Using a Reference Color Plate to Correct Soil Color Measurements with Different Smartphones Under Different Lighting Conditions

Soil color has long been used as an indicator for soil properties such as soil organic carbon and soil moisture. Recent developments in citizen science have seen increased use of smartphone cameras for soil color measurements. However, there are high errors associ-ated with this technique. Two major sources of errors are smartphone camera and lighting condition. In this study we tested the effectiveness of using a color plate as a reference to reduce these errors on color measurements for the squares on the color plate itself, color chips in a Munsell soil color book and soil samples. The results show that the raw values of color parameters showed different patterns of biases with different smartphones and lighting conditions. The calibration reduced the errors consistently for all smartphones under all lighting conditions for the color plate squares. For the Munsell book chips or the soil samples, the calibration did not always reduce the bias but it did reduce the variations of all color parameters among smartphone and lighting condition, therefore, improved the precision of color measurements.