Efficiency of Potassium Uptake and Utilization in Cotton Cultivars Differing in Maturity.

A K-efficient cultivar may either take up K more efficiently from the soil or utilize plant K more efficiently in producing yield.  We evaluated K uptake and utilization in two cotton cultivars differing in maturity, under two K fertility regimes at Jackson, TN.  Plots fertilized with 56 kg K ha^-1 yr^-1 received the recommended rate of K fertilization, while plots receiving 112 kg K ha^-1 yr^-1 were overfertilized with K.  Early and late maturing cultivars were ‘FM960BR’ and ‘DP555BG/RR’ respectively; both sampled at early bloom and cutout.  We calculated K-uptake efficiency as the ratio of aboveground plant K content to the sum of fertilizer K applied each year plus residual soil K from the previous season.  K-utilization ratio was calculated as the mass of lint produced per unit plant K.  We hypothesized that the earlier, more determinate cultivar would absorb K faster or utilize K more efficiently during boll filling.  Total aboveground dry matter and K accumulated faster in FM960BR than in DP555BG/RR during boll filling in 2006, but not in 2007.  K-uptake efficiency did not differ significantly between cultivars, but decreased in both cultivars with higher K fertility in 2006.  Dry matter and K also accumulated faster in fruit of FM960BR than DP555BG/RR in 2006, but not in 2007.  DP555BG/RR tended (p=0.084) to produce more lint per unit plant K than FM960BR in 2006, but not in 2007.  The two cultivars produced equivalent lint yields, but tended (p=0.025) to have opposite yield responses to K.  The K-utilization ratio of both cultivars decreased at the higher K rate in both years, indicating that K was probably not limiting yield at 56 kg K ha^-1 yr^-1.  Results indicate that cultivar differences in K uptake or utilization efficiency may not be apparent with sufficient K fertility.