A field experiment was carried out to study the effect of different rates of potassium (K) fertilizer [0, 50, 100, and 150 potassium oxide (K2O) ha(-1)] in the presence of increased supply of nitrogen (N) (120, 180, and 240 kg N ha(-1)) on cotton (Gossypium hirsutum L.) yield and the N and K use efficiencies using the N-15 isotopic dilution technique. Potassium fertilizer increased cotton yield, which was significant and more pronounced with the application of N in the high level (N3). The greatest cotton yield (6442 kg ha(-1)) was obtained in N2K3 treatment with an increase of 14% over the control. In addition, K fertilizer significantly increased N uptake efficiency in the N2 and N3 treatments. The greatest N uptake efficiency (98%) was in N2K3 treatment. The greatest K uptake efficiency (42%) was occurred in N3K1 treatment. In conclusion, the use of K fertilizer could be useful when growing cotton in soils of moderate to high N content to improve N uptake efficiency and consequently increase cotton yield.
