Research has demonstrated that for the determination of soil C diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) is generally more accurate and produces more robust calibrations than near-infrared reflectance spectroscopy (NIRS) when analyzing ground, dry soils under laboratory conditions. However, DRIFTS is known to be more affected by moisture and sample preparation than NIRS even with the spectrometer being sealed or purged with dry nitrogen gas to eliminate effects of moisture on the optics, and ambient carbon dioxide and moisture on the spectra. Also, DRIFTS is not considered to be feasible on samples containing high levels of moisture due to the strong water absorptions in the mid-infrared, although the presence of water is also known to degrade even near-infrared spectra and subsequent calibrations. While DRIFTS has been shown to be advantageous in the laboratory, if samples need to be ground and dried and instruments purged to obtain useable data, it may not be practical for on-site use. Studies were therefore undertaken to determine the effect of ambient atmospheric conditions and soil state (ground, dried, etc.) on DRIFTS and NIRS calibrations for soil C. Results using a portable Fourier transform mid-infrared spectrometer (FTIR) over a wide range of ambient temperatures and humidity levels have demonstrated that purging of the FTIR is not necessary to obtain calibrations for inorganic or organic C in soils equal to those obtained in the laboratory under ideal conditions. Results will also be shown for the effects of sample state (wet and non-ground, dry and non-ground and dry and ground) on DRIFTS and NIRS calibrations for the same samples. Preliminary efforts have already demonstrated that drying of samples on site occurs rapidly under even moderate temperatures (70's) and should not present a problem if dried samples are required.