Potassium (K) deficiency may reduce upper-canopy light interception and reduce yield from late-set bolls, thus inducing earliness of maturity. Objectives of this research were to determine the influence of soil-applied and foliar K fertilization on interception of photosynthetically active radiation (PAR) and earliness, and to describe the relationship between light interception at different in-canopy heights and earliness of maturity, as influenced by K. Research was conducted on a Memphis silt loam with low extractable K at Ames Plantation TN. Cotton 'DPL50' was planted on 29 Apr 1993 and 6 May 1994 in 40" rows, using no tillage. A split-plot RCB design was used, with 0 and 120 lb K₂O/acre soil-applied as main-plot treatments, and 0 and 4.4 lb K₂O/acre foliar applied four times per season as sub-plot treatments. All plots received broadcast applications of 80 lb N and 60 lb P₂O₅/acre each year before planting. Interception of PAR was measured on 7 Aug 1993 and 25 Aug 1994 at 9" vertical increments through the canopy. In 1993, a LI-COR LI-191 line quantum sensor was used above and within the canopy. In 1994, the line sensor was used in-canopy while a LI-COR LI-190 point quantum sensor was used above the canopy. Plots were spindle-picked on 12 Oct and 2 Nov 1993, and on 4 Oct and 31 Oct 1994. Samples were ginned to determine lint yields, and earliness was measured as the percent of total yield picked at first harvest.