Appropriate Soil Thickness Can Improve Growth of Machine-Transplanted Seedlings in Factory Seedling Raising Mode

The aim of this study was to investigate the impact of soil thickness on seedling growth in rice machine transplanting. Zhongzu 53 was selected as the test variety, and three different soil thickness treatments were applied: 0 cm (CK), 0.5 cm (T1), and 1 cm (T2). The emergence rate, plant height, root length, leaf age, number of green leaves, total root length, projected area, root surface area, and total root volume were measured. The results demonstrated that, compared with the CK treatment, the seedling emergence rate of the T1 treatment increased significantly by 68.6%, while no significant difference was observed in the emergence rate between the T1 and T2 treatments. The plant height, root length, and leaf age of the T1 treatment were significantly higher than those of both the CK and T2 treatments. In terms of root morphological indicators, the total root length, total root projected area, and number of root tips in the T1 treatment were significantly greater than those in the CK and T2 treatments. Correlation analysis revealed that the seedling emergence rate was extremely significantly positively correlated with the total root number (p < 0.01) and significantly positively correlated with the number of white roots, number of root tips, and total root length (p < 0.05). Grey correlation analysis indicated that the total root number had the highest correlation degree with the seedling emergence rate. Principal component analysis (PCA) results showed that the cumulative contribution rate of PCA1 and PCA2 reached 67.5%. Membership function analysis revealed that the T1 treatment had the highest average membership value, whereas the CK treatment exhibited the poorest performance. In conclusion, an appropriate cover soil thickness can effectively improve the growth performance of mechanically transplanted rice seedlings.