January 2014

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.

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Field Science | By Ken Carey Using canopy reflectance tools in turfgrass management Turfgrass managers can tell a lot about turf just by looking at it—and the more experienced they are, the better their judgment. Nutrient status, pest damage, abiotic stresses (drought, traffic, etc.) are all visible to the trained eye. Sometimes, however, it's good to have some tools to help; the highly trained manager may not be available to see everything, or the problem may produce very subtle effects. This article discusses some recent innovations in assessing turfgrass, developed and widely used in turf research, which might be useful to the turfgrass manager. What we see when we look at a turfgrass sward could be termed "canopy reflectance"; it's just the ambient sunlight reflected off the leaves in the full visible light spectrum. A trained Figure 1. Top: The peaks show the wavelengths of visible light that are absorbed by chlorophyll (Photosynthetically active radiation). Figure 3. Bottom: Measurement (660 nm) and reference (770 nm) wavelengths used by the Greenseeker to calculate NDVI. 32 SportsTurf | January 2014 researcher or turfgrass manager learns to record and interpret the details of what they see, whether it's the off color of nutrient deficiency or spray damage, or the darkening of drought stress. However, both in research and in Figure 2. Measurement ( VIS) and reference ( NIR) waves of light reflecting off bare practical management situsoil and turf. ations, we work with less well-trained helpers, and will benefit from techniques that remove the subjectivity and observer bias, and reduce the need for training. One very familiar tool is a camera, and with improved digital cameras this is a very useful adjunct to assessing problems. However, even though they can form an important permanent record, the digital photos still need to be interpreted. Researchers are working on improving software to analyze digital images to document and quantify turf characteristics (weed and disease infestation, drought and nutrient stress), but these full spectrum techniques are still relatively early in development for widespread turf use. A more mature, and somewhat simpler, technology for assessing turf involves restricting the wavelengths observed to ones that we have learned through experience are indicative of turfgrass problems. Photosynthesis in plants involves chlorophyll absorbing light to power the plant, and the wavelengths that chlorophyll absorbs are a subset of the sunlight hitting the plant (Fig. 1). Light that chlorophyll absorbs is not reflected, and the light hitting the plant looks different from that reflected. Of the visible wavelengths, chlorophyll absorbs red light, generally, so the light reflected is white minus red = green. The wavelengths that chlorophyll absorbs are often termed photosynthetically active radiation or PAR. Various sensors have been developed which all function in a similar fashion, comparing the reflectance off a surface (e.g. turf ) of a wavelength that chlorophyll absorbs (measurement wavelength), with one that chlorophyll does not absorb (reference wavelength). Fig. 2 shows light reflecting from turf and bare soil. The longer (reference) wavelength is not absorbed by chlorophyll and is reflected equally from both surfaces; the shorter (measurement) wavelength is partly absorbed by the plant, and the reflected amount is reduced. Usually the meas-

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