ABSTRACT

Late-season potassium deficiencies have occurred in many cotton fields across South Carolina over the past several years, with some varieties showing deficiency symptoms more frequently than other varieties. New, higher-yielding earlier-maturing cotton varieties appear to develop more of their total boll load over a shorter period of time, which leads to a more condensed boll filling period and an increased demand for the uptake and mobilization of potassium from the soil and leaf to the developing lint. Southeastern Coastal Plain soils typically have accumulations of potassium in clayey subsoil layers due to leaching of potassium incorporated into sandy surface soil layers. The extent of downward potassium movement during the growing season and access to subsoil potassium may govern potassium availability in Coastal Plain soils. Current potassium fertilizer recommendations in South Carolina are based on pre-season potassium levels of the topsoil that is adjusted by depth and potassium content of the subsoil. The data establishing the subsoil adjustment to fertilizer recommendations preceded development of these high potassium demanding cotton varieties. Research was conducted to determine if current soil testing procedures and recommendations are valid to optimize yield of modern cotton varieties. A replicated field experiment was conducted in 2002 at the Pee Dee Research and Education Center located in Florence, SC, on a Norfolk-Bonneau soil complex identified as potassium deficient last growing season. Potassium treatments were broadcast applied prior to planting at 0, 50, 75, 100, and 125 lb K₂0/acre. Five cotton varieties released between the years 1919 and 2001 (Dixie Triumph, DPL 90, DES 119, Paymaster 1218BR, and DPL 555BR) were evaluated.

No differences in flowering occurred among varieties fertilized with 0, 50, 75, 100, and 125 lb K₂0/acre. As expected, varieties differed in their flowering patterns. PM 1218BR developed the majority of its flowers during the first 3 to 4 weeks of the flowering period. The other four varieties did not reach peak bloom until week 5 or 6 of the flowering period. Leaf and petiole potassium levels were positively related to the sum of the initial soil potassium level of the A-horizon plus 50% of the potassium fertilization rate. Including E- or B-horizon potassium levels and/or a higher or lower percentage of potassium fertilization rate did not improve these relationships. Leaf potassium was a better indicator of potassium supply than petiole potassium. Leaf potassium concentrations were low throughout boll development (especially with the low potassium fertilizer treatments), attaining deficiency levels of less than 1.5% at early bloom and less than 0.75% at cutout. Although leaf potassium concentrations were at or below sufficiency levels during boll development, no differences in lint yield or fiber quality due to potassium fertilization rates occurred. No significant potassium rate or potassium x variety interactions were found for lint yield or lint quality, but visible differences in deficiency symptoms in the field occurred among varieties and potassium rates. All varieties responded favorably to increased levels of leaf potassium, with recently released higher-yielding varieties such as PM 1218BR and DPL 555BR responding more to potassium than older, lower-yielding varieties such as Dixie Triumph, DES 119, and DPL 90. Differences in variety maturity levels appeared to be less important when compared to the yield capacity of the variety during the season.