in CF. Overall, our results suggest that a comparable yield is expected by using the same nitrogen amount with a conventional recipe of chemical fertilization in autumn cultivation. However, further improvement of hydroponic management is needed in summer cultivation.

Using organic fertilizer made from waste materials is beneficial for both the economy and the environment, promoting sustainability and reducing pollution. In hydroponics, decomposition converts these materials into fertilizer, with multiple parallel mineralization (MPM) enabling efficient nutrient conversion by microorganisms. The tomato cultivar “Momotaro Next” was cultivated hydroponically in order to determine whether organic fertilizer derived from soluble bonito fish waste (OF) through MPM could be used in tomato hydroponic cultivation compared with a chemical nutrient solution treatment (CF). In this study, ammonium (NH4+) was generated through the OF decomposition process. During cultivation, the ammonium concentration tended to increase, while the nitrate (NO3−) concentration tended to decrease. The total yield (TY), total dry matter (TDM), and leaf area index (LAI) were lower after OF treatment than after CF treatment. Notably, the TY, TDM, and LAI were 5.4 kg m−2, 594 g plant−1, and 1.7 for OF and 6.8 kg m−2, 895 g plant−1, and 3.8 for CF, respectively. The results of the tomato fruit qualities show no significant differences for total soluble solids (TSS) (%Brix), lycopene, glucose, fructose, or sucrose. However, significant differences were observed for gamma-aminobutyric acid (GABA), glutamate, aspartate, and citric acid. The lower yield and quality of the tomato crop with the OF treatment were caused by the high concentration of NH4+ that occurred during cultivation due to a nonoptimal mineralization process. Therefore, a well-managed MPM process could improve crop quality by reducing the risk of high NH4+.

The rhizosphere microbial community in a hydroponics system with multiple parallel mineralization (MPM) can potentially suppress root‐borne diseases. This study focused on revealing the biological nature of the suppression against Fusarium wilt disease, which is caused by the fungus Fusarium oxysporum, and describing the factors that may influence the fungal pathogen in the MPM system. We demonstrated that the rhizosphere microbiota that developed in the MPM system could suppress Fusarium wilt disease under in vitro and greenhouse conditions. The microbiological characteristics of the MPM system were able to control the population dynamics of F. oxysporum, but did not eradicate the fungal pathogen. The roles of the microbiological agents underlying the disease suppression and the magnitude of the disease suppression in the MPM system appear to depend on the microbial density. F. oxysporum that survived in the MPM system formed chlamydospores when exposed to the rhizosphere microbiota. These results suggest that the microbiota suppresses proliferation of F. oxysporum by controlling the pathogen's morphogenesis and by developing an ecosystem that permits coexistence with F. oxysporum. The rhizosphere microbial community in a hydroponics system with multiple parallel mineralization (MPM) can potentially suppress root‐borne diseases. This study focused on the suppression of Fusarium wilt disease caused by the fungus Fusarium oxysporum. Our data indicated that the microbiota suppresses the disease by controlling the pathogen's morphogenesis and by developing an ecosystem that permits coexistence with F. oxysporum.