Key Takeaways:

• Researchers at the University of Connecticut have investigated how novel soil tests can guide nitrogen fertilization of creeping bentgrass fairways.
• Nitrogen mineralization potential of fairways are site specific and will affect the responsiveness of the fairways to fertilization.
• The mineralization potential and nitrogen responsiveness of fairways can be categorized using the new soil tests when compared with a standard fertilization program.
• Results from the tests are positively correlated to creeping bentgrass fairway quality and growth responses and show good potential to guide nitrogen fertilization.
• The tests were equally predictive for the nitrogen response of trafficked or non-trafficked creeping bentgrass fairways.

Soil Mineralization Potential and How It Affects Nitrogen Fertilization

Turfgrass responses to nitrogen fertilization are often obvious. However, when available soil nitrogen is near optimum or even beyond, it is harder to judge this visual response, or to know how long the response can be maintained without additional nitrogen. Often this is achieved through trial and error before a superintendent has a good sense of how a particular location on the course will respond. Superintendents should adjust their nitrogen rates according to how the site has responded to prior applications, the soil’s mineralization potential, and growing conditions.

Methods to determine soil mineralization potential have been developed but they can be expensive, entail long wait times for results, are only available from a limited number of laboratories, or are rarely used for turfgrass. The Solvita company has developed two test kits to measure the active carbon and nitrogen fractions in soil organic matter where results can be obtained in 24 hours—the Soil Carbon Dioxide (CO2)-Burst (CO2B) and Soil Labile Amino Nitrogen (SLAN) tests, respectively. There is increasing research evaluating these tests, particularly in agricultural and horticultural systems, and this will probably drive the demand for the tests to be a service provided by soil testing laboratories. Additionally, the kits are straightforward enough to be used on site by a golf course superintendent, without the need to send soil samples to a laboratory. Test kits are available from the company with instructions on how to perform the analyses and interpret the results. Unfortunately, there have been few studies investigating these tests in turfgrass or whether they can help guide fertilization.

What Do the Tests Measure?

The CO2B test measures the amount of CO₂ that is presumably emitted from respiration due to soil microbial activity and the breakdown of the easily degradable carbon fractions of soil organic matter at the end of a 24-hour incubation. The SLAN test measures the labile – i.e., active and easily degraded – amino-nitrogen forms in the soil organic matter that are released after a 24-hour incubation. Research at the University of Connecticut suggests that the SLAN and CO2B tests can categorize the probability of nitrogen fertilizer response of high-cut (3-inch) Kentucky bluegrass and tall fescue lawn turf (Moore et al., 2019 a and b).

Connecting Mineralization with Nitrogen Fertilization

The SLAN and CO2B tests could provide a cost-effective and convenient way for superintendents to determine the nitrogen responsiveness of fairways based on quantitative measures that are not subjective, like visual cues, and thereby provide a more reliable guide to fertilization. This will save money, provide better turf, and reduce the potential for nonpoint source pollution.

Research Study

USGA-funded research conducted at the University of Connecticut investigated the response of a creeping bentgrass fairway grown on a native sandy loam soil that received various rates of nitrogen. Treatments included:

• A compost/organic fertilizer applied at 0 to 2.25 pounds of nitrogen per 1,000 square feet per year (in 0.25-pound increments).
• A standard nitrogen treatment of 0.2 pound of nitrogen per 1,000 square feet as liquid urea approximately every 21 days during the growing season for a total of 2.25 to 2.5 pounds of nitrogen per 1,000 square feet per year.
   

At the start of the study, compost from the University of Connecticut Compost Facility (average total nitrogen and phosphorus concentrations of 0.86% and 0.29%, respectively; carbon:nitrogen ratio 9:1) was spread across the plots then incorporated into the top 4 inches of the soil profile by rototilling. After incorporation, the surface was smoothed and seeded with ‘13M’ creeping bentgrass. In the following three years, an organic fertilizer (Suståne 5-2-4) was applied every November or early December to respective plots at the designated rates. Half of each plot received simulated golf cart traffic treatment three times a week. Turf quality and growth (clippings yield) responses were measured every month from each plot and correlated to the plot’s SLAN and CO2B measurements. The research was conducted for three consecutive growing seasons (2019 through 2021).

Trafficked versus Non-Trafficked Bentgrass Response

Predictably, the non-trafficked plots had higher creeping bentgrass quality and growth responses than the trafficked plots, and bentgrass responses increased with increasing compost/organic rates. The midrange to higher compost/organic nitrogen rates produced results that were equal to or greater than responses of the standard nitrogen fertilization treatment, but equivalency varied a bit for each response measured. This was expected, and the results were like many previous studies for creeping bentgrass response to nitrogen fertilization. Our data suggest that fairway areas under trafficked conditions would need, on average, about 50%–60% more nitrogen to match responses of non-trafficked areas. This result is important for superintendents to consider given the increased play many golf courses are experiencing.

To obtain a probability-of-response guide, we compared the responses of the compost/organic fertilizer treatments to the responses of the standard nitrogen treatment, then constructed curves based on a statistical procedure for the probability of equaling or exceeding the standard response in relation to the concentrations of SLAN and CO2B. Our results across three consecutive growing seasons (May through November) compiled with all measured variables receiving equal weight and combined are shown in Figure 1.

Even though the quality and growth responses were all lower for the trafficked plots, the probability of quality and growth responses equaling or exceeding the response to the standard nitrogen fertilization treatment within each traffic treatment was nearly identical. Based on the curves, we set out to categorize the responsiveness of the creeping bentgrass fairway in relation to the SLAN and CO2B concentrations. Although the probability-of-response ranges for the categories are arbitrary, we think they represent useful working guidance and are summarized in Table 1.

We acknowledge that the above proposed fertilization guide is arbitrarily set within selected probability ranges, but superintendents can select their own course of action based on their tolerance of uncertainty. Using our proposed categories and recommendations, they can slide any fraction of their standard nitrogen rate up or down within the probability ranges based on their own experience or comfort with the chance of obtaining the expected result.

Another more quantitative approach to guide nitrogen fertilization of creeping bentgrass fairways based on SLAN and CO2B concentrations would be to use the actual probability values generated by the statistical software (that produced the curves in Figure 1) and apply them in the formula: Apply (1 – P) × standard rate of nitrogen, where P = probability of equaling or exceeding the response of the standard treatment. This results in a guidance of fertilization shown in Table 2.

How Many Soil Samples are Needed and When to Sample?

Using our dataset of nearly 1,400 samples, we estimate that a bulk sample of 5 to 10 samples every 1,000 square feet of the fairway would provide very high confidence and low margins of error for reliable results. We found both tests to be good predictors of mineralization potential and probability-of-response, and either should be a reliable guide. However, it was observed that the SLAN test had slightly lower variability (relative standard deviation 11%) versus the CO2B test (relative standard deviation 16%) and may be able to distinguish smaller differences between sampling locations.

If the fairway is not receiving consistent yearly applications of compost or an organic fertilizer, the SLAN or CO2B results will likely not change much from year to year. If this is the case, then a test at the beginning of the growing season and another at the end of the summer may be all that is needed to give an estimate of the mineralization potential for any fairway area associated with the representative soil sample. Our results showed that the average monthly CO2B concentrations were similar across the growing season, so sampling could occur anytime when there is active grass growth. Whereas the SLAN concentrations were highest in May through September, and this suggests sampling during these months to capture peak mineralization.

Using the Tests in Variable Rate Application Technology

The tests should allow for a quicker estimation and monitoring of the fairway soil’s mineralization potential and this information could be used in variable rate application technology in conjunction with the information provided in Table 1 or 2. GPS-linked and computer-controlled fertilizer spreaders and sprayers coupled with a fairway map of SLAN or CO2B concentrations, geo-referenced to specific locations throughout the fairway, will greatly improve the economics and efficiency for fairway fertilization. With 20 to 30 acres of fairways on most courses, this could result in substantial cost savings and more efficient use of labor associated with fertilization practices. GPS-controlled variable rate fertilizer applications are common with agricultural and horticultural systems and should be just as applicable to golf course fairways, although the costs for such equipment may be prohibitive for many golf courses at this point.

More demand for this equipment should bring the costs down and provide opportunities for more courses to consider its use. With the increasing availability of online apps and cloud services to generate maps of soil test results needed for GPS-linked application equipment, the SLAN and CO2B tests could markedly change the way nutrient management plans for fairways are developed and implemented. A simple field map of SLAN and CO2B results of the study plots is shown in Figure 2. The data show within the study area low and high soil test concentrations that then could be used to customize nitrogen rates if this represented a fairway. For those courses not using or considering variable rate applications, the SLAN and CO2B test results still provide objective guidance for conventional fertilization practices.

Final Thoughts

Our results suggest that creeping bentgrass fairways can be categorized with the SLAN and CO2B tests as to their responsiveness to nitrogen fertilization when compared with a standard nitrogen treatment response benchmark. This information can be used to guide fertilization. Our research demonstrated proof-of-concept validation for these tests, and they should be applicable across different locations, soils, climate and turfgrasses. The results can be developed into accessible guides that superintendents can use to adjust their nitrogen rates to creeping bentgrass fairways based on SLAN and CO2B test results. The test kits are also straightforward enough that the analyses could be conducted at the course without sending samples to a laboratory.

Although our research indicates a good potential for these tests to guide nitrogen fertilization of creeping bentgrass fairways, this needs to be validated with more research at different locations, soils, species and varieties, and different nitrogen rates. Our results can be used as a starting benchmark but then modified based on local conditions and responses. The tests offer promise for more objective guidance of fairway fertilization and more accurate site-specific nitrogen rates based on inherent mineralization potential and local conditions.

This research was funded by the Mike Davis Program for Advancing Golf Course Management and New England Regional Turfgrass Foundation.

References

Moore, D.B., K. Guillard, X. Geng, T.F. Morris, and W.F. Brinton. 2019a. Predicting cool-season turfgrass response with Solvita soil tests, Part 1: Labile amino-nitrogen concentrations. Crop Sci. 59:1779–1788. doi:10.2135/cropsci2018.11.0706

Moore, D.B., K. Guillard, T.F. Morris, and W.F. Brinton. 2019b. Predicting cool-season turfgrass response with Solvita soil tests, Part 2: CO₂-burst carbon concentrations. Crop Sci. 59:2237–2248. doi:10.2135/cropsci2018.11.0707