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In Situ Characterization of Soil Clay Content with Visible Near-Infrared Diffuse Reflectance Spectroscopy

^a Dep. of Soil and Crop Sciences, Texas A&M Univ., 2474 TAMU, College Station, TX 77843-2474
^b Dep. of Land Resources and Environ. Sciences, Montana State Univ., PO Box 173120, Bozeman, MT 59717-3120
^c Dep. of Soil and Crop Sciences, Texas A&M Univ., 2474 TAMU, College Station, TX 77843-2474

View larger version (26K): [in this window] [in a new window]   Fig. 1. Schematic of a vertically sliced soil core. Columns and rows indicate locations scanned using the contact probe for in situ scans. Dried ground soil samples represent the soil from one row, both columns.

View larger version (45K): [in this window] [in a new window]   Fig. 2. Predicted vs. measured clay content of the validation data set for (a) air-dried ground, (b) air-dried in situ, (c) field-moist in situ, and (d) field-moist in situ smeared samples. Clay content predictions were made from models built with partial least square regression using the first derivative of visible near-infrared reflectance (VNIR) spectra (350–2500 nm). RMSD is root mean squared deviation.

View larger version (27K): [in this window] [in a new window]   Fig. 3. The wavelengths that contributed to significant regression coefficients (P 0.05) in the prediction of clay content are shown for field-moist and air-dry in situ and air-dry ground models. The relative magnitude of each regression coefficient indicates the strength of the correlation. The upper graph shows the mean of the regression coefficients common in all three models. All plots are on the same x axis. Values of the y axis are not shown, but all y axes are on the same scale.

View larger version (22K): [in this window] [in a new window]   Fig. 4. The wavelengths that contributed to significant regression coefficients (P 0.05) in the prediction of clay content are shown for field-moist, smeared and unsmeared, in situ models. The relative magnitude of each regression coefficient indicates the strength of the correlation. The upper graph shows the mean of the regression coefficients common in both models. All plots are on the same x axis. Values of the y axis are not shown, but all y axes are on the same scale.

View larger version (16K): [in this window] [in a new window]   Fig. 5. Laboratory-measured (+) and visible near-infrared (VNIR) predicted () clay content for three soil cores: (a) a dark-colored Vertisol, (b) an Alfisol with distinct A, E, and Bt horizons, and (c) a Mollisol with secondary CaCO3 concentrations in the B horizons. Laboratory measurements were taken to represent a row within a horizon, and VNIR predictions were made every 3 cm.