Effects of different NPK ratio combinations on growth, physiology, and soil physicochemical properties of daylily seedlings

To address the issues of imbalanced fertilizer application ratios and low nutrient use efficiency during the seedling stage of daylily, this study used ‘Datong Huanghua’ as the plant material. A pot experiment was conducted based on local conventional fertilization practices, with seven treatments of different nitrogen-phosphorus-potassium (NPK) ratios. The effects of NPK combinations were systematically investigated from three dimensions: plant growth, physiological characteristics, and soil physicochemical properties. Principal component analysis combined with the membership function method was used to establish a multi-index comprehensive evaluation system, in order to identify the most suitable fertilization scheme under the conditions of this experiment from among the seven tested treatments. The results showed that different NPK ratios had significant effects on the seedling growth, physiological characteristics, and soil physicochemical properties of daylily. Treatment B3 (with an NPK ratio of 2.13:0.34:1.21) exhibited the best overall performance. It significantly improved growth indicators such as plant height and leaf length, optimized photosynthetic parameters and photosynthetic pigment accumulation, promoted the synthesis of soluble sugars and soluble proteins, and coordinated the root-to-shoot ratio. In terms of soil environment, treatment B3 significantly increased the content of available potassium and significantly enhanced the activities of protease and urease; however, this treatment had no significant effect on available phosphorus content or invertase activity. Ratios with high N but low P, or with high N but low K, can disrupt plant metabolic balance and inhibit growth. This study identified the optimal nutrient ratio for daylily at the seedling stage that balances high yield and soil health, providing a theoretical basis and technical reference for precision fertilization of daylily.