Applying nutrients in the form of fertilizer to turfgrass is a key component to having healthy turf. The goal of any fertilizer application is for the plant to take up the fertilizer and minimize the loss of nutrients, specifically nitrogen (N) and phosphorus (P), to ground and surface water. In many Mid-Atlantic States, there are regulations that limit fertilizer applications during the winter months.

In 2007, the Delaware Nutrient Management Commission adopted regulations that prohibit fertilizer applicators who are certified through the Delaware Nutrient Management Program from applying commercial or manure-based fertilizer between Dec. 7 and Feb. 15. Nutrient applications are prohibited on the frozen or snow-covered ground. Also, fertilizer or manure should not be applied to impervious surfaces like sidewalks, driveways, parking lots or roadways. All of these rules are designed to protect surface and groundwater nutrient loss during the winter months when plants are not actively growing and taking up nutrients.

Turfgrass is the largest crop grown in the Chesapeake Bay Watershed, so we must understand that our fertilizer applications can have a major impact on water quality. Nitrogen and phosphorus are the primary nutrients of concern related to water quality. The fate of P is a bit different than that of N. Phosphates are typically lost to the system during erosion or runoff events. The chemical properties of P are such that it binds tightly with iron and aluminum that is found in the soil. As such, P has a relatively low potential to leach through the profile. Measuring soil P can be performed using conventional soil testing methods due to its ability to be held in the soil. Phosphorus recommendations are based on soil test results in order to curb over-application and minimize environmental impact. In contrast, nitrate is negatively charged (anion) and is not held in the soil and thus, has a higher leaching potential than P. Measuring N in soils is difficult due to its instability in the soil (i.e., can undergo rapid changes in the chemical form). 

Fall is a respite for turf and fertilization at this time can aid in turf recovery and increase plant vigor. Fall is the best time to fertilize your cool-season turf. These fall N applications are the most important ones of the year as they set your turf up for future success by increasing root mass, which leads to increased carbohydrate storage. Overall, this carbohydrate and root mass increase leads to better turf resiliency in the summer months. Plants will draw on those fall carbohydrate storages as environmental conditions become strenuous in the summer months. The larger the root mass going into the summer, the better chance of maintaining acceptable turf through this summer stress period. 

While we know that fall is the best time to fertilize turf, the conventional thinking about the timing of fall nitrogen applications may not result in the strongest turf possible while keeping our environmental footprint as small as possible. The conventional thinking (and industry standard) is that N can be applied anytime from the beginning of September to the end of November. However, recent research from Dr. Doug Soldat et al. (2017), a turfgrass soil scientist at the University of Wisconsin-Madison, suggests that the fall N fertilization window is much smaller than previously thought. His novel research shows a direct correlation between temperature and N leaching potential. Based on this research, Dr. Soldat suggests that our N applications should end much sooner than our current industry norms. Late season N applications can lead to large losses of nitrates into the system, thus affecting water quality.

Based on the results of their research, Soldat et al. (2017) suggest that there is a need to reconsider when we are actually applying fall N.  Evapotranspiration (ET) is a key component of photosynthesis, and it is dictated by temperature. Nitrogen is used by cool-season grasses at a much higher rate during periods of high ET, i.e., temperatures above 60°F. During this period of growth, the leaching potential will decrease due to plants using the soil-available nitrogen as opposed to it being lost via leaching. The N demand of plants will, in turn, decrease as temperatures decline, while nitrate leaching potential will increase. Essentially, nitrogen has no place to go other than lost to the system when the turfgrass plants are not actively taking up nutrients from the soil. This is the critical moment when we need to be aware that N is vulnerable to losses, and we want to try to avoid making N applications. Our N efficiency drops off significantly and Soldat et al. (2017) quantifies these potential losses.

In Soldat et al. (2017), the researchers showed an increase in nitrate leaching from 35 percent of applied N at 50°F to 71 percent of applied N when temperatures were 32°F. In other words, approximately 65 percent of applied N was taken up by the plants at 50°F, while only 29 percent of applied N was taken up by the plants at temperatures 32°F. This is a significant difference in N use efficiency! Essentially we could be wasting 70 peercent or more of applied N if we wait to fertilize until sometime in November when temperatures are much cooler. Soldat et al. (2017) also reported no statistically significant effect between early or late fall N applications on spring green-up, chlorophyll content, clipping yield, shoot density, or root mass in the following spring. Since there is no practical or physiological benefit to fertilizing later in the fall, we strongly recommend making N applications much sooner in the fall, with plans to wrap up in late October (even though this is well in advance of the mandated Dec. 15 start of the no fertilizer blackout period). Earlier fall N fertilization will increase plant N use efficiency without any detriment to turf quality and reduce the potential for water quality issues related to nitrate leaching.

If you are looking for more information on the Nutrient Management Program or turfgrass fertility, please contact the nurient management team.