FIGURE 4.6 The June 1991 status map of the soil survey geographic databases (SSURGO) (USDA 1991a).

The Soil Interpretation Record (SIR or SOIL-5) database contains estimated values for more than 25 properties of more than 30,000 soil series and phases of soil series in the United States. These property or attribute values define the expected range of values for the site and major layer (horizon) of a soil series. The properties included are site characteristics including mean annual air temperature, precipitation, elevation, frost free days, and drainage; and horizon attributes including particle size distribution, bulk density, permeability, organic matter, available water capacity, soil reaction, salinity, cation exchange capacity, sodium absorption ratio, gypsum, and calcium carbonate equivalent. Estimates of flooding potential, water table depth, depth to bedrock, shrink-swell, and potential frost action characteristics are also listed.

Soil property data are recorded in SIR as an estimated range of values because soils in the landscape occur naturally in a continuum, not as discrete entities. Hence, all map units have inclusions of other soil series with similar and/or dissimilar properties. Soil survey guidelines (USDA 1983) specify that no more than 25 percent of a mapping unit should be comprised of dissimilar soils. Wilding (1988) and colleagues attempted to quantify the spatial variability of a number of soil properties within mapping units of