Influence of Potassium Fertilizer Application Timing on Cotton Production as Related to Soil Potassium on U.S. Coastal Plain Soils

Understanding soil K dynamics is highly significant in cotton production owing to its prominent role in cotton fiber quality. About 31 % of cotton production in the U.S. is concentrated in SE states, with coastal plain soils having low innate K availability. Crop fertilizer-K recommendations are primarily made worldwide and across the U.S. using pre-plant STK concentrations. A literature review on cotton K studies suggests that fertilizer-K recommendations based on pre-plant STK concentrations alone need fine-tuning to meet the increasing K demands in modern cultivars, variations in crop K requirement patterns, and varied soil K supplying capacity. Studies have been conducted to find the optimum fertilizer-K rate and split fertilizer-K application impact in cotton. However, there is a lack of studies assessing if whole K application at high nutrient requirement growth stages can improve K-use efficiency compared to the application at planting. Field studies were conducted at Edisto REC in Blackville, SC, from 2018 to 2021 on soils with varying STK concentrations to evaluate the impact of different fertilizer-K application timing on cotton growth, yield, fiber quality and K-use efficiency. Treatments included fertilizer-K application rates (KR) of 0, 46, 92, and 138 kg K ha-1 applied either at planting, first square growth stage, or first flower growth stage. In the 2018 to 2020 field studies, pre-plant STK concentrations were in the low category (16-30 mg kg-1 M-1 K), but a yield response was noted only in the 2020 study, with the highest yield recorded with KR of 92 kg K ha-1 applied at the time of cotton planting. In the 2018 and 2019 trials with low STK, the utilization of subsoil K in the clayey subsoil horizon (Bt) could have caused no yield response to the K application. Cotton growth was highest, with a 92 kg ha-1 rate, when applied at planting with no impact on lint percentage. The yield and fiber quality parameters showed similar trends across five years for parameters like fiber length, length uniformity, elongation, and contrasting trends concerning micronaire. However, fiber quality was reduced due to late planting and exposure to unfavorable weather conditions. Agronomic K-use efficiency increased two-fold with the single whole application at planting compared to the first square and flower stage application. Continued research on the impact of varying fertilizer-K rates and application timing in different soil types across South Carolina can give more insight into the soil-plant K dynamics existing in the region to further validate the fertilizer-K recommendations adopted in the state