Because of its physical, chemical, and biological stability, biochar could have a significant role in increasing soil C storage, improving soil fertility and crop yield, and maintaining the balance of the soil ecosystem. However, experimental results are variable and dependent on the experimental set-up, soil properties, and fertilizer application rates. We conducted a pot experiment to study the influence of biochar and mineral fertilizer application rates on soil quality. The biochar used in this study was prepared by pyrolyzing pruned apple tree branches at 450 °C. Two soil types were used in the study: loess soil and alluvial soil. These two soils are commonly cultivated in the Guanzhong district of Shaanxi Province. The effect of biochar application on wheat and millet productivity (both yield and above-ground biomass) was also observed. Wheat was planted in the pots first. After wheat harvest, millet was sown. Biochar was applied at five rates before planting each of the crops: B0 (0 t/hm²), B5 (5 t/hm²), B10 (10 t/hm²), B15 (15 t/hm²) and B20 (20 t/hm²). Mineral fertilizers were applied basally at the rates of 225 kg N/hm², 180 kg P₂O₅/hm², and 150 kg K₂O/hm². Soil pH, cation exchange content (CEC), organic C concentration, and available nutrient concentrations were determined in the laboratory. Crop biomass and yield were also determined. Differences among the soil quality indices, among crop biomass, and among crop yields were analyzed by analysis of variance and multiple comparisons.The results showed that compared to the control treatment, biochar application increased the pH of the alluvial soil after millet harvest. The soil pH of the loess soil tended to increase as the biochar application increased; however, the increases were not significant. Biochar application increased the CEC of alluvial soil by 1.5% to 58.2%. The increases tended to be larger after wheat harvest than after millet harvest. Organic C concentrations increased in both soils as the biochar application rate increased. The increases, which ranged from 31.1% to 272.2%, tended to be larger after millet harvest than after wheat harvest. The increases were also larger in the alluvial soil than in the loess soil. Amendment with biochar increased mineral N concentrations in both soils by 6% to 112.8%, available P concentrations in alluvial soil by 3.8% to 38.5% and available K concentrations in alluvial soil by 6.1% to 47.2%. Biochar amendment increased N uptake by wheat and millet on loess soil; however, the effect of biochar on crop P and K uptake was inconsistent. Biochar amendment also had inconsistent effects on biomass production and yield of both wheat and millet. The maximum biochar application rate (20 t/hm²) resulted in a slight decrease in wheat and millet growth. In conclusion, biochar amendment improved soil chemical properties and increased available nutrient concentrations to some extent; however, biochar amendment had negative or insignificant effects on crop yield. The effects of biochar application were related to the fertility of the soil and to the crop type. Specifically, biochar had greater effect on the low fertility soil than on the high fertility soil. The benefits of biochar on the low fertility soil increased as the biochar application rate increased.