Quantifying the Relationship between Soil Organic Carbon and Soil Color in Nebraska

Soil color is easily measured in the field and holds potential to be used as an indirect measurement of soil organic carbon (SOC). The main limitation to this approach is knowledge about the specific color-SOC relationship in a region, which often varies in relation to parent material, soil texture, climate, and land use. Furthermore, the Munsell color data is subjective in nature. The objectives of this study are: 1) to develop and evaluate the accuracy of pedotransfer functions (PTFs) for the prediction of SOC based on soil color and texture in the state of Nebraska and 2) to evaluate digital based color measurements methods as field predictors of SOC in Nebraska. To address the first objective, data were obtained from the National Soil Information System (NASIS) database. The dataset consisted of 1576 soil pedon descriptions and samples of various soil textures, Munsell color, and SOC. The second objective was addressed using digital color measurements of 50 soil samples from Kellogg Soil Survey Laboratory archive. Methods used included a portable color sensor (PCS) and smartphone cameras (SPCs). Regressions of moist Munsell value versus SOC using the NASIS data had R2 = 0.23 to 0.69 for individual MLRAs. Regression developed using the PCS for three selected MLRAs had R2 = 0.49 to 0.81. Various PTFs based on the NASIS data resulted in RMSE of prediction = 0.795 to 2.1. The results indicate the potential of using soil color as a predictor for SOC, especially when PCS are used to measure soil color. Advisor: Judith Tur

The Impact of Sampling Methodology on Soil Bulk Density Measurement by the Clod Method

The clod method is a widely used and accurate bulk density method. However, its use is limited to sampling from soil pits. This study was conducted to: 1) determine whether clods collected from cores provide similar bulk density measurements to those collected from soil pits and 2) evaluate the impact of various clod bulk density methods on carbon stock calculation. Clods were collected from soil pits, 5.1 cm soil cores, and 8.9 cm soil cores. Three-dimensional laser scanning was used to measure the volume of the soil clods before and after oven-drying and bulk density was calculated as the dry mass of the clod divided by volume. Of 24 horizons sampled, 17 showed no significant effect of sampling method (core vs. pit) on dry bulk density. The error introduced by collecting clods from a core was small enough that it had no significant effect on carbon stock (p = .683). However, the moisture at which clods were scanned did have a significant effect on carbon stocks (p = .002 to 0.016). Data from the study suggest that collecting clods from soil cores introduces minimal error, but attention should be given to the moisture state at which clod volume is determined