1 23 Plant height, stem diameter, leaf number, and leaf size are key indicators for assessing crop growth. Plant height and stem diameter reflect the overall growth vigor during development, while leaf number and size indicate the rate of meristematic activity and expansion of each plant. During the entire growth period of pak choi, the growth performance was recorded every ten days. Figure 2 illustrates the ten-day growth progression of pak choi from emergence to maturity across different substrates. Table 3 shows the measured 40-day growth indicators of pak choi. Table 3 - Measured 40-day growth indicators of pak choi in different substrates Substrate No. R1 R2 R3 R4 R5 R6 R7 R8 R9 CK Plant height (cm) 18.23 20.41 19.84 20.96 23.92 22.40 22.06 26.97 23.41 24.92 Number of leaves (Nr.) 15 16 15 16 17 17 19 20 20 20 Figure 2 - Growth of plant height of pak choi in different substrates Figure 2 indicates that pak choi is able to grow in all substrates, with a generally similar trend in plant height over time. The growth is relatively slow in 10 d-20 d, accelerates in 20 d-30 d, and then slows again in 30 d-40 d. The trend aligns with the typical growth cycle of pak choi. This suggests that the microbial fertilizer promotes the conversion of nutrients, ensuring rapid crop growth. However, due to the varying ratios of microbial fertilizer added, differences in growth rate are observed among the substrates. The overall trend in plant height follows R8 > CK > R5 > R2, R9 > R6 > R3, and R7 > R4 > R1, with the plant height of R5 close to that of the CK, and that of R8 slightly larger than that of the CK. This indicates that in the same ratio, a higher ratio of microbial fertilizer results in a better effect. When the ratio of microbial fertilizer is fixed, but the ratio of other substrates varies, the growth performance trend follows R2 > R3 > R1, R5 > R6 > R4, and R8 > R9 > R7. This demonstrates that a 1:1 ratio of fine-grained gangue to tailing coal slime provides the most favorable conditions for crop growth. 3.2 Analysis of Properties of Different Substrates Before and After Crop Planting The pak choi stubbles were turned over and returned to the field after harvest. After one year of planting, the pH value, organic matter, available nitrogen, and rapidly available
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