Guiding Fall Fertilization Of Cool-Season Turfgrass Lawns With Ndvi Sufficiency Index

Fall N fertilizers are applied to turfgrass at predetermined rates instead of being based on plant-N status. Normalized difference vegetative index (NDVI) and the sufficiency index (SI) concept are used to assess plant-N status in some agronomic crops but has not been evaluated for turfgrass. This study was conducted across 3 yr in Connecticut to determine fall and spring NDVI SI values in a mixed species, cool-season lawn. Field experiments were set out with varying fall N rates with NDVI measured weekly in October and November, and in late April. Quadratic and Linear-Response and Plateau (LRP) models were significant (P < .0001) for all fall and spring SI values in response to fall N rates. Fall SI was maximized between 140 and 155 kg N ha(-1) for the quadratic model and plateaued between 79 and 102 kg N ha(-1) for the LRP model. Spring SI was maximized at 189 and plateaued at >= 116 kg N ha(-1) for quadratic and LRP models, respectively. In relation to commonly recommended fall N rates of 24.5, 49, and 98 kg N ha(-1), the models suggest that no supplemental N is needed when SI is >= 92, >= 95, and >= 99% of the N-rich reference in the fall, respectively. Late April SI >89% of spring N-rich reference would indicate that excess fertilizer was applied the previous fall at rates >49 kg N h(-1). The NDVI SI can provide site-specific fall N fertilization guidance of cool-season turfgrass lawns.