Objectives Potassium and boron play pivotal roles in the growth and development of oilseed rapes (Brassica napus). We compared the effects of combined application of potassium with different boron fertilizers on the growth, nutrient absorption and seed yield formation of oilseed rape.

Methods A field experiment was conducted in Xianning City, Hubei Province from September 2022 to May 2023, using a two factor completely randomized block design. Factor potassium was applying potassium chloride at K₂O 0 kg/hm² (K₀) and 120 kg/hm² (K₁₂₀); Factor boron contained five boron fertilizer basal application treatments, no boron fertilization (B₀), Colemanite 23.57 kg/hm² (B_a), Borax 15.00 kg/hm² (B_b), Boric acid 9.70 kg/hm² (B_c), and Sodium octaborate 7.86 kg/hm² (B_d), with the equal pure boron input of 1.65 kg/hm². On the 50 days since transplanting, the plant were sampled for measuring photosynthetic parameters and nutrient absorption such as boron and potassium. At harvesting period, the oilseed rape yield, and yield components and nutrient contents such as boron and potassium were investigated.

Results As compared with K₁₂₀B₀, all the boron fertilizer treatments significantly increased the yield of oilseed rape by 70.9% to 84.6%, enhanced the total plant biomass, as well as the accumulation of potassium, boron, and other nutrients. The most pronounced yield-increasing effects were observed under the basal application of B_b and Bc. Under K₀ condition, boron fertilizers significantly increased oilseed rape biomass and yield, with yield improvements ranging from 16.2% to 24.1%, which were notably lower than those achieved under K₁₂₀. The absence of either potassium or boron fertilizer significantly impaired the photosynthetic capacity of oilseed rape leaves. Under K₀ treatment, boron application did not significantly impact the net photosynthetic rate but significantly increased leaf number and leaf area. While under K₁₂₀ treatment, boron application significantly enhanced the net photosynthetic rate, leaf number, and leaf area of oilseed rape leaves.

Conclusions The combined application of potassium and boron fertilizers can enhance photosynthetic efficiency, promote leaf growth, facilitate the accumulation of nutrients such as potassium and boron, and significantly increase oilseed rape yield. Among the rested boron fertilizers, both borax and boric acid demonstrated more pronounced production-enhancing results than the others in this region.