A system and a method are disclosed for combining a soil sample and extractant in a mixing chamber to produce a liquid extractant-soil mixture. A portion of the liquid extractant-soil mixture is directed from the mixing chamber through a sample measurement chamber coupled to the mixing chamber. The sample measurement chamber is coupled to a light source so that light propagates from the light source through the portion of the liquid extractant-soil mixture to an optical detector which generates an attenuation spectrum indicating light received by the detector at different wavelengths. The sample measurement chamber may include an attenuation cell having a specified optical path between a first measurement window and a second measurement window and angular surface directing particulates in the liquid extractant-soil mixture away from the attenuation cell.