Relationship Between Autumn Verdure Nitrate-N Concentrations and Spring Color in Kentucky Bluegrass.

Measurement of nitrate (NO₃-N) in various annual crops has been used as an indicator of soil N availability. Few studies, however, have been reported relating tissue NO₃-N concentrations to growth or quality responses in perennial crops. The objective of this research was to determine if any relationship exists between autumn NO₃-N concentrations in the verdure (all aboveground portions of the turf plant remaining after clippings removal) of Kentucky bluegrass (Poa pratensis L.) turf and color responses in the following spring. Randomized complete block field experiments were conducted in 2006-07 and 2007-08 with 13 N rate treatments ranging from 0 to 196 kg ha^-1, applied as urea in September. Beginning in October and through early November at approximately weekly intervals, verdure samples were removed from each plot immediately after mowing. Samples were dried and analyzed for concentrations of NO₃-N. In the following spring, turf color was measured at approximately weekly intervals with reflectance meters beginning in late March to early May. Bivariate plots were constructed between autumn verdure NO₃-N concentrations and relative spring meter readings. Plateau models were applied to determine if a critical level for autumn verdure NO₃-N could be found for spring color. In this case, the critical verdure NO₃-N value marks the concentration where no further change in spring turf color is observed with increasing previous autumn verdure NO₃-N concentration. When data were pooled across both years and all autumn and spring sampling dates, significant linear- and quadratic-plateau models suggested critical values for autumn verdure NO₃-N concentrations between 300 and 600 mg kg^-1 that maximized spring color of turf. Delineation of a critical value for autumn verdure NO₃-N concentrations may be useful in guiding late-season N fertilization of Kentucky bluegrass turf.