Effects of humic acid-containing water-soluble fertilizers on the growth of water spinach and soil nutrient properties

In order to investigate the effects of different application rates and methods of humic acid-containing water-soluble fertilizers on the water spinach quality, nutrient contents, soil carbon contents, and soil enzyme activities, eight treatments were established. These treatments included: control group (CK), conventional fertilization (CF), root-applied humic acid water-soluble fertilizer with full substitution (RT₁), root-applied humic acid water-soluble fertilizer with 50% substitution (RT_0.5), root-applied humic acid water-soluble fertilizer with 25% substitution (RT_0.25), leaf application of humic acid water-soluble fertilizer with full substitution (LT₁), leaf application with 50% substitution (LT_0.5), and leaf application with 25% substitution (LT_0.25). The physical and chemical properties of the soil after harvest were determined, alongside the quality indices of water spinach yield, and the impact of fertilization methods was comprehensively evaluated using principal component analysis. The results indicated that RT₁ enhanced yield, soil organic carbon (SOC) content, and soluble solids content by 11.5%, 8.7%, and 8.8%, respectively, compared to CF. On the other hand, LT₁ improved soluble solids, crude protein, and vitamin C content by 17.6%, 15.5%, and 11.0%, respectively, relative to CF, although yield and SOC content decreased by 2.9% and 11.3%, respectively. Significant positive correlations (P<0.05) were found between water spinach yield and quick-acting nitrogen (AN) content as well as SOC, while significant negative correlations (P<0.05) were observed with pH, quick-acting potassium (AK), and quick-acting phosphorus (AP) content. These findings suggested that quick-acting nitrogen nutrients and organic matter played pivotal roles in enhancing the growth and yield of water spinach. Principal component analysis underscored that applying humic acid-containing water-soluble fertilizers was beneficial for producing high-quality vegetables and improving soil quality. Specifically, applying the full amount of humic acid-containing water-soluble fertilizers at the root level (RT₁) yielded the most favorable outcomes compared to conventional chemical fertilizers (CF). Overall, this study provides valuable insights into enhancing crop yield quality, improving soil health, reducing chemical fertilizer dependency, and boosting soil carbon sequestration capacity.