Widespread late-season potassium (K) deficiency in the US Cotton Belt has focused attention on K nutrition of cotton (Gossypium hirsutum L.). Cotton is more sensitive to low K availability than most other major field crops, and often shows signs of K deficiency on soils not considered K deficient. Preplant soil tests provide a means for estimating overall K fertilizer requirements, whereas petiole analysis has become a valuable diagnostic tool for assessing nutrient status and determining K requirements during the growing season. However, petiole analysis has not always been reliable, and there is some uncertainty about K threshold levels. This report describes studies conducted in Arkansas from 1992 to 1996 on the K nutrition of cotton. Specific objectives addressed are: (a) K partitioning in plant components in relation to tissue sampling, (b) changes in various physiological processes with the onset of K deficiency, and (c) the influence of K deficiency on gas exchange and carbon discrimination.