Reflectance spectra of soil can be used to estimate the concentrations of organic carbon in soil (SOC). The estimates are more or less imprecise, but spectroscopy is quicker, less laborious and cheaper than conventional dry combustion analysis. Are the greater economy and efficiency sufficient to justify the loss of information arising from errors in estimation? We measured soil spectra with three instruments: a bench-top mid-infrared (mid-IR) (mid-IR b ) spectrometer, a portable mid-IR (mid-IR p ) spectrometer and a portable visiblenear infrared (vis-NIR p ) spectrometer. We calculated a quantity E to express the cost-effectiveness of spectroscopic estimates relative to the conventional analysis, by accounting for their inaccuracy, their cost and their capacity, namely the maximum number of samples that can be prepared and measured daily. In all, 562 samples of soil were collected from 150 locations at four depths on a farm. The samples were dried and ground to particle sizes of ≤2 and ≤0.5 mm before measurements were made by dry-combustion analysis.The machine learning algorithm Cubist was used to derive spectroscopic models of SOC concentrations and their uncertainties. We found that the mid-IR b on the ≤0.5 mm samples was the most accurate and expensive but nevertheless sufficiently cost-effective (large value of E) for determining the organic C. The mid-IR p was somewhat more accurate, but its E was smaller than vis-NIR p on corresponding samples because it required more time to record the spectra. We also found that, with the portable spectrometers, the SOC predictions made on the ≤0.5 mm samples were somewhat more accurate than those made on the ≤2 mm samples, but their E was smaller because of the additional cost of sample preparation. The vis-NIR p on the ≤2 mm samples was the most cost-effective for estimating SOC because it is cheap, accurate and has a large capacity for measurements. Highlights• Concentrations of soil organic carbon (SOC) were determined by standard dry combustion and estimated from reflectance spectra recorded by three instruments.• The labour required for each of the techniques and the cost, including that of the equipment, were recorded.

No abstract