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Nature of Clay–Humic Complexes in an Agricultural Soil

II. Scanning Electron Microscopy Analysis

USDA-ARS, National Soil Tilth Lab., 2150 Pammel Drive, Ames, IA 50011

View larger version (92K): [in a new window]   Fig. 1. Scanning electron micrographs of the whole clay fraction from the Webster soil; (A) low magnification scanning electron microscopy (SEM) illustrating large (5–20 µm) soil microaggregates and small (0.5–2 µm) discrete particles, (B) medium magnification SEM stereo pair of a typical soil microaggregate, and (C) high magnification SEM stereo pair illustrating small particles with platy (p), rounded (r), and nodular (n) morphologies.

View larger version (83K): [in a new window]   Fig. 2. Scanning electron micrographs of the fine clay fraction from the Webster soil; (A) low magnification scanning electron microscopy (SEM) illustrating numerous large (10–50 µm) and only a few small (0.5–5 µm) particles, (B) medium magnification SEM stereo pair of typical large smectite particles, and (C) high magnification SEM stereo pair of thin crumpled and thicker more rigid platy particles.

View larger version (142K): [in a new window]   Fig. 3. High resolution stereo scanning electron micrographs of particle surfaces from the untreated (A) and H2O2-treated (B) fine clay fraction of the Webster soil. The prominent ropy structures are believed to be wrinkles in the surficial smectite layers. Subtle blotchy features between the ropy structures seen in A but not seen in B are believed to be diffuse filamentous coatings of humic materials. The volcano was probably caused by a gas bubble that burst through the top smectite layer when the sample was evacuated.

View larger version (92K): [in a new window]   Fig. 4. Scanning electron micrographs of the medium clay fraction from the Webster soil; (A) low magnification scanning electron microscopy (SEM) illustrating numerous large (5–20 µm) and small (1–2 µm) particles, (B) medium magnification SEM stereo pair illustrating typical large particles covered with small particles, and (C) high magnification SEM stereo pair of small protoillite and smectite particles.

View larger version (87K): [in a new window]   Fig. 5. Scanning electron micrographs of the coarse clay fraction from the Webster soil; (A) low magnification scanning electron microscopy (SEM) illustrating numerous small (0.2–2 µm) discrete particles and only a few small (1–3 µm) aggregates, (B) and (C) high magnification SEM stereo pairs illustrating platy (p), rounded (r), angular (a), and nodular (n) morphologies of the small discrete particles. Most of the surfaces of the platy and rounded particles appear smooth indicating that these particles are either uncoated or have very thin coatings of humic substances.

View larger version (178K): [in a new window]   Fig. 6. Energy dispersive x-ray adsorption maps of elemental distributions in small discrete particles from the coarse clay fraction from the Webster soil. Three discrete particles with high Cu concentrations (assumed to be discrete humic particles) also have high Fe but low Si and O levels. The elemental maps are 10 µm on each side. The elemental maps were electronically filtered to reduce noise and enhance contrast.