A onfarm experiment of potassium (K) fertilizer oneoff application was conducted in Jingzhou City,Hubei Province from 2017 to 2018. The aim was to assess the K rate on yield,K apparent balance,K efficiency of ricerapeseed rotation in Jianghan Plain,and combined with the fertilizer efficiency model to clarify the appropriate amount of K fertilizer. The results showed that the application of K fertilizer could significantly increase the yield of rice and rapeseed,the dry matter amount and K uptake of the aboveground,where the increasement came from the density,number of kernels,seed setting rate of rice and density,number of 1st branches and pods of rapeseed. When the K fertilizer input (K2O) reached 360 kg/hm2 per year,the K apparent balance in the rotation was still negative,and the deficit was 85.4 kg/hm2 without straw returning. On the contrary,the annual K balance of farmland could be achieved positively by inputting 90 kg/hm2 K fertilizer with straw incorporation. Compared with no K fertilizer application,the agronomic K fertilizer efficiency (KfAE) of rice and rapeseed applied with K fertilizer were 10.6 kg/kg and 2.9 kg/kg,where the K fertilizer recovery efficiency (KfRE) of those were 45.5% and 54.1%,respectively,indicating that the yield production of K fertilizer applied to direct seeding rice was significantly higher than that of rapeseed,and it also reflected from the side that rapeseed was more dependent on external K fertilizer than rice. Considering the input of straw K,the K efficiency (KAE and KRE) in rice season was consistent with that of KfAE and KfRE,while the KAE and KRE in rape season were 2.1 kg/kg and 38.5%,which were significantly lower than that of KfAE and KfRE. The comprehensive linear plus platform model and the K apparent balance of farmland showed that the suitable K fertilizer rate for direct seeding rice and rapeseed were 62.31 kg/hm2,70.18 kg/hm2 and yield for 10 066 kg/hm2,2 182 kg/hm2 under straw returning,respectively,which was lower than the recommended K fertilizer amount of 90 kg/hm2 at an average of 44.4% and 28.2%. The result indicated that direct seeding rapeseed required more K fertilizer to support population growth. Therefore,in view of the differences in crop growth and nutrient requirements under direct seeding and transplanting methods,the management of direct seeding nutrients should be lightly simplified according to agricultural production to improve crop yield,nutrient utilization and economic benefits.