Tyler Carr
Assistant Professor and Turfgrass Extension Specialist
Introduction
Turfgrass managers aim for consistent turf growth and color. Nitrogen fertilization is typically used to achieve these goals, but frequent applications of quick-release nitrogen (like urea or ammonium sulfate) can result in uneven growth and color. While “spoon feeding” nitrogen—applying small amounts frequently—can address this, it’s impractical for large turf areas due to high labor costs and the limitations of typical lawn care service schedules. Slow-release fertilizers offer a solution by gradually releasing nitrogen, reducing the need for frequent applications and minimizing the growth spikes associated with quick-release sources.
Slow-release fertilizers reduce the frequency of fertilizer applications per year. However, it remains unclear whether turfgrass can maintain consistent growth and quality with only one or two applications annually compared to four applications of quick-release fertilizers.
Materials and Methods
This experiment was initiated in May 2023 on a mature Kentucky bluegrass lawn and continues through May 2025. The experimental area was mowed at 2.5 inches weekly, with clipping returned. Irrigation was not provided except for four fertilizer applications per year, each followed by a 0.25-inch watering to incorporate the product.
Four nitrogen sources were applied were applied at 2 lb N/1000 ft2/year according to the following structure:
| Source | Rate (lb N/1000 ft2) | Timing |
| Std. control – Urea (46-0-0) | 0.5+0.5+0.5+0.5 | May+July+September+October |
| Urea (46-0-0) | 2 | May |
| Urea (46-0-0) | 2 | September |
| Urea (46-0-0) | 1+1 | May+September |
| Polyon 44 (44-0-0) | 2 | May |
| Polyon 44 (44-0-0) | 2 | September |
| Polyon 44 (44-0-0) | 1+1 | May+September |
| Polyon 42 (42-0-0) | 2 | May |
| Polyon 42 (42-0-0) | 2 | September |
| Polyon 42 (42-0-0) | 1+1 | May+September |
| Nature Safe (13-0-0) | 2 | May |
| Nature Safe (13-0-0) | 2 | September |
| Nature Safe (13-0-0) | 1+1 | May+September |
Evaluations:
- Turfgrass color is assessed every two weeks using digital image analysis
- Turfgrass height is measured every two weeks using a Turf Height Tester
Results
Turfgrass color
2023
2024
- All spring applications had color similar to or greater than the standard urea treatment.
- Applying 2 lb N/1000 ft2 in a single application resulted in the greatest magnitude of turfgrass color
- Sources differed in their duration of color
- Fall applications in 2023 generally improved color through mid-April 2024.
Turfgrass height
2023
2024
- In 2023, spring treatments at higher nitrogen rates resulted in the greatest magnitude of growth. This growth could be thought of as excessive.
- 1 lb N/1000 ft2 treatments in both May and September provided more consistent growth and generally were similar to the standard urea control.
- Fall applications resulted in similar growth in the fall, but had greater growth in the spring.
Conclusions
Current data suggests that a single 2 lb N/1000 ft2 of any of the fertilizer sources tested in either spring or fall will likely not provide consistent turf color or growth. However, 1 lb N/1000 ft2 in both May and September can reduce fluctuations in color and growth, while providing labor benefits over four applications of urea at 0.5 lb N/1000 ft2. Also, since some of the slow-release fertilizers elicit a turf response later than others, custom blends of quick- and slow-release sources can provide more consistent color and growth.