Digital Mapping of Soil Organic Matter and Cation Exchange Capacity in a Low Relief Landscape Using LiDAR Data

Soil organic matter content (SOM) and cation exchange capacity (CEC) are important agronomic soil properties. Accurate, high-resolution spatial information of SOM and CEC are needed for precision farm management. The objectives of this study were to: (1) map SOM and CEC in a low relief area using only lidar elevation-based terrain attributes, and (2) compare the prediction accuracy of SOM and CEC maps created by universal kriging, Cubist, and random forest with Soil Survey Geographic (SSURGO) database. For this study, 174 soil samples were collected from a depth from 0 to 10 cm. The topographic wetness index, topographic position index, multi resolution valley bottom flatness, and multi resolution ridge top flatness indices generated from the lidar data were used as covariates in model predictions. No major differences were found in the prediction performance of all selected models. For SOM, the predictive models provided results with coefficient of determination (R2) (0.44–0.45), root mean square error (RMSE) (0.8–0.83%), bias (0–0.22%), and concordance correlation coefficient (ρc) (0.56–0.58). For CEC, the R2 ranged from 0.39 to 0.44, RMSE ranged from 3.62 to 3.74 cmolc kg−1, bias ranged from 0–0.17 cmolc kg−1, and ρc ranged from 0.55 to 0.57. We also compared the results to the USDA Soil Survey Geographic (SSURGO) data. For both SOM and CEC, SSURGO was comparable with our predictive models, except for few map units where both SOM and CEC were either under or over predicted

Effect of Air- and Water-Filled Voids on Neutron Moisture Meter Measurements of Clay Soil

Air- and water-filled voids around neutron moisture meter (NMM) access tubes have been cited as sources of volumetric water content (θ) measurement error in cracking clay soils. The objectives of this study were to experimentally quantify this potential error stemming from (i) uncertainty in bulk density (ρ) sampling and (ii) the impact of air- and water-filled voids. Air- and water-filled voids were simulated using ∼0.6-cm (small) and ∼1.9-cm (large) annuli around access tubes. After NMM measurements were taken in a tightly installed access tube, either a small or large annulus was installed in the same borehole. Additional NMM measurements were taken with the annulus filled with air, and then water and ρ and θ were measured. The RMSE of the NMM calibration using all 11 installations was 0.02 m m. However, if two cores were used for calibration, the ratio of NMM-measured θ to in situ θ was significantly different ( < 0.05) from measured θ half the time (RMSE, 0.012–0.05 m m). Small air-filled voids created drier estimates of θ (bias, −0.039 m m; < 0.001), wherease small water-filled voids were not significantly different from the calibration. Air- and water-filled voids from larger annuli were significantly lower and higher ( < 0.001) than core-measured θ, with biases of −0.068 and 0.080 m m, respectively. Although this work does not correct NMM-predicted θ to matrix θ, it does bound NMM error under field conditions in a cracking clay soil