A soil mapping system for collecting and mapping soil reflectance data in a field includes an implement having a furrow opener for creating a furrow and an optical module. The optical module is arranged to collect soil reflectance data at a predetermined depth within the furrow as the implement traverses a field. The optical module includes two monochromatic light sources, a window arranged to press against the soil, and a photodiode for receiving light reflected back from the soil through the window. The two light sources have different wavelengths and are modulated at different frequencies. The photodiode provides a modulated voltage output signal that contains reflectance data from both of the light sources. Additional measurement devices are carried by the implement for collecting additional soil property data, such as electrical conductivity, pH, and elevation, which can be used together with the optical data to determine variations in soil organic matter.