Fertilizers and the Lawn Care Program
The goal of any fertilization program is to compensate for soil nutrient deficiencies that are needed by the grass plant to sustain healthy growth and remain competitive against disease, insect, and weed invasion. While phosphorus (P) and potassium (K) are needed for a healthy lawn, it is nitrogen (N) that is required in the largest amounts by the grass plant.
New research results from the University of Minnesota and the University of Wisconsin-Madison necessitate updating our current lawn/turfgrass fertilizer recommendations. For the past 20 to 30 years, one of the more important fertilizer application times was considered to be the end of October and into early November in the Twin Cities and southern Minnesota. Indeed, lawns respond positively with good green color and active growth significantly earlier the following spring when given about 1 pound of nitrogen per 1000 square feet late in the previous growing season. This application came to be known as a late fall or more accurately a late season fertilization. In most years, this typically coincided with about the last mowing of year.
Even though this research was conducted on creeping bentgrass these findings demonstrate that nitrogen uptake late in the season is significantly less than when applied around Labor Day to the middle of September. So, what happens (or potentially can happen) to the remaining nitrogen not used or taken up by the grass plants? Other recent research on Kentucky bluegrass at Michigan State University points to increased leaching of N fertilizers when plants are not actively or efficiently growing which is often the case with the late season fertilization. Nitrogen can also be converted to a gaseous form and lost back to the atmosphere. In some situations it can even be lost through runoff, particularly when soils are frozen. Some may also be taken up by other landscape plants that happen to share the same rootzone as the turfgrass (e.g., trees and shrubs).
So, back to our original question, "If only a small portion of available nitrogen is utilized by the grass plant, what happens to the rest of it?" A more complete answer to that question rests with additional ongoing research. Nonetheless, available and unused nitrogen can pose additional environmental risks as noted above and be uneconomical for the user. After all, no one wants to be spending money on fertilizer and the labor to apply it if only a small fraction of that material is being utilized by the grass plant with the rest potentially being wasted.
It should be noted that results from the University of Minnesota soil testing lab may indicate significantly less nitrogen be applied on an annual basis depending on information provided about the care and use of the turfgrass area tested as well as the level of soil organic matter present. Leaving clippings on the lawn typically results in about one application of a complete fertilizer (i.e., a fertilizer containing nitrogen, phosphorus and potassium) annually back to the lawn. Hence, when clippings are returned, the amount of nitrogen needed is also reduced. The four options below assume clippings are returned to the lawn.
Likewise, areas with soil test levels of organic matter above 3.1 percent will also indicate a reduced need for nitrogen. As soil organic matter breaks down over time, there is some nitrogen released back to the soil that in turn becomes available for the grass plants to use. The soil testing lab considers that when suggesting annual amounts of nitrogen to apply.
Based on this current research, the following revised lawn/turfgrass fertilizer programs are suggested (Table 7.16). These are applicable to lawns and other turfgrass areas predominantly composed of Kentucky bluegrass with varying amounts of fine fescue and perennial ryegrass. Whenever possible be sure to take a soil test to determine how much fertilizer is needed. It should be noted that turfgrass fertilizer programs are constantly being refined based on analysis and interpretation of these and other research results. Hence, it is important to check back periodically to the U of MN Extension website for the latest in lawn fertilizer recommendations.
Table 7.16. Lawn / Turfgrass Fertilizer Schedule
|Lawn Care Level||Amount of N Fertilizer||Timing of Applications|
Irrigated average quality lawns - full sun conditions; soil organic matter >3.1%; clippings not removed
|2 lbs. N annually||At first mowing in spring: 0.5 lbs N per 1000 ft2
Memorial Day: 0.5 lbs N per 1000 ft2
Labor Day: 1.0 lb N per 1000 ft2
Irrigated average quality lawns - full sun conditions; soil organic matter <3.1%; clippings not removed
|2.5 lbs. N annually||At first mowing in spring: 0.5 lb. N per 1000 ft2 (50% slow release N)
Memorial Day: 0.5 lb N per 1000 ft2 (50% slow release N)
Labor Day: 1.0 lb. N per 1000 ft2 (50% slow release N)
Non-irrigated, average quality lawns - full sun to lightly shaded conditions; soil organic matter >3.1%; clippings not removed
|1.5 lb. N annually||Memorial Day: 0.5# N per 1000 ft2 (50% slow release N)
Labor Day: 1.0# N per 1000 ft2 (50% slow release N)
On occasion, an additional application of N to enhance growth and color may be needed: Apply at the rate of 0.5 lb. N per 1000 ft2 (50% slow release N; often this will be in late June to early July especially if weather conditions prior to that period have produced abundant and frequent rainfall)
Non-irrigated, average quality lawns - full sun to lightly shaded conditions; soil organic matter <3.1%; clippings not removed
|1 lb. N annually||Labor Day: 1.0 lb. N per 1000 ft2 (50% slow release N)
On occasion, an additional application of N to enhance growth and color may be needed: Apply at the rate of 0.5 lb. N per 1000 ft2 (50% slow release N; often this will be in late May to early July especially if weather conditions prior to that period have produced abundant and frequent rainfall)
Adapted from Home Lawn and Landscape Turfgrass Fertilizer Recommendations Being Revised to be More Environmentally Sensitive. University of Minnesota Extension, Yard and Garden News, August 1, 2010.