Arbor Age

www.greenmediaonline.com Green Media / August 2014 17 in a similar fashion, comparing the refl ectance off a surface (e.g. turf) of a wavelength that chlorophyll absorbs (mea- surement wavelength), with one that chlorophyll does not absorb (reference wavelength). Fig. 2 shows light refl ecting from turf and bare soil. The longer (reference) wavelength is not absorbed by chlorophyll and is refl ected equally from both surfaces; the shorter (measurement) wavelength is partly absorbed by the plant, and the refl ected amount is reduced. Usually the measurement wavelength used is in the red (vis- ible) part of the spectrum and the reference in the near- infrared (Fig. 3). Canopy refl ectance sensors like this will report/record an index which is usually of the form (ρNIR - ρVIS) / (ρNIR + ρVIS). Rho (ρ) is the refl ectance, and you can see from the formula that when there is no absor- bance 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), oth- ers like the Spectrum FieldScout (Fig. 5) multiply the index and report a value between 0 and 1000 (chlorophyll index). Canopy refl ectance in turf management The key feature of canopy refl ectance indices like NDVI and the chlorophyll index is that the values observed in turf- grass are very sensitive to a multitude of things of interest to a turf manager. Changes in nutrient status, moisture status, traffi c, insects, disease, rootzone problems, and other biotic and abi- otic stresses can all produce subtle shifts in canopy refl ectance, some of which are even undetectable by a trained human eye. Canopy refl ectance, 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 refl ectance. 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 cur- rently being done to examine the potential in water manage- ment, but most of the current use is in turf research. A few examples of the power and sensitivity of 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 ap- plications to the real world will be developed. 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 fi ne tune release characteristics of fertilizers, but the same data could help a turf manager track nutrient status. Figure 4. Greenseeker canopy refl ectance sensor system. Figure 5. Spectrum FieldScout chlorophyll meter. Figure 6. Change in canopy refl ectance following a single appli- cation of slow-release nitrogen fertilizer (points represent data from four replicate plots; the line is the best fi t release curve; DAT = days after treatment). All images and graphics provided by Dr. Ken Carey

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