Surface structure effects on close-range reflectance as a function of soil organic matter content

A study was conducted to develop an equation for soil organic matter content as a function of close-range red range (660 nm) reflectance. The effects of soil surface were included by passing the soil throuhg different sieves and scraping the soil surface flat or rolling the surface smooth before taking reflectance measurements. Two different sensor geometries were used. One sensor was designed to include both specular and diffuse reflectance (modified original sensor) while the other sensor was designed to include only diffuse reflectance (normal incidence sensor). The soils tested were from the delta region in eastern Arkansas. The organic matter content for these soils ranged from 0.47 to 2.1%. For these soils, the red range reflectance, organic matter content, and aggregate size (as measured by the geometric mean diameter) were interrelated. Surface structure affected close-range reflectance from soil. Attempts to minimize surface structure effects by passing the soil through small sieves and rolling the surface smooth resulted in weaker correlations between reflectance (or absorbance) and organic matter content. Surface structure effects appeared to increase the amount of information available in reflectance (or absorbance) measurements for determining organic matter content. The maximum r2 between reflectance and organic matter content for a single sieve/sensor/surface preparation combination was 0.609. An absorbance transformation of the reflectance and multiple linear regression analyses were used to determine a stronger relationship. A combination of absorbances from soil passed through the 0.50 mm sieve using the modified original sensor with a scraped soil surface and the normal incidence sensor with a rolled soil surface produced an r2 of 0.732 between measured and predicted organic matter content with a standard error of 0.270% organic matter.