Foliar factors do not flummox these facile faculty

 Editor's note: This post is the  “Verdure” column by Beth Guertal, Ph.D., as seen in the October 2014 issue of GCM.

 Foliar fertilization is widely used in the green industry to apply a small amount of nutrients directly to the leaf surface. One of the problems, however, in studying foliar fertilization is the methods by which we can measure nutrient uptake into the leaf. A long-used method (specifically for nitrogen) uses labeled nitrogen (¹⁵N), which provides a tracer that enables the scientist to follow the path of the nitrogen through the plant. The problem with ¹⁵N is that it is very expensive, it’s often hard to find labeled product, and it requires specialized equipment not lying around everyone’s laboratory. Regardless, it is a pretty accurate and standard method for measuring nitrogen uptake by plants, and it is also an excellent way to keep a Ph.D. student occupied for a great part of his or her academic student experience.

For this research, that student was a member of a group headed by Bruce Branham, Ph.D., at the University of Illinois. The objective was to answer some of the questions often posed by superintendents about foliar nitrogen fertilization: How quickly is the applied nitrogen taken up (absorbed) by the leaf? How does spray volume affect this uptake? And what happens when I add other things to my spray tank?

These questions were answered via a series of studies conducted on a stand of Pennlinks creeping bentgrass that was maintained at a cutting height of 0.5 inch (1.3 centimeters). For most of the studies, ¹⁵N labeled urea was the nitrogen source, but in two cases, labeled ammonium sulfate and calcium nitrate were also added to the studies.

The first two studies looked at the impact of spray volume, with spray volumes of 20, 40, 60, 80 or 100 gallons/acre (190, 375, 560, 750 or 935 liters/hectare). The second two studies examined the nitrogen sources: urea, ammonium sulfate and calcium nitrate. The third set of studies examined the inclusion of other things in the tank mix, including chlorothalonil (4.5 kilograms a.i./hectare), trinexapac-ethyl (0.1 kilogram a.i./hectare), a biostimulant, an indicator dye and, finally, all of those together (the kitchen sink treatment). The last set of experiments evaluated four adjuvants: a nonionic surfactant, an organosilicate adjuvant, methylated seed oil and a crop oil concentrate.

In general, many of the factors that superintendents think about when applying foliar nitrogen had no impact on nitrogen uptake in this study. Across all the experiments, from 6 to 34 percent of foliar-applied nitrogen was taken up by the bentgrass leaf. This nitrogen uptake often occurred within the first two hours after application, and it was always achieved within six hours after application. The type of adjuvant, nitrogen source or materials added to the tank mix had no effect on nitrogen uptake, even when the fungicide, growth regulator, dye and biostimulant were all added together with the fertilizer.

The authors recognized that their measured nitrogen uptake numbers were lower than those found by other researchers (who often measured nitrogen uptake in the 50 percent range). Such differences could be attributed to many factors, including location (a lot of the earlier work was not done outside, but in a greenhouse), relative humidity, air and soil temperatures, and nozzle droplet size.

The only treatment that affected nitrogen uptake was the spray volume. Once the spray volume increased above 40 gallons/acre (375 liters/hectare), nitrogen uptake decreased, from an average of 14.9 percent at 40 gallons/acre to 11.3 percent nitrogen uptake (when the spray volume was 60 or 80 gallons/acre) to 6.2 percent nitrogen uptake when the spray volume was 100 gallons/acre. The authors concluded that if foliar nitrogen uptake is the objective, spray volumes should be kept as low as practical to maximize foliar nitrogen uptake. Further, the authors concluded that by bypassing the soil and introducing some of the nitrogen directly into the plant, foliar nitrogen applications are more efficient than traditional soil application.

 Source: Henning, S.W., R.L. Mulvaney and B.E. Branham. 2013. Factors affecting foliar nitrogen uptake by creeping bentgrass. Crop Science 53:1778-1783.

Beth Guertal, Ph.D., is a professor in the department of agronomy and soils at Auburn University in Auburn, Ala., and the incoming editor-in-chief for the Agronomy Society of America. She is a 17-year member of GCSAA.