The objective of this study was to compare the effects of biochar extract (BE) and mineral potassium fulvic acid (MPFA) on the salt tolerance and growth of Bok Choy (Shanghai Bok Choy) under saline conditions, aiming to utilize saline groundwater resources in the Yellow River Delta region. Based on the mineralization level of saline groundwater in the Yellow River Delta region, all the treatments were cultured in a nutrient solution containing 6 g·L−1 NaCl. Nine treatments were set up: BE (C1: 1 g, C2: 5 g, C3: 10 g, C4: 15 g·L−1), MPFA (H1: 0.2 g, H2: 0.4 g, H3: 0.6 g, H4: 0.8 g·L−1), and a control (CK). The results showed that both the BE and MPFA reduced MDA accumulation, increased SOD and CAT enzyme activities, thereby increasing the number of effective leaves, the maximum leaf length, and the maximum leaf width of Shanghai Bok Choy. However, MPFA was better than the BE at increasing the antioxidant enzyme activities and total chlorophyll content. Correlation analysis revealed that the hydrogen peroxide content and CAT enzyme activity were the most important factors affecting Shanghai Bok Choy yield. Compared to the CK, the BE treatments reduced the hydrogen peroxide content by 1.1–46.4% and increased the CAT enzyme activity by 4.1–68.0%, while the MPFA treatments increased the hydrogen peroxide content and decreased the CAT enzyme activity. Consequently, the fresh weight of Shanghai Bok Choy treated with the BE was 11.01–112.21% higher than the MPFA treatments and 5.17–49.70% higher than the CK. Overall, C3 had significantly lower hydrogen peroxide content than the CK and the highest CAT enzyme activity, which was markedly better than the MPFA treatments. This suggests that C3 may be an effective method for improving the salt tolerance and growth of Shanghai Bok Choy under salt stress. This study provides favorable data to support the solution to the problem of agricultural water use and the realization of sustainable development in the Yellow River Delta.
