Precision agriculture technologies are pointing to the necessity of using sensors to collect soil information to assist in fertilizer dose evaluation. This study evaluates the effect of increasing doses of lime, thermophosphate, and organic matter on spectral reflectance of a tropical soil. We analyzed the most common tropical soil in Brazil, a medium texture Typic Haplortox occurring in the state of São Paulo, Brazil. Physical chemical and mineralogical analyses were carried out both before and after treatments. A spectroradiometer with a 450–2500-nm spectral range was used in the laboratory to capture spectral data. The application of increasing doses of lime increased the reflectance intensity as a result of the presence of calcium. The application of thermophosphate altered reflectance intensity primarily because of the presence of phosphorous. Organic matter reduced the reflectance when compared with the control. Chemical variations promoted by fertilizer application altered the intensity of reflected energy mainly between 826 and 2500 nm. However, there were no significant changes in soil absorption features except when applied to the organic product. The high values of determination coefficients for calcium and organic matter indicate that they can be estimated through soil-reflected energy, and, consequently, will be useful in precision farming systems.