Lawn Turf Color and Density in Relation to Soil Nitrate Concentration Xingyuan Geng, Karl Guillard and Thomas Morris Department of Plant Science and Landscape Architecture, University of Connecticut, Storrs, CT 06269, USA Lawn Turf Color and Density in Relation to Soil Nitrate Concentration Xingyuan Geng, Karl Guillard and Thomas Morris Department of Plant Science and Landscape Architecture, University of Connecticut, Storrs, CT 06269, USA Materials and Methods Turfgrasses 1-2 year old stands of: Kentucky bluegrass (Poa pratensis cv. America) Tall Fescue: (Festuca arundinacea cvs. Shortstop II, Dynasty, Crossfire II ) Treatments: N application rates: 0, 5, 10, 20, 30, 40, 50, 75, 100 kg ha -1 month -1 From May through October Arranged in RCBD for each species, 3 replicates Management: Irrigated after N use Mowed to 7.6 cm Clippings returned Measurements: Canopy reflectance using Spectrum CM-1000 Chlorophyll Meter and TCM-500 Turf Color NDVI meter every two weeks, May through October Soil samples collected to 10-cm depth below thatch every two weeks, May through October, analyzed for NO 3 -N concentrations In October, shoot counts made on two 26 cm 2 areas per plot and averaged Results Relationship Between Soil Nitrate-N and Turf Color: Better fit of the models to data for mid-summer/fall periods than for spring/early summer (data not shown) Significant (p < 0.01) Linear-Plateau and Quadratic- Plateau models for turf color suggested critical levels of soil NO 3 -N ranging between 13.4 and 18.4 mg kg -1 (Figs 1 and 3) for CM1000 and ranging between 13.2 and 20.0 mg kg -1 (Figs 2 and 4 ) for NDVI for Kentucky bluegrass when two-year data pooled. Significant (p < 0.01) Linear-Plateau and Quadratic- Plateau models for turf color suggested critical levels of soil NO 3 -N ranging between 4.4 and 7.2 mg kg -1 (Figs 5 and 7) for CM1000 and ranging between 4.6 and 8.2 mg kg -1 (Figs. 6 and 8) for NDVI for tall fescue when two-year data pooled. Critical soil NO 3 -N concentrations for the two-year pooled data were higher for Kentucky bluegrass than for tall fescue. Relationship Between Soil Nitrate-N and Turf Density: For Kentucky bluegrass, there seems to be a peak of density (number of shoots per m 2 ) slightly before the critical levels that were obtained for reflectance meter readings (Fig. 9). For tall fescue, there seems a peak of density (number of shoots per m 2 ) falling into the range of critical levels that were obtained for turf color (Figs. 10). Shoot density drops off as soil NO 3 -N concentration increases beyond the critical value for each species. Conclusions A direct measure of soil NO 3 -N was used successfully to predict the color quality and density responses of Kentucky bluegrass and tall fescue. The two-year results suggest that less soil NO 3 -N is needed for tall fescue than Kentucky bluegrass in maximizing turf color and density response. Introduction Nitrogen fertilization recommendations for lawns are not based on objective testing results. Instead, N is usually applied at a set rate on a set schedule (usually 49 kg N ha -1 and 3 to 5 times a year), rather than being based on nutrient availability provided by an objective method like a soil or tissue test. It has been generally considered that a direct measure of soil nitrate would not be a reliable predictor of turf response. The objective of this experiment was to determine if frequent sampling for soil nitrate (every 2 weeks) could relate to turf response, and therefore be used to guide N fertilization recommendations. This research funded by U.S.D.A. National Integrated Water Quality Program, award number Figs Relative response of Kentucky bluegrass turf color as measured with the Spectrum CM1000 Chlorophyll Meter (Left) and TCM500 NDVI Turf Color Meter (Right) in relation to mean soil NO 3 -N using all data across 12 sampling dates from 23 May to 2 Nov, 2007 and using all data across 11 sampling dates from May 22 to 27 Oct, Critical levels ranging between 13.4 and 18.4 mg kg -1 for CM1000 and between 13.2 and 20.0 mg kg -1 for NDVI. Figs Relative response of tall fescue turf color as measured with the Spectrum CM1000 Chlorophyll Meter (Left) and TCM500 NDVI Turf Color Meter (Right) in relation to mean soil NO 3 -N using all data across 12 sampling dates from 23 May to 2 Nov, 2007 and using all data across 11 sampling dates from 22 May to 27 Oct, Critical levels ranging between 4.4 and 7.2 mg kg -1 for CM1000 and between 4.6 and 8.2 mg kg -1 for NDVI. Figs Shoots counts of Kentucky bluegrass and tall fescue turf color in relation to mean soil NO 3 -N across 12 sampling dates from 23 May to 2 Nov, 2007 and across 11 sampling dates from 22 May to 27 Oct, Data Analysis Linear-plateau (LP) and Quadratic-Plateau (QP) models used to determine a critical level for soil nitrate-N relative to reflectance meter readings Meter responses for each individual sampling date converted to a relative scale Data pooled across the growing season and across two years for each species