MADISON, WI--Among turfgrass professionals, convention wisdom has held that fertilizing turfgrasses in the fall is beneficial for cooler climate production. Until recently, however, concrete information about nitrogen (N) uptake in turfgrasses during cold temperatures has been limited. Motivated by new environmental concerns about nitrate leaching, researchers are increasing their emphasis on best management practices for nitrogen fertilization of popular turfgrasses.
Nitrogen, the most important mineral nutrient required for growing cool-season turfgrass, is often applied as fertilizer to supplement available nitrogen in the soil. Fertilizer nitrogen is applied to turfgrass at various times during the growing season. In temperate climates, fall is widely considered to be the most important time for nitrogen fertilization, and is often the season when up to half of annual N is applied. Many practitioners and researchers consider the ideal N application timing to be shortly after shoot growth ceases, a regimen commonly referred to as a "late fall" N fertilization.
According to a new study published in HortScience, the conventional wisdom is that the cooler air temperatures in fall allow a greater portion of assimilated N to be used for carbohydrate accumulation and root and rhizome development instead of being partitioned into shoot growth. Daniel Lloyd, a researcher at the University of Missouri's Turfgrass Research Center, along with colleagues Douglas Soldat and John Stier from the Department of Soil Science at the University of Wisconsin–Madison, published the study that contains new recommendations for growing cool-season turfgrasses. The scientists evaluated nitrogen (N) uptake potential, use, and plant metabolic response in a climate-controlled environment by studying the responses of various cool-season turfgrass species to variable N rates and temperature regimens.
The researchers used turfgrasses 'Penncross' creeping bentgrass (Agrostis stolonifera L.), 'Midnight' kentucky bluegrass (Poa pratensis L.), and 'True Putt' annual bluegrass (Poa annua var. reptans L.) in the study. The three grasses were seeded and grown for 3 months, then acclimated in a growth chamber to one of three climate regimens corresponding to typical fall temperatures (September 15, October 15, and November 15) in Madison, Wisconsin. The turfgrasses were fertilized with N-labeled ammonium sulfate. The research team then collected data on verdure biomass, root mass, net canopy photosynthesis, and N fertilizer uptake.
For all of the turfgrasses studied, shoot growth increased in response to N application in the September regimen, but not in October or November regimens. Nitrogen uptake was significantly lower in the November regimen compared with September with an average of 73% of fertilizer recovery in September compared with 57% and 38% in October and November, respectively. Root mass and net canopy photosynthesis were greatest in the November treatments, although these responses were generally unaffected by N application rate.
The researchers recommended that late-fall fertilization regimes be reevaluated. "The convention for the past several decades has been to recommend N application in the fall. Our results suggest that some of the widely held views on the importance of fall fertilization may not be as well understood as was previously thought," they noted. "The N uptake capacity of creeping bentgrass, annual bluegrass, and kentucky bluegrass declines substantially as temperatures decrease, although N uptake potential appears to be relatively high near the time after shoot growth stops. Waiting until after this period greatly reduces N uptake potential."
The complete study and abstract are available on the ASHS HortScience electronic journal web site: http://hortsci.ashspublications.org/content/46/11/1545.abstract
Founded in 1903, the American Society for Horticultural Science (ASHS) is the largest organization dedicated to advancing all facets of horticultural research, education, and application. More information at ashs.org
Low-temperature Nitrogen Uptake and Use of Three Cool-season Turfgrasses under Controlled Environments
Daniel T. Lloyd, Douglas J. Soldat, and John C. Stier
HortScience 46:1545–1549. [Abstract][Full Text][PDF]