Potassium Rates and Sources for Coastal Bermudagrass

During the past few years stands of ‘Coastal’ Bermudagrass (Cynodon dactylon L. Pers) have been reduced even though recommended rates of fertilizer have been applied. Many of the soils tested have been low in potassium. The objectives of this study were to determine (i) if the recommended fertilization rates of K were sufficient for optimum yield, (ii) the effect of single and split applications of K on forage yield, and (iii) the advantages of sulfur-coated KCl or K₂SO₄ on forage yield production of Coastal Bermudagrass. Coastal bermudagrass was fertilized with nine potassium treatments on Cuthbert fine sandy loam (Typic Hapludult; clayey, mixed, thermic) and Darco fine sand (Grossarenic Paleudult; loamy, siliceous, thermic) soils for 3 years at Overton, Tex. Potassium treatments included rates of 0, 139, and 278 kg K/ha as KCl in single and split (four) applications. In addition, sulfur-coated muriate of potash (S-KCl) and sulfur-coated potassium sulfate (S-K₂SO₄) were applied at both 139 and 278 kg K/ha. On the Cuthbert soil a slight forage yield increase due to the K treatments was measured in the 1st year. Highly significant forage yield responses were noted on this soil in both the 2nd and 3rd years. On the Darco soil, small but insignificant increases in forage yield were measured in the 1st and 2nd years. Forage yield was not significantly higher at 278 than at the 0 kg K rate until the 3rd year. Forage yield responses tended to occur late in the growing season usually at harvest three or four, indicating a depletion of available K reserves. Improved yields were not observed for split applications of K. Sulfur-coated KCl tended to produce highest forage yields annually and at several of the harvest dates. The S-coated treatments generally resulted in lowest soil pH values. Available K was reduced significantly in the surface 15 cm after 3 years on both the 0 and 139 kg K/ha rates. The 278 kg K/ha treatment resulted in exchangeable-K levels of 146 and 104 kg K/ha at the 0- to 15-cm depth for the Cuthbert and Darco soils, respectively, at the termination of the study. These K levels were below the original soil K levels for both soils. Results indicate that in the long term, soil K would be depleted by high yields of Coastal bermudagrass with the K rates used in this study.