Comparison of visual and instrumental measurements of soil color with different low-cost colorimeters

Soil color has been conventionally measured using a Munsell soil color chart. Recently launched colorimeters can also measure the color of an object at a reasonable cost. This study was undertaken to evaluate to what extent such low-cost colorimeters (< 500 USD) can be useful for soil color analysis in the laboratory as compared with conventional colorimeters (> 3000 USD) and a Munsell soil color chart (about 200 USD). Sixty two air-dried soil samples collected from rice fields in Madagascar were subjected to two pretreatments for homogenization (2-mm sieving or additional hand-grinding) and instrumental analysis using four low-cost colorimeters (CS-10, Cube, Nix Pro, and Color Muse) and four conventional colorimeters (SPAD-503, CR-20, CR-400, and CR-410). The color of 2-mm sieved samples was also measured visually using a color chart. The effects of pretreatments and the analytical conditions were evaluated by the repeatability and stability of the measurement, the comparability of the soil color data obtained, and the time required for analysis. Overall, instrumental measurement was much more repeatable than visual observation. Both the repeatability and stability of the low-cost colorimeters tended to be lower than those of the conventional colorimeters. Among the low-cost colorimeters examined, soil color parameters (L*, a*, and b*) obtained with Nix Pro were most comparable (r > 0.97 for all parameters of 2-mm sieved samples) with those obtained with SPAD-503, which is an instrument designed specifically for soil color analysis. Additional hand-grinding pretreatment improved the repeatability of the instrumental analysis and reduced the subsample weight to two grams. However, this pretreatment also increased the L* value (lightness) of the samples, decreased the comparability with the data from 2-mm sieved samples, and prolonged the time required to complete the whole analysis. Among the various methods tested, 2-mm sieving of air-dried samples followed by the color measurement with Nix Pro several times per sample was considered the most cost-effective approach for measuring soil color in the laboratory.