Determination of representative elementary areas for soil redoximorphic features identified by digital image processing

Photography has been a welcome tool in documenting and conveying qualitative soil information. When coupled with image analysis software, the usefulness of digital cameras can be increased to advance the field of micropedology. The determination of a representative elementary area (REA) still remains a critical information need for soil scientists so that field measurements are independent of sample size and account for spatial heterogeneity. An objective of this study was to define and determine an REA for Low Chroma and High Chroma soil redoximorphic features (SRFs) present in claypan soils of northeastern Missouri, USA using a digital camera and image classification techniques. An additional objective was to examine REA differences between these two SRF types, soil depths, and landscape positions to highlight sampling considerations when quantifying SRFs in the field. Three metrics were chosen to quantify SRF heterogeneity, including percent occurrence, mean Euclidean distance, and the Interspersion/Juxtaposition Index. The relative change in these metrics was determined for 16 image sizes ranging from 2.5 cm2 to 40 cm2 and used to identify an REA. Results showed REAs (mean ± SE) for Low Chroma (17.7 cm2 ± 0.4) and High Chroma (25.4 cm2 ± 0.7) were significantly different (α = 0.05). Further review of REAs indicated large sampling diameters (> 8 cm) are necessary to simultaneously capture REAs of Low Chroma and High Chroma SRFs. When SRFs were considered separately, a ≥ 5 cm diameter core is recommended to reach an REA for Low Chroma, allowing accurate quantification for soil classification purposes and hydric soil determinations. Federal and state agencies requiring quantifiable SRF measures for land management decisions may greatly benefit from determining these minimum measurement scales, ensuring appropriate data collection methods in the future.

Research Highlights
► A conceptual model of soil redoximorphic features was translated in quantitative measures used to define representative elementary areas (REAs) for soil description.
► Mean REAs significantly differed between High Chroma and Low Chroma soil redoximorphic features at six depth increments and four landscape positions in soil profiles sampled by cores.
► REAs values for Low Chroma indicate a ≥ 5 cm diameter core is recommended for core sampling to capture areas necessary for soil classification purposes and hydric soil determinations in northeastern Missouri, USA.