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Project Details:
Methodology
Objectives
Justification
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 cultivars develop more of their total boll load over a shorter period of time, which leads to a more condensed boll filling period (Oosterhuis et al., 1991). This fast-fruiting characteristic of newer cultivars has resulted in an intense demand for the uptake and mobilization of potassium from the soil and leaf to the developing lint – from 2 to 4 lb K/acre/day (Halevy, 1976; Mullins and Burmester, 1991). However, Pettigrew et al. (1996) found no differences in response to K deficiency among cotton varieties with varying genetic backgrounds and maturity ranges in the Mississippi Delta. Since potassium is the primary osmotic for fiber development and provides the turgor pressure necessary for fiber elongation (Dhindsa et al., 1975), optimum cotton yields and fiber quality are highly dependent upon an adequate supply of potassium throughout the growing season. Late-season potassium deficiencies appear to be extremely detrimental to cotton, with reduced fiber quality (especially fiber length, strength, and micronaire) and lint yield (Pettigrew, 1996; Bauer et al., 1998) often occurring as a result of late-season potassium deficiencies.
Early research in the Mississippi Delta region found that deep placement of potassium was needed to provide sufficient potassium for maximum boll filling of improved cultivars (Tupper et al., 1993). Excessive drying of the upper soil layers renders potassium unavailable to the crop, and deep soil layers have little potassium because downward leaching of potassium is limited in relatively high cation exchange capacity soils (Pate et al., 1994). Soils in the Coastal Plain region of South Carolina are much different than those in the Delta, and the distribution and availability of potassium is also quite different. 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 likely governs potassium availability in Coastal Plain soils. Current potassium fertilizer recommendations in South Carolina are based on pre-season potassium levels of the topsoils that are adjusted by depth and potassium content of the subsoil (Woodruff and Parks, 1980). The data establishing the subsoil adjustment to fertilizer recommendations preceded development of these high potassium demanding cotton cultivars.
A previous study was conducted during the 2002 and 2003 growing seasons at the Pee Dee Research and Education Center to evaluate the response of five cotton varieties released between the years 1919 and 2001 (Dixie Triumph, DPL 90, DES 119, Paymaster 1218BR, and DPL 555BR) to various potassium rates. In this study, cotton growth and development was significantly altered by the various potassium treatments. 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 appeared to be a better indicator of potassium supply than petiole potassium, but was also more affected by growth stage compared to petiole measurements. 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. Visible differences in deficiency symptoms in the field occurred among varieties and potassium rates. All varieties responded favorably to increased levels of leaf potassium, but recently released higher-yielding varieties such as PM 1218BR and DPL 555BR responded more to potassium than older, lower-yielding varieties such as Dixie Triumph, DES 119, and DPL 90. Lint yields increased 400 to 800 lb/acre with each 1% increase in leaf K, with lint yields of newly released varieties increasing more than older varieties. Research will be continued during the 2004 and 2005 growing seasons to further define the optimum potassium fertilization rate for these higher yielding modern cotton cultivars.
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