Fall Verdure Sap Nitrate-N Concentrations As a Predictor of Cool-Season Turfgrass Lawn Color Response in the Fall and Following Spring.

In northern climates, fall fertilization of cool-season turfgrasses with N has become the standard practice to maintain turfgrass color and density into the fall, increase root carbohydrate concentrations for stress tolerance, and optimize spring green-up after winter. However, there are no routine tests that guide fall N fertilization for these purposes. The objective of this study was to determine if relationships exist between fall sap nitrate-N concentrations in the verdure of a cool-season turfgrass lawn mixture and turf color during the fall and following spring. The study was set out as two randomized complete block design experiments with three replicates, and conducted across 3 consecutive fall-spring periods (2010-11, 2011-12, and 2012-13) on a turfgrass lawn consisting of 35% Kentucky bluegrass (Poa pratensis), 30% perennial ryegrass (Lolium perenne), and 35% creeping red fescue (Festuca rubra). Treatments in each experiment were 13 N application rates (from 0 to 196 kg N ha^-1) applied as urea. N was applied in September for the first blocked experiment, and in October for the second blocked experiment. Turf color and verdure sap nitrate concentrations were measured weekly in October (on the September-fertilized plots) and in November (on the October-fertilized plots) with an NDVI meter and a Cardy nitrate meter, respectively. NDVI response as a function of sap nitrate-N concentration was modeled with linear plateau models. The results suggest that fall and spring turf color is significantly correlated to fall verdure sap nitrate-N concentrations, with NDVI readings plateauing at concentrations between 200 and 300 mg nitrate-N L^-1. Sap verdure nitrate-N concentrations increased through the weekly November sampling, indicating that the turfgrass plants stored increasingly more nitrate as growth slowed with the onset of dormancy. These results suggest that sap nitrate-N concentrations in the verdure show promise as a guide for fall N fertilization of cool-season turfgrass.