Poultry litter is an effective N fertilizer for cotton. Litter contains substantial amounts of K and Mg also, but whether the K and Mg needs of cotton can be met by the commonly recommended litter rate has not been documented. The objectives of this research were to determine if cotton receives sufficient K from the application of the commonly recommended litter rate of 4.5 Mg ha-1 and if Mg derived from the same litter rate improves cotton Mg nutrition. The research was conducted from 2002 to 2004 in Mississippi at Coffeeville and Cruger, which had contrasting soil K and Mg levels. The soil at Coffeeville had approximately five times less extractable K and approximately 22 times less extractable Mg than the soil at Cruger. Cotton at each location was fertilized with 2.2, 4.5, or 6.7 Mg ha-1 broiler litter in an incomplete factorial combination with 0, 34, or 67 kg ha-1 N as urea-ammonium nitrate solution (UAN). The results showed cotton received sufficient K from 4.5 Mg ha-1 litter, a rate previously found to be insufficient in meeting the N requirement of cotton. Unlike K, Mg concentration in the plant did not respond to increased applied litter rate but showed a strong response to supplemental UAN-N rate, which suggests the external N supply might be more important to cotton Mg nutrition than the external Mg supply. The results showed that K nutrition of cotton depended on the rate of applied litter, whereas Mg nutrition is dependent on whether the cotton received sufficient N fertilization.