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Title: MID- AND NEAR-INFRARED SPECTROSCOPIC DETERMINATION OF SOIL COMPOSITIONAL PARAMETERS AND STRUCTURAL INDICES IN TWO FERRALSOLS

Author
item MADARI, B - EMBRAPA SOILS
item Reeves Iii, James
item MACHADO, P - EMBRAPA SOILS
item TORRES, E - EMBRAPA SOYBEAN
item GUIMARAES, C - EMBRAPA RICE AND BEAN
item McCarty, Gregory

Submitted to: Geoderma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/24/2006
Publication Date: 6/15/2006
Citation: Madari, B.E., Reeves III, J.B., Machado, P., Torres, E., Guimaraes, C.M., Mccarty, G.W. 2006. Mid- and near-infrared spectroscopic determination of soil compositional parameters and structural indices in two ferralsols. Geoderma. 136:248-259.

Interpretive Summary: The use of light in the near- and mid-infrared spectral ranges has come to be used to rapidly determine the composition of biological samples including, more recently, soils. However, many questions remain to be answered for soils including applicability to different soil types and even what characteristics of soils can be accurately determined by spectral analysis. The objective of this work was to examine the ability of near- and mid-infrared spectroscopy to determine the composition of soil aggregates (soils separated into different fractions based on particle size) which are important for good soil structure. The bulk soil samples had been collected from long-term tillage experiments in areas covered by secondary natural forest in the Brazilian savanna region and southern Atlantic forest. The soil aggregates were separated from the bulk soil samples (non-fractionated) for characterization. The different aggregate classes were analyzed for total C, N by conventional methods, and several measures of soil aggregation (indices) were also determined. The bulk samples were also analyzed for texture (sand, silt and clay contents). All samples were scanned in the mid- and near-infrared regions, and quantitative calibrations (development of mathematical equations relating spectral information to parameter of interest) for the various analytes were developed. Results for the set of bulk samples for total C and N were excellent using either spectral region. Similar results were obtained for determinations of the sand and clay components of texture. Efforts with the 700 fractionated samples (aggregates) were excellent for C and N using either spectral range. Calibration efforts to determine the values for the set of 700 fractionated samples using the set of 120 original bulk samples from which they were fractionated produced satisfactory results when 5% of the fractionated samples was added to the set of 120 bulk samples. Spectral discrimination of sample source was easily and very accurately determined for all data sets examined. These results demonstrate that both mid- and near-infrared can be used to accurately determine various characteristics of soil aggregate fractions although mid-infrared will provide more accurate results.

Technical Abstract: Bulk soil samples (BS) (n = 120) from long-term tillage experiments and areas covered by secondary natural forest were collected in the Brazilian savanna region (Cerrado) at Santo Antônio de Goiás, and southern Atlantic forest region at Londrina. Bulk soils were separated into 8 aggregate size fractions (ASF) by wet sieving (n = 700). BS and ASF were analyzed for total C, N by dry combustion. BS were also analyzed for sand, silt, and clay by densitometry, and several measures of soil aggregation were determined. All samples were scanned in the mid- and near-infrared (Mid-IR, NIR) by diffuse reflectance of ground, non-KBr diluted samples using a Digilab FTS-7000 FTIR spectrometer equipped with a Pike Auto-Diff autosampler. Quantitative calibrations for the various analytes were performed separately for the original set of 120 BS samples and the set of 700 ASF samples using partial least squares. Results for the set of 120 BS for total C and N were excellent using either spectral range with R2 > .97, except for C by Mid-IR (R2 = .93). Similar results were obtained for determinations of sand and clay (R2 > .94). Silt, determined by difference (100% - %sand -%clay), gave much lower R2 (.80 and .63 for Mid-IR and NIR, respectively). For aggregation indices, R2 of ~.8 were achieved in the Mid-IR, NIR results were not as good (R2 < 0.67). Efforts with the 700 ASF samples resulted in R2 of .98 and .96 for C, and .96 and .97 for N, for Mid-IR and NIR calibrations respectively. Calibration efforts to determine the values for the set of 700 ASF samples using set of 120 BS from which they were fractionated gave R2=0.90 and 0.87 in the Mid-IR and NIR respectively when 5% of the aggregate samples were added to set of 120 BS. Spectral discrimination of sample source was easily and very accurately determined for all data sets examined. These results demonstrate that both Mid-IR and NIR can be used to accurately determine various soil parameters of soil aggregate fractions although mid-infrared will provide more accurate results.