Granular nitrogen source and rate affect bermudagrass phytotoxicity

Abstract

Fertilizer burn is a common concern among turfgrass managers. Risk of burn is largely assumed by using the fertilizer salt index. However, the validity of the salt index as a predictor of turfgrass phytotoxicity under field conditions has not been confirmed. The objective of this study was to determine the phytotoxicity of 12 granular N sources applied at five rates to Latitude 36 hybrid bermudagrass [ Cynodon dactylon (L.) Pers. × Cynodon transvaalensis (Burtt‐Davy)] under field conditions at two locations. Treatments were applied as a single application at N rates of 0, 49, 98, 196, and 392 kg ha ^{− 1} to plots arranged in a split plot design with four replications in Jay, FL, and Lexington, KY, in August 2020, and were irrigated immediately following application. Phytotoxicity, turfgrass quality, normalized difference vegetation index, soil electrical conductivity (EC), and volumetric water content were measured for 22 days after initiation. Nitrogen source and rate influenced most response variables at both locations. Monoammonium phosphate (MAP) and diammonium phosphate (DAP) resulted in the greatest phytotoxicity, reaching 51.6% and 48.4% of the plot area affected at an N rate of 392 kg ha ⁻¹ in Jay, despite resulting in less soil EC than ammonium sulfate and calcium nitrate, which resulted in no phytotoxicity. Only MAP and DAP resulted in phytotoxicity in Lexington. No N source resulted in phytotoxicity using 49 kg ha ⁻¹ of N, and turfgrass managers should be able to apply any N source evaluated to bermudagrass maintained at 2.5 cm with little risk of phytotoxicity provided N applications are ≤49 kg ha ⁻¹ and the turfgrass is immediately irrigated with 0.6 cm of water. These results were not consistent with salt index rankings, suggesting the salt index may not be a valid predictor of nitrogen fertilizer burn risk in turfgrass.