Diffuse Reflectance Spectroscopy

The appearance of a soil results from the interaction of its different constituents with incident light. Color and various other attributes of the appearance of soil are highly sensitive to the nature, proportion, particle size and morphology, and spatial association of its mineral and organic components. In fact, color has been used for more than 75 yr to obtain information about these soil properties with a goal of characterizing and distinguishing soil types. The demand for a standardized method to describe soil color was met by the adoption of the Munsell notation by the USA Soil Survey Program in 1949 and, about 10 yr later, by the International Society of Soil Science (Simonson, 1993). Since then, Munsell Soil Color Charts (Munsell Color, 1975) have been systematically used by pedologists. Visual estimation of soil color, however, is subject to substantial error due to various psychophysical and physical factors. For this reason, the use of colorimeters and spectrophotometers has gained widespread acceptance among soil scientists as a means to measure color accurately and precisely. Moreover, different types of spectrophotometers afford the elucidation of the spectrum of light reflected by a soil illuminated in various ways. Reflectance, which is the base quantity that characterizes the process of reflection, is defined as the ratio of the reflected radiant flux (or power) to the incident radiant flux (or power) (Wyszecki and Stiles, 1982). Generally, the reflectance of a soil at any wavelength λ can be considered to be the sum of two components: regular (or specular, or mirror) reflectance and diffuse (or volume or nondirectional) reflectance (defined in more detail below). Reflectance measurements in the field are usually made on relatively large areas (>10 cm2). Under these conditions, both specular and diffuse reflectance usually contribute to the total reflectance of a soil surface, the magnitude of which depends on particle size, structure, microrelief, and other properties that define the “surface state” (Escadafal, 1989). In contrast, laboratory measurements of soil reflectance are usually made on small areas (<10 cm2) of disturbed soil materials that are usually sieved or ground to a small size. In this case diffuse reflectance predominates, which depends mainly on soil composition. This chapter describes available laboratory methods for recording diffuse reflectance spectra for soil materials and ways to handle the information to identify and characterize soil minerals. Only the visible and narrow vicinal ultraviolet (UV) and infrared (IR) regions of the spectrum are considered here because reflectance in the IR region is the subject of another chapter (Johnston and Aochi, 1996)).