Objectives The available potassium (K) content in most farmland soils in south China is low, and K deficiency is a major limiting factor for oilseed rape production in some areas. Here, we investigate the effect of different K fertilizer application rates on oilseed rape yield and seed quality to provide theoretical support for the rational application of K fertilizer in winter oilseed rape production for high yield and quality.

Methods A field trial was conducted for two consecutive years from 2019—2021 in Wuxue City, Hubei Province, with five K₂O application levels of 0 (K₀), 60 (K₆₀), 120 (K₁₂₀), 180 (K₁₈₀), and 240 K₂O kg/hm² (K₂₄₀) . Since the 15 day at full flowering stage, silique samples were collected every six days in K₀ and K₁₂₀ plots for determination of N and K contents. At maturing stage, the yield, yield components, oil content, protein content, fatty acid composition, glucosinolate content, seed N and K contents were analyzed.

Results 1) Compared with K₀, K application (P<0.05) increased rapeseed yield by 62.9%−103.7% and 31.0%−9.2% in 2020 and 2021, respectively. The rapeseed yield did not stop increasing until K₂O application rate beyond 120 kg/hm². K application increased yield through enhancing the number of siliques per plant and the seed number per silique. 2) K application improved rapeseed nutritional and feeding quality. Compared with K₀, K₁₈₀ increased rapeseed oil content in 2019−2020 and K₁₂₀ in 2020−2021 (P<0.05) by 4.8% and 6.9%, K₁₈₀ and K₂₄₀ in 2019−2020 and K₂₄₀ in 2020−2021 decreased the protein content by 9.1%, 14.2%, and 12.1%, respectively. With the increase in K₂O rate, the oleic acid in rapeseed showed an increasing trend, while the glucosinolate content decreased. K application (P<0.05) increased rapeseed oil and protein yield. There was no further increase in rapeseed oil yield at K₂O 180 kg/hm² and above. There was no significant difference (P>0.05) in protein yield among the K application treatments. K application (P<0.05) reduced seed N content, with 13.1%−21.9% and 15.9%−22.0% reduction in 2019−2020 and 2020−2021, respectively, compared to K₀. Seed K content (P<0.05) increased with increasing of K₂O rate, recording 8.1%−37.8% and 6.3%−27.1% in 2019−2020 and 2020−2021. During silique development, seed N content in K₀ treatment was higher than in K₁₂₀, while seed K content was lower than in K₁₂₀. K fertilizer application also increased the photosynthetic capacity of silique, and the net photosynthetic rate (P_n) of silique in K₁₂₀ was 35.5% higher than K₀.

Conclusions K fertilizer application increases rapeseed oil yield through enhancing the silique number per plant and the seed number per silique. K application reduces rapeseed N but significantly increasing K content and photosynthetic capacity of silique, thereby promoting oil and protein synthesis, improving the composition of fatty acids, and reducing glucosinolate content. K₂O rate between 131−160 kg/hm² improves rapeseed’s nutrition and feeding quality while recording higher oil and protein yields.