Several problems unique to the field work and mapping occur in soil surveys. Because soil series vary so much in slope, degree of erosion, number and size of stones, or some other feature affecting their use, practical suggestions about their management could not be made if they were shown on the soil map as one unit. Such soil series are divided into soil phases. The name of the soil phase indicates the surface feature most affecting its management. For example, Vina loam, 0 to 2 percent slopes; Vina loam, 2 to 9 percent slopes; Vina gravelly loam, 2 to 9 percent slopes; and Vina silty clay loam, 2 to 9 percent slopes are four soil phases of the Vina series.
The area enclosed within each continuous line on a soil map is the delineation of a map unit. The map unit includes the soil phase and all inclusions. A problem for the soil scientist arises in delineating areas where several different soils are so intricately associated and so small in size it is not practical to show them separately on the aerial photo maps. Therefore, they show this conglomerate of soils as one mapping unit and call it a soil complex.
Ordinarily, a soil complex is named for the major kinds of soils in it. For example, Castaic-Balcom complex, 9 to 15 percent slopes, eroded and Millsholm-Malibu complex, 30 to 50 percent slopes, eroded occur in the Ventura Area Soil Survey.
Another problem occurs when the soils have been disturbed by road work and other mining or construction activities to such an extent the original characteristics of the soil have been obliterated. These areas are delineated on the soil map as are other map units by being called "made land”, "pits and dumps”, and "fill land”. Other natural non-soil areas are given descriptive names such as: “badland”, "igneous rock land”, "landslides”, "sandy alluvial land", and “terrace escarpments”.