1 21 very good water retention, fertilizer retention, and moisture conservation performance, but it is viscous and has poor permeability. Fine-grained gangue (0.5-6 mm) can increase permeability, but has poor moisture conservation. Large gangue (+6 mm) cannot form a stable soil structure when used directly, and has slow rate of activation and decomposition, which is not conducive to plant growth. Therefore, in the soil utilization of coal gangue, particle sizes below 6 mm are selected for gradation. -6 mm coal gangue is divided into tailing coal slime (-0.5 mm) and fine-grained gangue (0.5-6 mm) according to the particle size. In Table 1, fine-grained gangue, tailing coal slime, soil, plant powder, and microbial fertilizer are mixed in a certain ratio for orthogonal test to form 9 different substrates, designated as R1-R9. Soil collected from the experimental site is used as the control check (CK). Table 1 - Test substrates and compounding ratios Substrate No. Fine-grained gangue Tailing coal slime Soil Plant powder Microbial fertilizer R1 22.3% 44.7% 20% 10% 3% R2 33.5% 33.5% 20% 10% 3% R3 44.7% 22.3% 20% 10% 3% R4 21.7% 43.3% 20% 10% 5% R5 32.5% 32.5% 20% 10% 5% R6 43.3% 21.7% 20% 10% 5% R7 20% 40% 20% 10% 10% R8 30% 30% 20% 10% 10% R9 40% 20% 20% 10% 10% CK 100% The pre-planting soil property indicators of different substrates are presented in Table 2. Table 2 - Pre-planting soil property indicators of different substrates Substrate No. pH Organic matter (g/kg) Rapidly available phosphorus (mg/kg) Available nitrogen (mg/kg) Rapidly available potassium (mg/kg) R1 8.52 59.06 5.57 55 120 R2 8.37 52.05 5.61 58 126 R3 8.41 45.05 5.59 57 131 R4 8.36 57.04 7.17 62 138 R5 8.20 50.49 7.32 65 143 R6 8.27 43.94 7.09 63 140 R7 8.02 52.53 9.41 70 158 R8 7.67 46.93 9.82 76 171 R9 7.73 41.33 9.75 72 166
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