Dark surface horizons

Umbrepts. Dark surface horizons medium to low base supply. Soils which contain little organic matter or nitrogen, They are usually dry for more than 6 months/year, In numerous areas, salts accumulate on or near the soil surface. Since the nutrient content, except nitrogen, of aridisols is often high, these soils can be productive with irrigation and nitrogen application. Salt accumulation can be a problem with some crops. 

Soils with dark, thick, organic-rich surface horizon, high base supply. Mollisols are highly fertile and can support a variety of crops. 

Mollisols Soils possessing a relatively dark and high C and base saturation surface horizon commonly occur in... 

Mollisol. A soil consisting of a relatively thick, dark-colored surface horizon that contains at least 0.58% organic carbon, has a base saturation of more than 50% (pH 7), and is predominantly saturated with bivalent cations. 

Thin Dark Surface. For use in LRRs P, R, S, T, and U for testing in LRRs K and L. A layer 5 cm (2 in.) or thicker within the npper 15 cm (6 in.) of the snrface, with valne 3 or less and chroma 1 or less. At least 70% of the visible soil particles in this layer mnst be covered, coated, or masked with organic material. This layer is underlain by a layer(s) with value 4 or less and chroma 1 or less to a depth of 30 cm (12 in.) or to the spodic horizon, whichever is less. 

Figure 13 Tall grassland (Mollisol) paleosol with thick, dark brown crumb-textured surface over a deep (79 cm) white nodular calcic horizon, over a thinner short grassland paleosols with carbonate nodules at a depth of 39 cm, in the Late Miocene (7 Ma) Ash Hollow Formation, 13 km north of Ellis, Kansas.

Soil was collected from two horizons (surface and O.S m depfli) in an uncontaminated area near the Tim s Branch stream system at the DOE SRS site, and stored at 4 °C in the dark until use. Our interest in working with this type of soil stems from the fact that Tim s Branch, a second order stream system flowing into a tributary of the Savannah River, experienced large influxes of depleted and natural uranium (U), nickel (Ni), and aluminum (Al) as well as other metals. Tim s Branch soils and vegetation have had a strong inq>act on flie deposition of metals and metal-laden sediments (28). 

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