Experimental Study on the Physical and Mechanical Properties of Combined Plug Seedlings of Pepper

To accommodate the downward-pressing seedling-picking method, this study designed a combination-type plug tray composed of a bottomless plug tray paired with a seedling base plate. The effects of peat ratio, substrate filling ratio, and nutrient solution concentration on pepper plug-seedling performance were evaluated through cultivation experiments and physical–mechanical tests, using seedling vigor index, root biomass, compressive yield strength, and substrate fragmentation rate as assessment indicators. In addition, soil-block detachment force tests were conducted to examine the influence of substrate moisture content on seedling-picking resistance. The results showed that seedling vigor index, root biomass, and compressive yield strength first increased and then declined as the levels of the three factors increased. Higher peat ratios and greater nutrient solution concentrations significantly reduced substrate fragmentation rates. The detachment force of the soil block was highly sensitive to moisture content, with an average value of 6.8 N when the substrate moisture content ranged from 65% to 75%. Based on a comprehensive evaluation approach, the optimal cultivation parameters were determined to be a substrate ratio of 3:1:1, a compaction coefficient of 1.2, and a nutrient solution concentration of 4.0‰. These results provide technical support for optimizing combined plug-seedling cultivation and for the design and parameter determination of seedling-picking mechanisms.