Abstract: In order to screen the suitable concentration of Bacillus subtilis for cucumber seedling growth, the two-factor test of nano-carbon sol and Bacillus subtilis was carried out with cucumber seedlings which as the material. The dilution ratio of nano-carbon sol was set at 0, 350 and 650 times, and the concentration of Bacillus subtilis was 0, 8×10⁷ and 1.6×10⁸ CFU·mL^-1, respectively. The effects of Bacillus subtilis and nano-carbon sol on cucumber seedling growth, root growth and soil fertility were analyzed. The results showed that suitable concentration of Bacillus subtilis and nano-carbon sol could significantly enhance the growth of aboveground and underground parts of cucumber seedlings and improve soil conditions. Under single application, treatment with 1.6×10⁸ CFU·mL^-1 Bacillus subtilis promoted root growth, significantly reduced soil EC value while increasing soil P content, enhancing soil enzyme activity, and increasing number of bacteria and actinomycetes. 650 dilution multiple of nano-carbon sol significantly promoted plant leaf growth, soil organic matter content and sucrase activity, and increased the quantity of soil actinomycetes by 57.36%. The treatment of 8×10⁷ CFU·mL^-1 Bacillus subtilis + 650 dilution multiple of nano-carbon sol was optimal. Root fresh weight, dry weight, surface area, diameter and volume were the highest in these treatments, beyond the treatment without nano-carbon sol and bacillus subtilis 56.63%, 57.14%, 66.15%, 56.55% and 21.94%. The relative growth rate of plant height was increased by 6.51%. The EC value of soil was reduced. The amount of bacteria in the soil was 1.85 times of the non-added nano-carbon sol and bacillus subtilis, and the overall performance was the best.

Key words: nano-carbon sol, Bacillus subtilis, cucumber, bio-organic fertilizer