Objective The aims were to explore the efficient utilization of nitrogen (N) and potassium (K) in flax after the combined application of N and K fertilizers, as well as the coupling mechanism between them and the lodging resistance characteristics, so as to provide a theoretical basis for optimizing the precise fertilization in the technical system of efficient management and utilization of flax nutrient resources.

Methods Taking 'Dingya 26', one of the main flax (Linum usitatissimum L.) cultivars as the test material in the northwest main production area, a completely randomized block experiment was set up with different N (pure N) application rates (N0: 0 kg hm⁻², N1: 150 kg hm⁻², N2: 225 kg hm⁻²) and K (K₂O) application rates (K0: 0 kg hm⁻², K1: 60 kg hm⁻², K2: 90 kg hm⁻²) to study the effects of N and K fertilizers on dry matter accumulation and distribution, N and K utilization, lodging resistance of flax stems, lodging resistance index and yield components.

Results The interaction between N and K can significantly increase the dry matter accumulation and yield of flax. Compared with the no-fertilization treatment, the dry matter accumulation increased significantly by 24.52% and 53.36% during the budding stage and green fruit stage, respectively, and the grain yield increased by 45.45%. The interaction between N and K promoted the accumulation of N and K in flax. The increases in N and K accumulation in the early and late growth stages reached 67.12%-104.02% and 130.82%-180.99%, and the utilization rates of N and K decreased gradually with the increase in fertilization amount. The interaction of N and K fertilizers further improved the bending resistance and lodging resistance of the stem. The bending resistance increased by 42.24%-60.49%, and the lodging resistance index increased by 153.03%-312.61%. With the increase in fertilization amount, the utilization rates of N and K both showed a gradually decreasing trend. Correlation analysis and structural equation models indicated that dry matter accumulation and stem bending resistance significantly affected the lodging resistance index, which in turn affected the flax grain yield.

Conclusion Applying 150 kg N hm⁻² combined with 60 kg K₂O hm⁻² can significantly increase the dry matter accumulation and the absorption and accumulation of N and K nutrients in flax, promote the improvement of stem bending strength, optimize the lodging resistance performance of the stem, and thereby increase yield.