Abstract: A method assures the quantitative or semiquantitative assessment of analytes in soils by in situ optical methods. A determination is made of the optical chemical response factors for chemicals of interest which are added in known amounts to discrete soil matrices (types) and conditions (moisture content) under controlled laboratory conditions to provide predetermined reference signals. A combination probe is provided with an optical chemical sensing device that produces chemical concentration signals representative of the concentration of a chemical of interest in a soil sample and further is provided with a strain gauge sensing device that produces strain gauge signals representative of soil type and optionally, condition of the soil sample. The combination probe is inserted into the soil sample and the optical chemical sensing device produces the chemical concentration signals and the optical strain gauge sensing device, simultaneously and in parallel, produces the optical strain gauge signals.