SportsTurf provides current, practical and technical content on issues relevant to sports turf managers, including facilities managers. Most readers are athletic field managers from the professional level through parks and recreation, universities.
Issue link: https://read.dmtmag.com/i/232221
Field Science Figure 4. Left: Greenseeker canopy reflectance sensor system. Figure 5. Right: Spectrum FieldScout chlorophyll meter. the system will, we hope, convince you that it is a technique to watch. The more the system is used in research, the faster the applications to the real world will be developed. urement wavelength used is in the red (visible) part of the spectrum and the reference in the near-infrared (Fig. 3). Canopy reflectance sensors like this will report/record an index which is usually of the form (ρNIR - ρVIS) / (ρNIR + ρVIS). Rho (ρ) is the reflectance, and you can see from the formula that when there is no absorbance by chlorophyll (ρNIR = ρVIS) the top of the ratio is zero, and the index is zero. When all of the measurement wavelength is absorbed (ρVIS = 0), the ratio becomes ρNIR / ρNIR or 1. Some sensor systems, like the Greenseeker (Fig. 4), will report an index between 0 and 1 (sometimes called the normalized-difference vegetation index, or NDVI), others like the Spectrum FieldScout (Fig. 5) multiply the index and report a value between 0 and 1000 (chlorophyll index). CANOPY REFLECTANCE IN TURF MANAGEMENT The key feature of canopy reflectance indices like NDVI and the chlorophyll index is that the values observed in turfgrass are very sensitive to a multitude of things of interest to a turf manager. Changes in nutrient status, moisture status, traffic, insects, disease, rootzone problems, and other biotic and abiotic stresses can all produce subtle shifts in canopy reflectance, some of which are even undetectable by a trained human eye. Canopy reflectance, especially as it is affected by nutrient status, has become an important tool in precision agriculture, where maximizing yields and optimizing fertilizer inputs is tied to systems that measure reflectance. The uses in turfgrass management will likely be more complex as they develop, since yield and nutrient stress are only a small part of the stresses that turf experiences. For example, research is currently being done to examine the potential in water management, but most of the current use is in turf research. Figure 6. Change in canopy reflectance following a single A few examapplication of slow-release nitrogen fertilizer (points represent ples of the power data from four replicate plots; the line is the best fit release and sensitivity of curve; DAT = days after treatment). 34 SportsTurf | January 2014 FERTILIZER PERFORMANCE AND RELEASE CHARACTERISTICS Fig. 6 shows a sample of data collected from recent fertilizer performance trials at the Guelph Turfgrass Institute (GTI). The points show the change in NVI as a fertilizer application at day 0 gradually releases and increases the absorption of PAR to the maximum at ~25 days after treatment, then gradually declines as the fertilizer runs out at ~100 days. Using these techniques we can help fine tune release characteristics of fertilizers, but the same data could help a turf manager track nutrient status. GERMINATION, ESTABLISHMENT AND COVER DEVELOPMENT IN TURF Canopy reflectance can be used to track the establishment of newly seeded turf. In research trials, we can use this to assess different cultivars, blends and mixtures, or different management techniques in establishment. Fig. 7 shows cover development in a recent trial at the GTI, and Fig. 8 shows the change in canopy reflectance in one of the entries over the first 26 days after seeding. Fig. 9 shows data from an earlier trial, in this case using the chlorophyll index rather than NDVI. Sixteen Kentucky bluegrass cultivars show clear differences in speed of establishment as measured by canopy Figure 7. Cover development in turf trial. reflectance. DROUGHT STRESS, WATER USE AND LOCALIZED DRY SPOT Fig. 10 shows localized dry spot and treatment effects of wetting agents in a recent trial. If we look at the canopy reflectance and independent assessments of soil moisture (Fig. 11) and localized dry spot (Fig. 12), the potential of canopy reflectance to detect and help manage water problems is clear. We have also used the technique in assessing the effectiveness of different irrigation regimes in establishing turf from dry seeding, hydroseeding, and sod. OTHER BIOTIC AND ABIOTIC STRESSES As mentioned above, many stresses that affect turfgrass will be detectable in changes in canopy reflectance. We routinely use the www.sportsturfonline.com