Determining Economical and Efficient Nitrogen Application Strategies for Sod Production Ben Wherley, PhD Texas A&M University Department of Soil and Crop Sciences Background The recent downturn in sod sales combined with escalating fertilizer prices has caused sod producers to scrutinize their current practices in while identifying the most efficient and costeffective cultural programs. Sluggish sales and delayed turnover of production fields has necessitated greater efficiency in use of all resources, including nitrogen (N). A 2005 survey of Texas sod producers found that ‘cost of production and high input prices’ was identified as the one of the greatest problems facing the industry. In addition to being an economic concern, efficient use of N is also important from an environmental standpoint, as improperly timed or excessive N applications may move off-site into ground or surface waters. Given the relatively diminished turnover in production fields, long growing season, and limited resources with which to operate, Texas sod producers could greatly benefit from research that seeks to identify how to best allocate fertilizer resources. Frequent broadcast applications of quick release N sources such as ammonium sulfate or urea have been a common practice in the past, but less-frequent applications of more costly controlled release N sources or lower N application rates may be a more resource-efficient, cost cutting strategy during times of diminished demand. Additionally, application strategies that maximize plant uptake and minimize runoff or gaseous loss, whether through banding applications to ribbons or through soil-injection, offer potential for even greater fertilizer use-efficiency. Questions Some of the questions we were attempting to answer with this research were: 1) How low can we go? What is the impact on sod re-growth, quality, and tensile strength at harvest when N inputs are reduced to 75% or 50% levels? 2) Urea vs. Ammonium sulfate: Pound per pound of N, which offers the best ‘bang for the buck’ for sod regrowth? 3) Slow vs. quick release: Is there any benefit to slow release fertilizers? 4) Soil injection vs. broadcast granular: How does a single, high rate liquid soil injection compare to multiple granular broadcast applications? 5) Ribbon banding: Can fertilizer be applied only to ribbons for a time following harvest without delaying re-growth? Methodology This study was conducted from May through November 2012, at Horizon Turfgrass Farm, near Somerville, TX. Studies were carried out on recently harvested fields of both ‘Raleigh’ St. Augustinegrass and ‘Tifway’ bermudagrass (Figure 1). Following an April harvest, replicated 25 ft2 treatment plots were laid out in a factorial design which included combinations of N source, N rate, and N application method. Initial soil testing of soil within the plot area revealed difficient

levels of phosphorus, so plots were amended with recommended levels of P, K, as well as micronutrients prior to initiating the study. Treatments were replicated four times and consisted of testing quick release fertilizers urea (46-0-0) and ammonium sulfate (21-0-0) as well as slow release sulfur-coated urea (24-0-11). Granular N rate treatments included testing of 100% (60 lb. N/A), 75% (45 lb. N/A), 50% (30 lb. N/A), and 0% (unfertilized control plots) rates, applied on 6 week intervals from May through October. ‘Ribbon-banded’ treatments received the 100% application rate, but fertilizer was applied directly to a 1 ft. strip of ribbon, rather than to the entire plot, during the initial 3 of 5 applications, resulting in a 25% reduction in N use through the study. ‘Soil-injected’ fertilizer treatments (Figure 2) were injected on 9” centers to a depth of 8” once at the beginning of the study at a rate of 85 lbs. N per acre (100% rate) or 42.5 lbs. per acre (50% rate). A summary of the fertilizer methods and total rates applied is shown (Figure 4). No pre-emergence herbicides were applied at any time during the study. Plots were mowed and irrigated according to Horizon Turfgrass Farm’s standard practices for promoting optimal quality and recovery of plots. Evaluations Every six weeks over the course of the growing season (5 dates), visual assesment of percent green cover (0 to 100%) as well as color (1-9, 5= minimally acceptable) was made within all plots. Additionally, digital images were taken within plots using a light box. These images were analyzed using Sigma Scan to determine change in percent cover within plots. At the conclusion of the study in late November, plots were havested using a professional sod harvester. Slabs of sod from each plot were evaluated for their relative tensile strength using a custom built sod tesile strength tester with loadcell and display that registered maximum force during tearing of the sod. While bermudagrass sod was easily harvested, St. Augustinegrass sod was not ready to be harvested at the completion of the study. Study Findings Differences in turf color in bermudagrass occurred due to the fertilizer treatments. Overall color ranged from 6.6/9 (100% urea) to 3.5/9 (unfertilized). With the exception of 50% injected and unfertilized plots, all N sources and rates produced acceptable (≥5) bermudagrass turf color during the study (Figure 5). The most superior color was achieved at the 100% rates of urea and sulfur coated urea, which resulted in mean ratings 6.6 and 6.4 out of 9, respectively during the study. A general decline in color was observed with decreasing N rate. St. Augustinegrass exhibited much more plot-to-plot variability in color (Figure 6), and there were no significant differences in color among all sources and rates, with the exception of the unfertilized control. All but the unfertilized control plots produced acceptable (≥5) color during the study; however, there were no statistical differences between any of the sources and rates with regard to their effects on color. Fertilizer source and rate effects on percent green cover of plots is arguably of greatest importance in terms of fertilizer efficiency to producers. Bermudagrass responded well to N fertilizer inputs, as final percent green cover ranged from as low as 65% (unfertilized) to as high as 90% (urea 100%) by sod harvest in November (Figure 7). In St. Augustinegrass, final green cover ranged from 90% (urea 100%) to 50% (unfertilized), and again, there was considerably more variation in the data than with bermudagrass (Figure 8). Interestingly, in terms of color

response, both quick release sources urea and ammonium sulfate outperformed sulfur coated urea, although the differences were not statistically significant. Soil injection (at 100% levels) resulted in significantly lower final cover in St. Augustinegrass (32% decrease) than bermudagrass (8% decrease). This is probably due to the lack of active roots early on, per unit area of St. Augustinegrass, which is recovering only from stolons. Conversely, bermudagrass would have a greater presence of viable rhizomes and roots remaining in the soil after harvest, and be capable of greater utilization of the injected N fertilizer. At the conclusion of the study, following 7 months of growth, bermudagrass plots were harvested and tested for tensile strength (Figure 9). Based on the results of this testing, sulfur coated urea produced noticeably greater sod tensile strength (~45 kg force required to shear), regardless of application rate (100%, 75%, or 50%) relative to all quick-release N sources (~3040 kg force required to shear). This suggest that slow-release sources of N, while perhaps not stimulating as rapid top growth recovery, may provide a more complete feeding that promotes greater allocation of resources to production of rhizomes, and roots, resulting in greater tensile strength. Our results also showed that soil-injection resulted in noticeably lower sod strength. The lower strength of this treatment was apparently due to the slicing created through rhizomes by the injection unit during the May fertilizer application. It should be noted that this loss of strength did not affect the ability of the sod to be loaded onto or off of the pallet during testing. It should also be noted that all fertilized plots blocked and handled well, regardless of treatment. Unfertilized plots had to be handled with extreme caution.

How low can we go? Based on our results, there is certainly a benefit to minimal rates of N applied for promoting recovery of both bermudagrass and St. Augustinegrass (relative to unfertilized plots). However, reducing N inputs from 100% (300 lbs. N/A) to 50% (150 lbs. N/A) levels in both species resulted in only a 10-15% decrease in rate of recovery. This may represent an acceptable level of reduction for short-term periods where sluggish sales result in delayed turnover of sod fields. How this response would change following multiple years of reduced N inputs is not known. Urea vs. Ammonium Sulfate In terms of color and recovery, urea slightly outperformed ammonium sulfate in this study, although differences were not always statistically significant. For many areas of Texas were high pH soils present potential for volatilization from urea, a similar response may not be expected. However, it should be noted that volatilization losses are more likely to occur where turfgrass thatch layers exist. Minimal and to no thatch was present in these plots for this initial year following harvest, and fertilizer was watered into soil soon after application.

Slow vs. Quick Release In terms of this study, where fertilizer was supplied every 6 weeks, there was no clear benefit to using slow release N in terms of promoting color or recovery. However, in terms of sod tensile strength at harvest, SCU significantly enhanced sod tensile strength.

Soil Injection vs. Broadcast Granular The single soil injection of 85 lbs. N/A at the beginning of the study represents significantly less N than was applied through either the 100% (300 lbs. N/A) or 50% (150 lbs. N/A) broadcast applications, yet this method provided excellent results that outperformed the 50% granular and was nearly equivalent in some cases to 100% N rates. This suggests that considerably greater efficiency of N utilization results from this method, possibly due to decreased volatilization. The rhizomatous bermudagrass responded much better to the injection method of fertilization than did the stoloniferous St. Augustinegrass. It should also be noted that these soils were of relatively fine texture, and similar results may not be expected on coarser soils where leaching is likely. Ribbon banding vs. Broadcast Application Ribbon banding fertilizer to St. Augustinegrass during the initial 3 of 5 fertilizer applications led to a 25% savings in total N use, but also resulted in a 15% decrease in final percent cover by the end of the season. This technique offered some N savings, but the resulting decline in percent green cover may be more than desired. While this technique may offer greater precision in fertilizer application, it may be difficult, in practice to implement. Ribbons were fertilized by hand with a shaker jar in this study. Acknowledgement We wish to thank the Turfgrass Producers of Texas for their generous and continued support of turfgrass research at Texas A&M University.

Figure 1. Recently harvested area of Tifway bermudagrass being strung out for research plots in the fertilizer study.

Figure 2. Soil injection of liquid N performed early on in the recovery phase. Injection was to an 8” depth on 9” centers.

Figure 3. Fertilizer treatment application rates and dates.

Figure 4. Harvesting of sod from treatment plots at the conclusion of the study.

Figure 5. Seasonal average Tifway bermudagrass color in response to N fertilizer treatments. Error bars denote standard error of the mean.

Figure 6. Seasonal average Raleigh St. Augustinegrass color in response to N fertilizer treatments. Error bars denote standard error of the mean.

Figure 7. Seasonal change in Tifway bermudagrass percent green cover in response to the N fertilizer treatments over the course of the study.

Figure 8. Seasonal change in Raleigh St. Augustinegrass percent green cover in response to the N fertilizer treatments over the course of the study.

Figure 9. N treatment effects on Tifway bermudagrass sod tensile strength at the conclusion of the study.