In order to gain useful insight from cotton plant analysis data, it is essential that growth and nutrient uptake patterns of cotton be clearly recognized. The purpose of this work was to assess the usefulness of plant tissue analysis for monitoring the K status of cotton during its morphological development. The main criterion for this assessment was the identification of a critical K level as a function of yield. The experiment was conducted on a Loring silt loam at the Cotton Branch Experiment Station located in the Arkansas Delta near Marianna. These plots and treatments have been in place since 1983. The data used in this work have been taken from the years 1988 to 1990. Preplant fertilizer potassium rates were 0, 30, 60, 90, 120, and 150 lb K2O/A. Twenty upper fourth true leaves and their associated petioles were collected from alternate inside rows for nine weeks starting at the week before first bloom (periods 1 through 9). Surface (0-6") soil samples were collected in the spring prior to any fertilizer applications. There was no statistically significant yield increase after 30 lb K2O/A of applied fertilizer. Means of the leaf, petiole and soil test K concentrations were statistically different for each K2O fertilizer rate. Leaf, petiole, and soil test K concentrations increased for each increase in applied K2O fertilizer for most time periods. Leaf tissue K was correlated with petiole K levels. Petiole K values were initially higher and decreased much more rapidly. Although yields were responsive below a soil test level of 210 lb K/A, no relationship with yield was observed for leaf or petiole K concentrations. Because there was no relationship between tissue K concentration and yield, the statistically different mean K concentrations at the 0 lb K2O/A rate was inferred as the lower range of sufficient K for plant growth and development. Based on this assumption, early mid and late season critical petiole K levels would be 4.9 to 4.3, 2.6 to 2.2 and 1.0 to 0.6% K. Critical K concentrations for leaves would be 1.4 to 1.2% K from early through mid season followed by 0.9 to 0.5% K late season. The poor correlation of plant K levels with yields suggests that these critical levels are qualitative and should be used in conjunction with soil tests, visual identification and plant vigor indices for a more successful diagnosis.