Objectives The effects of long-term biochar application on soil potassium (K) availability, grain yield, K uptake and utilization, as well as grain quality of foxtail millet, were investigated to provide a theoretical basis for the agricultural application of biochar in calcareous soil.

Methods A long-term field experiment was initiated in 2014 with five biochar application rates: 0 (B0), 6.5 (B1), 13.0 (B2), 19.5 (B4) and 26 t/(hm²·yr) (B4), arranged in a randomized design with three replicates. After harvest in 2021 and 2022, soil and plant samples were collected to evaluate the effects of biochar application on foxtail millet grain yield, soil K forms, K utilization and grain quality.

Results Compared with B0, both B1 and B2 treatments increased straw biomass, grain yield, and grain number per ear. Whereas, the B3 and B4 treatments reduced ear number per hectare, resulting in grain yield and straw biomass similar to those of B0 but significantly lower than those of B1 and B2 treatments. With increasing biochar application rates, soil water soluble K and exchangeable K contents increased, whereas the effect on exchangeable K contents was relatively minor. Biochar application promoted K content and K accumulation in different parts of the foxtail millet, with the most significant effects observed in straw. Compared with B0, the B1, B2, B3 and B4 treatments increased straw potassium content by 45.0%–69.0%, 76.1%–83.0%, 88.0%–113.0% and 96.7%–143.9%, and increased straw K accumulation by 115.8%−124.5%, 130.7%−172.5%, 137.7%−175.4% and 144.2%−202.5%, respectively. Consequently, K harvest index and K physiological efficiency decreased by 36.9%−55.5% and 38.1%−58.46% respectively. Biochar application had no significant effect on the gelatinization time or gelatinization temperature of foxtail millet flour; however, peak viscosity and final viscosity increased with increasing biochar application rates. Correlation and Mantel analysis indicated that grain yield was positively correlated with straw K content and grain K accumulation. In addition, peak viscosity and final viscosity of millet flour were positively related to K content and accumulation in all parts of foxtail millet.

Conclusions Long-term biochar application effectively enhanced water soluble K and exchangeable K contents in calcareous soil and promoted K uptake by foxtail millet. Compared with grain, K luxury accumulation in straw reduced K use efficiency. Elevated plant K content and accumulation, induced by biochar application contributed to higher yield and increased flour viscosity characteristics. However, long-term annual biochar application rate exceeding 13.0 t/hm² reduced ear number per hectare, thereby limiting grain yield profoundly. Therefore, successive biochar application at a rate of 6.5−13.0 t/hm² is recommended for increasing grain yield and improving the cooking quality of foxtail millet in calcareous soil. These findings offer guidance for biochar application in foxtail millet production and a theoretical basis for efficient K management in biochar-amended soils.