Objectives In order to search the reasonable potassium application method for high and stable root tuber yield of sweet potato and clarify the yield-increasing mechanism, the effect of potassium application methods on yield formation of sweet potato was studied through field experiment and lab analysis.

Methods A filed experiment was conducted with two typical edible sweet potato cultivars of Beijing 553 and Hongxiangjiao in 2013 and 2014. The potash application treatments included no K (CK), all applied as basal fertilizer (JS), all applied when the field was completely covered by crop (FS), and half potassium applied as basal fertilizer and half potassium applied when the field was completely covered by crop (¹/₂JS + ¹/₂FS). ¹³CO₂was labeled when the fourth or fifth leaves fully unfolded. Samples for ¹³C detection using Isoprime 100 were collected in 24 h, 48 h and 96 h after labeling. Since the 50 days after transplanting, samples were collected until harvest in interval of 20 days. The samples were divided into four parts:root, stem, leaf and petiole, and weighed separately. The main stems were parted up, mid and bottom from the functional leaf to the bottom, and the sucrose content was determined. The biomass and yield were investigated and the economic index was calculated at harvest.

Results All the K application treatments increased the biomass, economic coefficiency and root tuber yields significantly, and the root tuber yields were increased from 21.33% to 34.38%. The biomass of sweet potato were similar in all the K application treatments, but the economic coefficiencies and root tuber yields were significantly different, with the highest in treatment of all potash basal applied (JS). The dry matter accumulation and photosynthate distribution rate in root tuber were significantly higher in JS than in the other treatments at 50 d since planting. Compared with treatment of FS and ¹/₂JS + ¹/₂FS, the photosynthate loading and unloading rate was significantly accelerated in JS at the 50 d and 110 d after planting, and became similar at 150 d after planting; the photosynthate distribution rate to root tubers was significantly higher in JS at 50 d after planting and became similar at 110 d and 150 d after planting; the bulking rate of root tubers in JS was much higher from 50 d to 130 d since planting.

Conclusions Earlier application of potassium is in favor of transportation of photosynthate from functional leaves to root tubers, and the rapid transformation could last longer, which will stimulate the expansion of tuber and formation of tuber yield. Therefore, all the potash fertilizer should be basal applied under the tested soil and climate conditions.