Automation of vegetable seedling transplanting has provided opportunities for saving labors and improving productivity. Some changes in seedling agronomy are necessary for efficient transplanting. In this study, the local nursery substrates were added with the herbaceous peat, the sphagnum peat and the coir peat, respectively. Effects of the new compound substrates were investigated on the seedling qualities and the root-substrate strength. In the results, we found that the addition of these horticultural mediums significantly affected the physiochemical properties of the original substrates. Under the same nursery conditions, some appropriate additions could promote the seedling growth. And the deficient or excessive additions were to inhibit the growing development of seedlings and their roots. The corresponding additions would also improve the structural characteristics of the root lumps. Generally, the nursery substrates added with the sphagnum peat were relatively optimized in contribution to the seedling qualities and the root-substrate strength. Especially as the commercial substrate and the sphagnum peat were mixed at the volume ratio of 2:1, the dry matter accumulation of seedlings was 2.18 times more than the original. Their root lumps had the best consolidation strength, which may be an effective application for the necessary qualities of seedlings for automatic transplanting.