Measuring soil carbon is currently a subject of significant interest due to soil’s ability to sequester carbon and reduce atmospheric CO2. The cost of conventional soil sampling and analysis along with the number of samples required make proximal sensing an appealing option.  To properly evaluate the performance of proximal sensing of soil carbon, a detailed lab-analyzed carbon inventory is needed to serve as the ‘gold standard’ in evaluating sensor estimations.  Four US midwestern fields were intensively sampled with two sets of lab analyses per field and at locations adjacent to the lab-analyzed cores, a sensor probe collected VisNIR, EC, and insertion force data to a depth of 0-90 cm. Estimations from each of the lab-analyzed inventories were compared to each other and to the estimation of the proximal sensor probes.  The results of the project revealed several important considerations for carbon inventorying efforts.