Soil Potassium Dynamics of Upland Cotton: A Three Year Study

A multi-year potassium study was conducted from 2015-2018 across the United States Cotton Belt to evaluate soil potassium (K) dynamics from a continuous cotton rotation. The objectives of the study were: 1) measure the draw down and/or accumulation of soil test K concentrations over a three year when fertilized with varying K application rates and placement strategies and 2) quantify the impact of K fertilization strategies on in-season nutrient status, lint yield, and lint quality of cotton. A total of seven sites were chosen for the study with one in Arizona, four in the Mid-South and two in the Southeast growing region. Fertilizer K rates were 0, 45, 90, 135, and 180 kg K₂O ha^-1 applied in a preplant broadcast (0-0-60) and a fluid solution (0-0-15) in a subsurface band prior to planting. Plots were maintained for the 2015, 2016, 2017 and spring 2018 growing seasons. Soil samples were taken at depths of 0-15, 15-30, and 30-60 cm each spring to determine fluctuations of Mehlich III extactable K.   Soil test K concentrations varied with region in the following order Southwest > Mid-South > Southeast. Two of the four Mid-South showed a soil test K response with K application rate at the 0-15 cm depths. The soil test K decreased at each sampling depth across K applications rates, with the highest K application rates decreasing less in soil test K in the Southeast. The overall decrease in soil test K across 6 of the seven locations at high K application rates was striking and indicates a net mining of soil K in a continuous cotton rotation across the cotton belt. Lint yield was impacted by environmental conditions across years and locations with only one location (Tennessee) having a position correlation between lint yield and K application rate during the study.