Turfgrass fertilization sometimes seems to have devolved into two camps: the foliar folks and the granular gang.
So it’s always nice when you find a research paper that examines those two methods for nutrient delivery. Recently, turfgrass researchers at Michigan State University used Penn A-4 creeping bentgrass putting greens to examine various combinations of granular and foliar fertilizers, and to determine the fertilizers’ effects on turfgrass quality and soil nutrient status.
The six fertilizer treatments were: 1) granular organic fertilizer (soy protein and blood meal) (10N-2P-4K); 2) granular methylene urea (40N-0P-0K); 3) granular urea (46N-0P-0K); 4) foliar nitrogen (N) (18N-3P-4K) applied at 8 ounces product/1,000 square feet (2.5 milliliters/square meter), the recommended labeled rate for the product; 5) the same foliar N product applied at 16 ounces/1,000 square feet (5.0 milliliters/square meter); and 6) a granular and foliar fertilizer combination.
All the granular products were applied at 0.5 pound N/1,000 square feet (2.4 grams/square meter) (per month, May-October) to supply a total of 3 pounds N/1,000 square feet (14.7 grams/square meter) per year. The foliar products were applied twice weekly, applying 0.125 or 0.25 pound N/1,000 square feet per application (0.6 or 1.2 grams/square meter) to supply a total of 1.5 and 3 pounds N/1,000 square feet (7.4 and 14.7 grams/square meter) per year for the low and high rates, respectively.
Finally, the foliar/granular combination treatment had a total of 3 pounds N/1,000 square feet per year applied, with half that N applied as split monthly granular treatments, and the remaining 1.5 pounds N divided as twice-monthly foliar applications (at 0.125 pound N/1,000 square feet per application).
So, with the exception of the lowest rate of the foliar-only treatment (which received only 1.5 pounds N/1,000 square feet/year), all the other treatments received 3 pounds N per year.
Last, there was an unfertilized control treatment. All of this was done on two different putting green mixes: a native sandy clay loam root zone and a USGA-type root-zone mix.
All of these various treatments also applied different rates of phosphorus (P) and potassium (K). Confounding nutrient application with mixed fertilizers is always an issue in fertilizer studies. In this case, the extra applied nutrients were not adjusted to uniformity across the various treatments and were simply applied as supplied in the various fertilizers. Even with these different rates of P and K addition, there were no increasing or decreasing trends in soil P or K over the length of the study, even in the treatments that consistently supplied P or K.
The quality of turfgrass growing in the native soil was almost always better than that of the bentgrass growing in the USGA-type greens mix. The fertilizer treatments affected turfgrass quality in every year of the three-year study. However, there were some turf quality differences between the treatments from year to year.
In the first year, quality was consistently highest in bentgrass to which granular urea had been applied. As the summer progressed, plots that received methylene urea, the foliar/granular mix or the higher rate of foliar fertilizer, also had similarly high quality.
As the second and third years of the study were completed, bentgrass that had received the twice-monthly applications of the foliar/granular combination or the high rate of foliar fertilizer consistently had higher quality than that observed in bentgrass that had received monthly applications of urea, methylene urea or organic fertilizer.
So, for highest turfgrass quality, the key was applying a higher rate of foliar fertilizer or a foliar/granular application every two weeks rather than monthly applications of granular products. And the lower rate of the foliar product, which was also applied twice monthly? Well, even though it did not produce turf of the highest quality, it did produce bentgrass of acceptable quality and was thus considered a successful treatment.
Source: Xiao, M., K.W. Frank and T.A. Nikolai. 2018. Foliar and granular effects on creeping bentgrass and soil nutrient levels. Crop, Forage and Turfgrass Management 4:170539 doi:10.2134/cftm2017.05.0039
Beth Guertal, Ph.D., is a professor in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., and the 2019 president of the Crop Science Society of America. She is a 20-year member of GCSAA.