Previous studies have related yield and quality of turfgrass and perennial grasslands to soil NO₃–N extracted with in situ anion exchange membranes (AEMs) using linear plateau and quadratic plateau models. These models suggested critical soil NO₃–N concentrations above which turf or grasses had a low probability of quality response or growth response to higher soil NO₃–N concentrations. However, these studies developed different models among sample dates, which resulted in critical concentrations varying by as much as two orders of magnitude among sample dates. Similar difficulties in studies relating corn (Zea mays L.) yield to soil NO₃–N concentrations have been ameliorated by converting absolute yields to relative yield values for each site–year, pooling data across site–years, and determining critical concentrations from pooled data with Cate–Nelson models. These models suggested critical concentrations applicable across sites and years. Data from a two-year field study and a two-year greenhouse study comparing turf color and yield to soil NO₃–N desorbed from AEMs were analyzed by converting color and yield measurements to relative values by sample date, and pooling data across dates. Cate–Nelson models suggested critical AEM NO₃–N concentrations applicable across all sample dates. Critical AEM NO₃–N concentrations were consistent among measured color and yield parameters and between studies. These results suggest that pooling relative color or yield measurements across sample dates for analysis in Cate–Nelson models may provide consistent critical concentrations for sufficiency for turfgrass of NO₃–N or other soil nutrients.