【Objective】 To explore potential causes of the occurrence of potassium deficiency symptoms in the middle and upper leaves, rather than lower leaves, of normally fertilized tobacco plants in the field during their growth, a field experiment was carried out in this study. 【Method】 The field experiment was laid out in a tobacco field of sandy loam planted with flue-cured tobacco of variety K326, designed to have three treatments, i.e. K0 ( plants applied with N P at the conventional rate and without K, 0 kg·hm^-2 K₂O), CF (plants applied with NPK at the conventional rate, 364 kkg·hm^-2 K₂O, and only those among the plants exhibiting potassium deficiency symptoms during the middle and late stages were sorted as Treatment CF), and CK (sharing the same plants with Treatment CF, applied with NPK at the same rate, and only those free from any potassium deficiency symptoms during the middle and late stages were sorted as Treatment CK). Observation of the plants began after their transplantation for growth and possible appearance of potassium deficiency symptoms in leaves). Leaves were sampled timely for determination of dry matter and potassium content, and rhizosphere soils were for analysis of available potassium content. 【Result】 (1) The tobacco plants in Treatments CF and K0 did not show any symptoms of potassium deficiency during the first 33 days after transplanting. However, when soil available potassium in the root zone was gradually depleted and dropped below 99.86 mg·kg^-1 in content on D42, symptoms appeared one by one on the 8th ~ 15th leaves of the plants in Treatment K0 ; and when soil available potassium in the root zone was lowered down below 131.1 mg·kg^-1 on D57, symptoms appeared one by one on the 12th ~ 16th leaves of the plant in Treatment CF, but not on the other leaves; (2) During the first 42 days after transplanting, dry matter and potassium accumulation significantly increased in Treatments CK and K0, but the trend of the total weight of the tobacco plants in Treatment K0 getting lower became more and more apparent; and the two indexes kept on rising till D57, and during this period of time, the two treatments exhibited a rising trend in dry matter accumulation, similar to that Treatment CF did, but Treatment CF was a bit lower than Treatment CK in potassium during the first 42 days after transplantation; (3) From D42 to D57, net potassium export was significant from the upper and middle leaves of the plants in Treatment CF and the upper leaves in Treatment K0, while net potassium import was in the stems, and potassium in the other organs or parts was generally kept in balance or slightly increased; and in Treatment CK, potassium stayed in balance in the upper leaves of the tobacco plants and slightly increased in the other organs; and (4) During the growth period, potassium content increased with rising position of the leaf but declined in overall in the plants of Treatment CK; similar patterns were observed Treatments K0 and CF in the early stage, but Treatment K0 showed a “rise-decline-rise” trend after D42 and D57, while Treatment CF did a “decline-rise” trend after D57. 【Conclusion】 All the findings indicate that the appearance of potassium deficiency symptoms in the middle leaves of the plants in Treatment K0 on D42 was attributed to (1) insufficient potassium absorption as a whole and (2) high net export of potassium in the upper leaves in the later stage. The appearance of potassium deficiency symptoms in the middle and upper leaves rather than lower leaves of the tobacco plants in Treatment CF was because (1) the on-going fast dry matter accumulation diluted potassium in the plants in the later stage; (2) potassium accumulation of the whole plant declined instead of going on in the later stage and (3) the net potassium export increased in middle and upper leaves of the plants.