The soils in the area, primarily used for cropland, in the Sacramento Valley, and rangeland, in the Coast Range foothills, are rated in Table 10a according to the Storie index (Storie, 1933; 1976). The soils in the higher Coast Range, primarily used for timberland, are in Table 10b according to the Storie index (Storie, 1948). These indexes express numerically the relative degree of suitability of a soil for general intensive agricultural uses or timber uses at the time of the evaluation.

The rating is based on soil characteristics and is obtained by evaluating soil surface and subsurface chemical and physical properties, as well as landscape surface features. The original "Soil-Rating Chart" (Storie 1976) is in the Appendices. Some of the terminology used in this chart is not used today. However, when the ratings were developed, they were compared to today’s terminology to create the most logical estimate for the rating.

Not considered in the rating are availability of water for irrigation, local climate, size and accessibility of mapped areas, distance to markets and other factors that might determine the desirability of growing certain plants in a given locality. Therefore, the index should not be used as the only indicator of land value. Where the local economic and geographic factors are known to the user, however, the Storie index may provide additional objective information for land tract value comparisons.

Four general factors are used in determining the index rating:

1. Permeability, available water capacity, and depth of the soil
2. Texture of the surface soil
3. Dominant slope of the soil body
4. Other conditions more readily subject to management or modification by the land user. In this area these conditions include drainage and flooding, salinity and alkalinity, fertility, acidity, erosion, and microrelief. For some soils, more than one of these X conditions are used in determining the rating.

A rating of 100 percent expresses the most favorable, or ideal, condition for general crop production. Lower percentage ratings are assigned for less favorable conditions or characteristics. Factor ratings, in percentages, are selected from tables prepared from data and yields. Certain properties are assigned a range of values to allow for variations in the properties to plant growth and crop yields. Certain properties are assigned a range of values to allow for variations in the properties that affect the suitability of the soil for general agricultural purposes.

The index rating for a soil component of a map unit is obtained by multiplying the percentage rating values given to its four factors, A, B, C, and X. If more than one condition is recognized for the X factor for a soil, the value for each condition acts as a multiplier. Therefore any of the general factors or X factors conditions may dominate or control the final rating.

If a map unit consists primarily of one named soil series (a consociation), the index rating for the named soil component equals the index rating for the map unit. If a map unit consists of more than one named component (a complex), ratings are assigned to each named component (soil series or miscellaneous area, such as "Rock outcrop"). Inclusions of other soils or minor components not named in the map unit name, are not used in the calculations.

Map units are assigned grades according to their suitability for general intensive agriculture as shown by their Storie index ratings. The six grades and their range in index ratings are:

Grade 1—80 to 100

Grade 2—60 to 79

Grade 3—40 to 59

Grade 4—20 to 39

Grade 5—10 to 19

Grade 6—less than 10

Grade 1 are well suited to intensively cultivated crops that are climatically adapted to the region.

Grade 2 soils are good agricultural soils, although they are not so desirable as soils in grade 1 because of a less permeable subsoil, deep cemented layers (e.g., duripans), a gravelly or moderately fine textured surface layer, moderate or strong slopes, restricted drainage, low available water capacity, lower soil fertility, or a slight or moderate hazard of flooding.

Grade 3 soils are only fairly well suited to agriculture because of moderate soil depth; moderate to steep slopes, restricted permeability in the subsoil; a clayey, sandy, or gravelly surface layer; somewhat restricted drainage; acidity; low fertility; or a hazard of flooding.

Grade 4 soils are poorly suited to agriculture. They are more limited in their agricultural potential than the soils in grade 3 because of restrictions, such as a shallower depth; steeper slopes; poorer drainage; a less permeable subsoil; a gravelly, sandy, or clayey surface layer; channeled or hummocky microrelief; acidity.

Grade 5 soils are very poorly suited to agriculture and are seldom cultivated. They are more commonly used as pasture, rangeland, or woodland.

Grade 6 soils and miscellaneous areas are not suited to agriculture because of very severe or extreme limitations. They are better suited to limited use as rangeland, protective habitat, woodland, or watershed.

The rating is based on soil characteristics and is obtained by evaluating soil surface and subsurface chemical and physical properties, as well as climatic features. The original "Timber Soil Rating Chart" (Storie 1948) is in the Appendices. Some of the terminology used in this chart is not used today. However, when the ratings were developed, they were compared to today’s terminology to create the most logical estimate for the rating.

Not considered in the rating are size and accessibility of mapped areas, distance to markets and other factors that might determine the desirability of timber production in a given locality. Therefore, the index should not be used as the only indicator of timber production. Where the local economic and geographic factors are known to the user, however, the Storie index may provide additional objective information for timber production.

Five general factors are used in determining the index rating:

1. Soil depth and texture characteristics of the soil
2. Soil permeability
3. Soil chemical characteristics.
4. Drainage and runoff properties of the soil.
5. Climate.

The index rating for a soil component of a map unit is obtained by multiplying the percentage rating values given to its four factors, A, B, C, D, and E.

If a map unit consists primarily of one named soil series (a consociation), the index rating for the named soil component equals the index rating for the map unit. If a map unit consists of more than one named component (a complex), ratings are assigned to each named component, except rock out crop. Miscellaneous areas are considered to be unsuited for timber production, and are not rated or considered zero. Inclusions of other soils or minor components not named in the map unit name, are not used in the calculations.

Timber site ratings in percent were assigned adjective ratings according to their suitability for general intensive timber production as shown by figure 1 "Relationship Between Timber Site Rating and Height - Age Index" in the appendices showing the "Timber Soil Rating". The five ratings and their range in index ratings are:

Redwood—100 to 120

High—85 to 99

Medium—50 to 84

Low—30 to 49

Non-Timber—0 to 29