Objective This study is aimed to investigate the effects of bio-fertilizer (BF), Bacillus subtilis (BS), slow-released nitrogen fertilizer (SRN) on cassava growth, soil nutrients and biological activity through field experients.

Method A field trial was carried out including 13 treatments with 2 BF application amounts (BF1, 5 t/hm²; BF2, 10 t/hm²), 2 concentrations of BS (BS100, diluted 100 times; BS200, diluted 200 times), SRN and reduced chemical fertilizer (75%CF) and urea treatment (application once, UN1; application twice, UN2).

Result BF2+75%CF treatment lowered soil available nitrogen content (P < 0.05) compared with BF2+SRN treatment. Soil available potassium (AK) content was increased by BS100 with SRN. AK content was also increased significantly by BF2 when applied with UN1. BF2+75%CF significantly increased of AK content compared with SRN. Soil microbial biomass carbon (SMBC) was increased by BS+SRN, BF+SRN andBF2+75%CF, higher than SRN. With the application of SRN, soil acid phosphatase activity was increased by BS100 and BF2. The cassava yields were increased by 20.5% and 21.7% respectively under SRN and UN2 treatments, compared with that under UN1 treatment. Both BS and BF could increase cassava yield when applied with SRN or UN, or only 75% CF. Compared with conventional fertilization (UN1), BF and BS combined with SRN or 75%CF could increase nitrogen utilization rate by 10.9%-21.5% and potassium utilization rate by 11.4%-28.9%.

Conclusion In conclusion, cassava yields are increased approximately by SRN or UN2. With the plication of SRN, AK, SMBC, soil acid phosphatase activity and cassava yield are increased by BF and BS, and the soil sucrase activity by BF. By reducing 25% chemical fertilizer, BF can also increase AK, SMBC and cassava yield. The nitrogen and potassium utilization rates are both increased by BF and BS.