Foliar uptake of nitrogen on creeping bentgrass and bermudagrass greens

Creeping bentgrass and hybrid bermudagrass show similar uptake of foliar-applied nitrogen.

Chris Stiegler, Ph.D.; Mike Richardson, Ph.D.; and Doug Karcher, Ph.D.

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August 2013 Foliar uptake research: main

Foliar fertilization has become an increasingly common means
of providing supplemental nutrition to turfgrass on golf courses.
Photos by M. Richards

Foliar fertilization refers to the process of nutrient uptake through the leaves and is often used to deliver nutrients during periods when root uptake may be limiting. Anything that restricts root growth or function can potentially lead to reduced nutrient uptake, even in a nutrient-rich environment. Applying essential elements directly to the plant foliage can effectively bypass deficiencies associated with the roots or any soil issues that might prevent nutrient uptake. Foliar fertilization also provides turfgrass managers with increased flexibility, precision and convenience of application compared with traditional granular fertilizer application methods.

Foliar fertilization is an increasingly common supplemental nutritional practice in today’s golf course management. Recently conducted surveys of golf course superintendents indicated that almost all of those responding use foliar fertilization somewhere within their nutrient management programs and, in many cases, foliar fertilization often makes up a major portion of the total annual nitrogen inputs to putting greens (J.C. Stiegler, unpublished data). Because foliar fertilizer is routinely applied to intensively managed areas of the golf course, it is particularly important, from an efficiency standpoint, to obtain a greater understanding of the time it takes for applied fertilizer nitrogen to be absorbed by the turfgrass foliage.

August 2013 Foliar uptake research: trials

The research plots were located at the University of Arkansas Agricultural Research and Extension Center in Fayetteville, Ark.

Although various researchers have examined the practical aspects (growth and color response) of foliar- and liquid-applied fertilizers (7,8,10,11), few studies have investigated foliar nutrient uptake dynamics or efficiency and only recently have scientists addressed this topic under field conditions (6,9). The majority of the earlier foliar nutrient uptake data in turf looked at nitrogen absorption by cool-season turfgrass leaves grown in controlled, moderate temperature environments (3,4,5,12). The previous studies have demonstrated that 30% to 60% of the nitrogen applied can be absorbed by the leaves. Although these basic studies have made a significant contribution, more research is needed to improve foliar nutritional strategies for superintendents who wish to maximize plant uptake and reduce losses to the environment.

August 2013 Foliar uptake research: holes

A cup cutter was used to remove turfgrass plugs from the research plots at various intervals after foliar fertilization in order to determine how much of the fertilizer had been absorbed and how quickly it had been absorbed.

Studies to evaluate foliar absorption of nitrogen in the field would be beneficial to understand how seasonal environmental conditions might affect this practice. Previous agricultural crop research has demonstrated that environmental factors and seasonal dynamics of leaf cuticle characteristics can influence the foliar absorption of nitrogen solutions (1,2). The goals of the present study were to directly quantify the rate of foliar absorption of urea nitrogen by putting green turf under field conditions and to determine the effect of season on the uptake efficiency of foliar nitrogen fertilizer.

Experimental area and nitrogen treatments

This study was conducted at the University of Arkansas Agricultural Research and Extension Center in Fayetteville, Ark., on both Penn A-1 creeping bentgrass (Agrostis stoloniofera L.) and TifEagle hybrid bermudagrass (Cynodon dactylon × C. transvaalensis) putting greens maintained according to putting green management practices typical for the region. Mowing was performed six days per week at a 0.125-inch (3.175-millimeter) cutting height.

Plant-available phosphorus and potassium levels in the root zone were tested at the beginning of the study and supplemented accordingly. A mixture of quick- and slow-release nitrogen sources was used to provide the creeping bentgrass a base fertility program of 4 pounds nitrogen/1,000 square feet/year (19.52 grams/square meter); the bermudagrass area received 6 pounds nitrogen/ 1,000 square feet/year (29.29 grams/square meter). Routine maintenance fertilizer applications were not made during the same week that foliar nitrogen uptake studies were conducted.

August 2013 Foliar uptake research: plot

Plots were fertilized at a high and a low rate of nitrogen, but rates of nitrogen uptake efficiency were significantly lower in turf receiving the higher rate.

Because urea is one of the most common forms of nitrogen included in foliar fertilizer products, it was chosen as the nitrogen source for this foliaruptake field trial. Throughout the two years of the study, urea enriched with a heavier isotope of nitrogen (15N) was used for fertilization to allow precise measurement of nitrogen uptake in the leaves. During the 2007 and 2008 growing seasons, foliar urea-nitrogen with 15N-labeled urea was applied once a month, May through September, to 2-foot × 4-foot (0.60 meter × 1.21-meter) plots with 1-foot (0.30-meter) borders. Foliar nitrogen was applied at 50 gallons/acre (467.69 liters/hectare) with the aid of a spray shield and a single-nozzle CO2-pressurized sprayer. Spray treatments were made at approximately 7:30 a.m. after the experimental areas were mowed. Rates of 0.1 and 0.25 pound nitrogen/1,000 square feet (0.5 and 1.25 grams/square meter) were used and designated as a low and high rate, respectively. Plots received no irrigation or rainfall for a 24-hour period after treatment so that all nitrogen absorption was limited to foliar uptake.

Tissue collection and processing

Immediately before and then after nitrogen application, two uniformly sized — 4.25-inch (10.8-centimeter) diameter — turfgrass plugs were randomly taken with a standard golf course putting green cup cutter at 0-, 1-, 4-, 8- and 24-hour intervals, to assess nitrogen uptake over time. A procedure was developed to precisely remove a uniform, 0.125-inch (0.3175-centimeter) thick sample of verdure from the top portion of each sampling core obtained during field sampling. Total nitrogen was determined for turfgrass plant leaf and shoot tissue.

August 2013 Foliar uptake research: figure 1

The experimental design was a randomized complete block within each turfgrass species. Treatment factors included nitrogen rate and year, with sampling time after application and month of year added as repeated measures. All treatments were replicated four times.

Foliar uptake of nitrogen

Foliar absorption of urea by both species occurred rapidly and followed a curved pattern of uptake (Figure 1). Absorption was greatest between time zero and one hour after application, thereafter leveling off and approaching a numerical maximum by 24 hours. This was consistent for both species and demonstrates the effectiveness of foliar application to quickly supply nitrogen to the turf. Based on the amount absorbed at 24 hours, within the first four hours, creeping bentgrass absorbed 83% and bermudagrass absorbed 94% of the urea. Bermudagrass foliar uptake of nitrogen peaked at four hours after application (Figure 1), but foliar nitrogen continued to be absorbed by leaves of creeping bentgrass up until the last sampling period of 24 hours after treatment (Figure 1).

Absorption of foliar-applied nitrogen is a complex process (diffusion and/or mass flow) that is governed by time, solution and molecular characteristics, along with various other environmental and leaf surface factors. General principles dictate that foliar uptake should occur as long as the urea solution remains on the leaf surface. However, from an agronomic perspective, for a superintendent wishing to maximize foliar uptake of ureanitrogen on creeping bentgrass greens, delaying necessary management practices (for example, syringing greens, etc.) for longer spans of time (for example, 24 hours) in an effort to obtain an extra 9% to 10% of nitrogen from a light foliar application would not be practical or warranted.

August 2013 Foliar uptake research: figure 2

For both creeping bentgrass and bermudagrass, absorption of urea at 0.25 pound nitrogen/1,000 square feet (1.25 grams/square meter) (high rate) was less efficient than that at 0.1 pound nitrogen/ 1,000 square feet (0.5 gram/square meter) (low rate) when expressed as a percentage of applied nitrogen. When combined across all sampling times, application months and years, the low nitrogen rate treatments averaged 50% nitrogen uptake efficiency on creeping bentgrass, whereas the high nitrogen rate averaged 44%. Absorption by bermudagrass was similarly affected by nitrogen rate, most notably during June and August 2008 (Figure 2).

There are no published data on the effects of nitrogen rate on foliar absorption by turfgrasses, and we can only speculate about potential reasons for reduced fertilizer uptake efficiency at higher nitrogen rates. The significant rate effect could be due to differences in precipitation of nitrogen out of solution. As spray droplets dry on the foliage, the higher nitrogen rate should precipitate earlier, meaning that a smaller percentage of the urea remained in solution and, presumably, was able to be absorbed. It is also possible that the more concentrated urea spray caused minor epidermal cell damage that may have affected the uptake process; however, visual phytotoxicity was not observed in any of the experiments.

It should be noted that a greater total amount of fertilizer nitrogen (0.11 pound nitrogen/1,000 square feet vs. 0.05 pound nitrogen/1,000 square feet) (0.537 gram/square meter vs. 0.244 gram/ square meter) was recovered within creeping bentgrass plant tissue when nitrogen was applied at the higher rate; the amount of nitrogen recovered was, however, a significantly smaller percentage of the amount applied.

Effect of season on uptake of foliar nitrogen

Significant effects of time (both month and year) on foliar absorption of urea-nitrogen were found for creeping bentgrass (Figure 3), and bermudagrass (Figure 2). Fertilizer nitrogen recovered in creeping bentgrass plants was quite variable depending on month of application and year (Figure 3). Although the range of nitrogen recovered in creeping bentgrass was similar (36%–59% in 2007; 38%–69% in 2008) between years, trends based on month of application were markedly different.

August 2013 Foliar uptake research: figure 3

In 2007, there was a significant decline in absorption efficiency as the season progressed (Figure 3), dropping steadily from 59% in May to 37% in September. In contrast, absorption efficiency in May 2008 was relatively low at 38%; it peaked in July at 69%, and then declined to 45% in August and 47% in September. These data suggest that there will be times of the year when uptake efficiency is reduced, but it is not as simple as attributing these reductions to month of application within a given year. Many factors (both meteorological and physical) will likely influence foliar nitrogen uptake efficiency. However, analysis of select environmental conditions measured on site during each 24-hour sampling period failed to reveal any consistent trends to help explain the variability in the data (data not shown). For example, relative humidity is known to affect foliar uptake of nutrients by its influence on spray droplet retention and through its effects on hydrating the leaf cuticle for increased receptiveness to foliar absorption. However, relative humidity values taken at each application event did not consistently correlate with foliar nitrogen uptake efficiency values obtained from bermudagrass and creeping bentgrass (Figures 2, 3) throughout the two-year field trial.

Recently, other researchers (6) used an indirect (rinsate-based) method to estimate foliar uptake of ammonium nitrogen and nitrate nitrogen by putting green turfgrasses. They also found absorption differed significantly between months, and they attributed these differences to changes in ambient air temperature.

In our study, average air temperature during application dates in 2008 exhibited a bell-shaped curve from May through September, which coincided with foliar nitrogen absorption efficiency patterns observed on creeping bentgrass during that year (Figure 3). However, comparisons of ambient air temperatures and foliar absorption by creeping bentgrass during 2007 showed no such relationship. In this study, analysis of environmental conditions and meteorological data recorded on site failed to show a strong relationship between any single environmental factor and foliar nitrogen absorption efficiency.

Conclusions

Both creeping bentgrass and bermudagrass golf course greens are capable of rapidly absorbing urea-nitrogen applied to the foliage. Absorption efficiency is similar to that reported in previous studies performed on turfgrasses grown in controlled environments. Most of the foliar-applied urea-nitrogen was absorbed in the first four hours after application, with the greatest increase in fertilizer nitrogen within leaves and shoots occurring between time of application and one hour after application.

As a supplement to traditional root-feeding practices, foliar fertilization has become an important component of putting green nutritional programs. Based on our results, we make the following recommendations for greatest efficiency: use low application rates (0.1 pound nitrogen/1,000 square feet) (0.5 gram/square meter); wait several hours to maximize foliar uptake and then water-in, washing any unabsorbed urea-nitrogen or ammonium-nitrogen remaining on leaf surfaces into the soil or root zone for second-chance uptake by the root system.

Future research should focus on the effects of certain environmental influences, leaf cuticle characteristics and/or cultural practices on foliar nitrogen absorption. These studies could lead to the development of optimized protocols for turfgrass practitioners to improve the efficiency of foliar fertilizer applications.

Funding

Financial support of this research was provided by the O.J. Noer Foundation, the USGA Green Section and the University of Arkansas, Division of Agriculture.

Acknowledgments

This research was originally published as “Foliar nitrogen uptake following urea application to putting green turfgrass species” by J. Chris Stiegler, Michael D. Richardson and Douglas E. Karcher, Crop Science 2011 51(3):1253-1260 (doi:10.2135/ cropsci2010.06.0377).

Chris Stiegler and his wife, Jenny, were killed in a car accident on Christmas Eve, 2010. The Chris Stiegler Graduate Student Travel Award has been established in the C5 Division of the Crop Science Society of America to support yearly grants to deserving graduate students. For information about supporting this effort, visit: http://turfgrassscience.wordpress. com/2013/06/17/chris-stiegler-turfgrass-science-studenttravel- award/.

Literature cited

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Chris Stiegler carried out this research as a graduate student at the University of Arkansas. At the time of his death, he was an assistant professor in the soil and crop sciences department at Texas A&M University, College Station, Texas. Mike Richardson is a professor and Doug Karcher is an associate professor in the department of horticulture at the University of Arkansas, Fayetteville.