ABSTRACT
ReŸectance spectroscopy has become a very useful tool for the soil sciences in the past 20 years. This technique enables the extraction of quantitative and qualitative information on many soil attributes in real time and can shed light on the soil’s composition without the need for laborintensive wet-chemistry analyses (e.g., [1-3]). ReŸectance spectroscopy information is acquired in both point and image arrangements in the laboratory, œeld, and, recently, also from air and space domains. Whereas in the laboratory, soil-reŸectance measurements are performed under controlled conditions using standard protocols (and hence with minimum interference), in the œeld, reŸectance measurements are fraught with a variety of problems, such as variations in viewing angle, changes in illumination, soil roughness, and soil sealing (Ben-Dor et al. [4]). Acquiring soil reŸectance data from air and space involves additional difœculties, resulting from, for example, relatively low signal-to-noise ratio sensors and atmospheric attenuation. Laboratory-based measurements enable an understanding of the chemical and physical principles of soil reŸectance and are widely used for practical applications requiring a quantitative approach. As the sensitivity of portable œeld spectrometers increases, œeld soil spectroscopy is becoming a promising tool for rapid point-by-point monitoring of the soil environment. Recently, considerable effort has been invested in commercializing œeld spectroscopy-based sensors for agricultural applications (e.g., VERIS Technologies https://www.veristech.com/index.aspx), NovoSpec (www.novospec.com). Several papers have described the possibility of using soil reŸectance in the œeld for precision agriculture applications (e.g., [5,6]). In this regard, the future looks bright for the development of a new spectral technology, termed imaging spectroscopy (IS), which combines the spectral and spatial domains [7]. Due to the large number of airborne IS sensors operating today for many terrestrial applications, this technology is slowly but surely entering the œeld of soil science, where its use will rely heavily on the spectral foundation generated over the past two decades in soil analysis laboratories [8]. Understanding the principles and limitations of soil spectra is crucial to the use of the forthcoming soil-IS technology. Information about soils from reŸectance spectra in the visible-near-infrared (VIS-NIR) (400-1000 nm) and short-wave-infrared (1000-2500 nm) spectral regions constitutes almost all of the data that passive solar sensors can provide and therefore this chapter will only cover these regions. We provide a historical overview of soil reŸectance spectroscopy and a general overview of the chemical-physical principles of the soil-reŸectance spectrum in this spectral region. We also discuss the basic interactive processes between soils and electromagnetic radiation and shed light on the principle of quantitative soil-spectral approaches. This chapter will provide recent examples on how reŸectance spectroscopy of soil is used in a modern remote sensing arena, using both point and imaging sensors, as well as future notes on the potential of this methodology.
