ABSTRACT
Kawashima and Nakatani indices ........................................... 71 4.10 Nitrogen estimation based on histograms of RGB model
components .............................................................................................. 71 4.11 Applying the IHLS color space to nitrogen estimation in
plants ........................................................................................................ 72 4.12 Measuring nitrogen content using digital images ............................. 73 References .......................................................................................................... 74
are used as a scouting diagnosis to determine the nutrient stage of the crops. Both field and laboratory diagnoses can occur, and they each have their drawbacks. A field diagnosis is based on observations made after the symptoms are visible, when the plant has already suffered a nutrimental disorder. A laboratory diagnosis gives reliable results for most nutrients, but it is time-consuming and requires an available laboratory, which can also be problematic. Thus, when the laboratory results are available, the nutrimental stage of the plant has often already changed. No method exists that determines the nutrimental stage of a plant with precision and speed at a low cost. Some researchers have developed plant nutritional meters based on the interaction between light and the plant’s foliar structure. It is known that when plants suffer stress due to a lack of water or other nutrient, the foliar structure inside the leaf changes, altering its reflectance. These reflectance changes can be measured and have been correlated with the plant’s chlorophyll and nitrogen (N) levels. The goal of this chapter is to describe the basis of plant reflectance and its correlation with the nitrogen status of plants. A red, green, and blue (RGB) color model is the most frequently used in these types of studies. In this color model, the R and B bands are the best indicators of a plant’s nitrogen status. Greenness indices that are based on the RGB model are another way to accurately estimate the N content. However, some difficulties are associated with using light reflectance to determine the N content of plants. The biochemical components, pigments, proteins, carbohydrates, oils, water, leaf morphology, and leaf architecture of the plant influence its tendency to absorb, transmit, or reflect radiation. Additionally, not all plants have the same reflectance, and the same plant can even change its reflectance as a response to its development stage, leaf orientation, color soil effect, atmospheric conditions, and other factors. Thus, an improved hue, luminance, and saturation (IHLS) color model has been proposed to overcome some of the aforementioned issues. This method is based on the RGB model and is less susceptible to light changes. It uses RGB values that have not been normalized, so no information is lost. Combined with the IHLS model, digital images can be used to determine the plant’s nitrogen status, yielding a promising technique that is inexpensive, reliable, noninvasive, and quick.
