Standard layers for soil properties

Standard Layers for Soil Properties

The number, thickness, and depth to top and bottom of soil layers in the STATSGO dataset vary widely from one soil component to another, even within the same map unit. This wide variation of layer definitions makes it difficult to use the STATSGO data in models, especially if they require rasterized (gridded) data. To facilitate rasterization of the STATSGO data and the physical and hydraulic properties derived from them, we defined a set of 11 standard soil layers. All the derived soil properties were mapped into these standard layers; the method used for this remapping is described under "Processing methodology" for each of the derived properties.

The 11 standard layers are

   Layer     Thickness       Depth to Top    Depth to Bottom
        
     1      5 cm (2 in)        0 cm (0 in)     5 cm (2 in)
     2      5 cm (2 in)        5 cm (2 in)    10 cm (4 in)
     3     10 cm (4 in)       10 cm (4 in)    20 cm (8 in)
     4     10 cm (4 in)       20 cm (8 in)    30 cm (12 in)
     5     10 cm (4 in)       30 cm (12 in)   40 cm (16 in)
     6     20 cm (8 in)       40 cm (16 in)   60 cm (24 in)
     7     20 cm (8 in)       60 cm (24 in)   80 cm (31 in)
     8     20 cm (8 in)       80 cm (31 in)  100 cm (39 in)
     9     50 cm (20 in)     100 cm (39 in)  150 cm (59 in)
    10     50 cm (20 in)     150 cm (59 in)  200 cm (79 in)
    11     50 cm (20 in)     200 cm (79 in)  250 cm (98 in)

The above selection of the number and depths of these standard layers reflects three main considerations:

The wide variation of numbers, thicknesses, and depths of layers for different components means that there are no "natural" or "obvious" choices for the standard layers.
Many models are particularly sensitive to the properties of the top few centimenters of soil; hence extra priority should be given to preserving all available information for this region.
To minimize data volumes, layer thicknesses should not be much less than the thicknesses of "typical" component layers at similar depths.

To aid in the selection of standard layers, therefore, the frequencies of depths and thicknesses of layers were tabulated for all components. This tabulation indicated that roughly 50% of components have surface layers thicker than 20 cm (8 inches); only about 4% of surface layers have a thickness of 5 cm (2 inches) or less, and about 16%, 10 cm (4 inches) or less. Deeper layers are in general thicker -- roughly 60% of all layers were at least 50 cm (20 inches) thick. The majority of components did not record layers extending below 60 inches (approximately 1.5 m); only about 10% include layers extending beyond 2.0 m (79 inches).

Last change: 5 Feb 1996, R. A. White / raw@essc.psu.edu