The plasticity index is defined as "the numerical difference between the liquid limit and the plastic limit; the range of moisture content within which the soil remains plastic". The "plastic limit" is, in turn, defined as the moisture content at which a soil changes from semisolid to plastic, and the "liquid limit" as the moisture content at which the soil passes from a plastic to a liquid state.

The information below was compiled from the following web page: http://www.essc.psu.edu/soil_info/index.cgi?soil_data&conus&data_cov Determining the Plasticity Index for Standard Layers The mean plasticity index was determined for each of 11 standard layers for each map unit of each state using data from the STATSGO Comp and Layer tables. The standard layers were introduced because of the wide variation in the number, thickness, and depth to top and bottom of soil layers in the STATSGO data from one soil component to another, even within the same map unit. Variable layers cause problems for many environmental models and GIS operations. Determining the mean plasticity index for the 11 standard layers required three main steps: Computing the mean plasticity index for each component layer. For each component, determining the contribution of each component layer to the 11 standard layers. For each map unit, combining the contributions of all components to compute the mean plasticity index for each standard layer. For each layer of each map unit component, the STATSGO Layer table contains two values for the plasticity index, PIH and PIL, defined as the maximum and minimum, respectively, of "the range in plasticity index for the soil layer or horizon, expressed as percent of moisture by weight." NOTE that these values do not indicate the values of the liquid and plastic limits, but only the differences between them. The mean plasticity index for each component layer was computed as the arithmetic average of PIH and PIL. The Layer table variable TEXTURE1, which gives the dominant soil texture class for the layer, was also read; if it corresponded to neither mineral soil nor bedrock, (i.e., water, organic matter, or other), or if the component was identified as all water (COMPNAME = "WATER"), the component layer was excluded from the computation of mean plasticity. Approximatedly 11% of all component layers had no entries for PIH and PIL; in these cases, the plasticity index values were assumed to be zero. There were a total of 38 layers (0.01%) for which PIH and PIL were both zero or omitted and TEXTURE1 specified a clayey texture class (clay, clay loam, or silt clay loam) for which one would normally expect a non-zero plasticity index. For the eight cases in which the texture class was clay, it was accompanied by a rock-fragment modifier specifying an admixture of either gravel or shale. For the remaining 30 cases, the PIH and PIL values were omitted from the STATSGO record, and not specifically entered as zero. For texture classes having relatively little clay, the fraction of component layers with PIH/PIL zero or omitted generally increased as the amount of sand increased and the amount of clay decreased -- see the table, below. The contributions of each component layer to the standard layers for a given map unit were determined using the component layer depths specified by Layer table variables LAYDEPL and LAYDEPH, the mean depth to bedrock for each component calculated by averaging Comp table variables ROCKDEPL and ROCKDEPH, and the percent of the area of the map unit covered by each component as specified by COMPPCT. For each component, the layers defined in the Layer table were compared with each standard layer in turn. If the standard layer was entirely included within one of the component layers, the plasticity index value for the layer was multiplied by the COMPPCT value to determine the weighted contribution of the component to the standard layer. If the standard layer overlapped two or more component layers, the plasticity index values for each component layer were first weighted in proportion to the amount of overlap before multiplication by the COMPPCT value. The region from the bottom of the last component layer to the bottom of the last standard layer, if any, was assumed to be the same as the lowest component layer down to the mean bedrock depth. Below this depth, the plasticity index was set to 0. The weighted contributions of all components to each standard layer were then summed to obtain the mean plasticity index values for the map unit. If none of the component layers contributing to the standard layer were mineral soil or if the entire map unit was specified to be water, the plasticity index was set to zero. NOTE that for many STATSGO components, a depth-to-bedrock value of 60 inches (152 cm) was used to indicate that the soil was not examined below this depth, and bedrock was not actually encountered. In all cases, however, the value of plasticity was computed as if bedrock was encountered at the depth specified by the mean of ROCKDEPL and ROCKDEPH. Accordingly, the plasticity index values for the two lowest standard layers (1.5 to 2.5 m) are, in many cases, misleadingly low. The number of component layers having PIH/PIL values which are zero or omitted was tabulated for the six texture classes which do not contain "clay" in their names. The number of component layers for which the mean plasticity index is greater than zero was also tabulated, and the percent of layers with zero or omitted PI values was computed. The results are shown below: Texture PI = 0 PI > 0 % zero Loam 601 61793 1.0 Silt Loam 841 57070 1.5 Silt 12 176 6.4 Sandy Loam 7763 52859 12.8 Loamy Sand 8750 2951 74.8 Sand 18325 3882 82.5

It is important to emphasize that, in addition to the limitations associated with generalizing from detailed soil maps to representative soil profiles in the STATSGO data, another level of generalization has been added by taking area-weighted averages over all the components in each STATSGO mapunit. Hence, for most mapunits, the average soil profile will not closely match any actual soil profile.

Miller, D.A. and R.A. White, 1998: A Conterminous United States Multi-Layer Soil Characteristics Data Set for Regional Climate and Hydrology Modeling. Earth Interactions, 2. [Available on-line at http://EarthInteractions.org] http://www.essc.psu.edu/soil_info/index.cgi?soil_data&conus&data_cov&ph

Process_Description:

Determination of Bulk Density And Porosity

Determining the Plasticity Index for Standard Layers
The mean plasticity index was determined for each of 11 standard layers for each map unit of each state using data from the STATSGO Comp and Layer tables. The standard layers were introduced because of the wide variation in the number, thickness, and depth to top and bottom of soil layers in the STATSGO data from one soil component to another, even within the same map unit. Variable layers cause problems for many environmental models and GIS operations.
Determining the mean plasticity index for the 11 standard layers required three main steps: 

Computing the mean plasticity index for each component layer.

For each component, determining the contribution of each component layer to the 11 standard layers.

For each map unit, combining the contributions of all components to compute the mean plasticity index for each standard layer.

For each layer of each map unit component, the STATSGO Layer table contains two values for the plasticity index, PIH and PIL, defined as the maximum and minimum, respectively, of "the range in plasticity index for the soil layer or horizon, expressed as percent of moisture by weight." NOTE that these values do not indicate the values of the liquid and plastic limits, but only the differences between them. The mean plasticity index for each component layer was computed as the arithmetic average of PIH and PIL. 
The Layer table variable TEXTURE1, which gives the dominant soil texture class for the layer, was also read; if it corresponded to neither mineral soil nor bedrock, (i.e., water, organic matter, or other), or if the component was identified as all water (COMPNAME = "WATER"), the component layer was excluded from the computation of mean plasticity. Approximatedly 11% of all component layers had no entries for PIH and PIL; in these cases, the plasticity index values were assumed to be zero. There were a total of 38 layers (0.01%) for which PIH and PIL were both zero or omitted and TEXTURE1 specified a clayey texture class (clay, clay loam, or silt clay loam) for which one would normally expect a non-zero plasticity index. For the eight cases in which the texture class was clay, it was accompanied by a rock-fragment modifier specifying an admixture of either gravel or shale. For the remaining 30 cases, the PIH and PIL values were omitted from the STATSGO record, and not specifically entered as zero. For texture classes having relatively little clay, the fraction of component layers with PIH/PIL zero or omitted generally increased as the amount of sand increased and the amount of clay decreased -- see the table, below. 

The contributions of each component layer to the standard layers for a given map unit were determined using the component layer depths specified by Layer table variables LAYDEPL and LAYDEPH, the mean depth to bedrock for each component calculated by averaging Comp table variables ROCKDEPL and ROCKDEPH, and the percent of the area of the map unit covered by each component as specified by COMPPCT. For each component, the layers defined in the Layer table were compared with each standard layer in turn. If the standard layer was entirely included within one of the component layers, the plasticity index value for the layer was multiplied by the COMPPCT value to determine the weighted contribution of the component to the standard layer. If the standard layer overlapped two or more component layers, the plasticity index values for each component layer were first weighted in proportion to the amount of overlap before multiplication by the COMPPCT value. The region from the bottom of the last component layer to the bottom of the last standard layer, if any, was assumed to be the same as the lowest component layer down to the mean bedrock depth. Below this depth, the plasticity index was set to 0. 

The weighted contributions of all components to each standard layer were then summed to obtain the mean plasticity index values for the map unit. If none of the component layers contributing to the standard layer were mineral soil or if the entire map unit was specified to be water, the plasticity index was set to zero. 

NOTE that for many STATSGO components, a depth-to-bedrock value of 60 inches (152 cm) was used to indicate that the soil was not examined below this depth, and bedrock was not actually encountered. In all cases, however, the value of plasticity was computed as if bedrock was encountered at the depth specified by the mean of ROCKDEPL and ROCKDEPH. Accordingly, the plasticity index values for the two lowest standard layers (1.5 to 2.5 m) are, in many cases, misleadingly low. 

The number of component layers having PIH/PIL values which are zero or omitted was tabulated for the six texture classes which do not contain "clay" in their names. The number of component layers for which the mean plasticity index is greater than zero was also tabulated, and the percent of layers with zero or omitted PI values was computed. The results are shown below: 

 Texture PI = 0   PI > 0  % zero   

Loam  601   61793  1.0  
Silt Loam  841   57070  1.5  
Silt  12   176  6.4  
Sandy Loam  7763   52859  12.8  
Loamy Sand  8750   2951  74.8  
Sand  18325   3882  82.5  

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).

plast_grid

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