Soil is divided into three major soil phases. The three soil phases are; complex, association and undifferentiated groups. Each soil phases has unique chemical and physical properties that learners and surveyors use to distinguish one soil phase from the other. The common physical properties that are used to distinguish the three soil phases are: depth, clay, moist bulk density, available water capacity and organic matter content. On the other had the chemical properties that are used to distinguish the soil phases are cation exchange capacity and pH. The table below shows the chemical and physical properties of the three soil phases which cover the best part of the AOI.
Moist bulk density
Available water capacity
Organic matter content
Cation exchange capacity
California sagebrush (ARCA11) - 16%
Bigpod ceanothus (CEME) - 44%
Red shank (ADSP) - 33%
Similarities between the soil phases
All the three soil phases encompass of clay, moist, water and organic matter even though their quantity vary from one soil phase to another. For example, from the above table the clay content of complex and undifferentiated soil phase is 12-27 percent while the content of association soil phase is 18-27 percent. Thus, all the soil phases are made up of clay content but the content of clay is less than 27 percent.
Differences between the soil phases
Associations and complexes soil phases consist of at least two dissimilar constituents that occur regularly in a repetitive pattern. However, the difference between the two soil phases is that the major elements of complex soil phase cannot be mapped individually at a scale of 1:24, 000; where else the major elements of associations soil phase can be mapped individually at a scale of 1:24, 000. On its part, the undifferentiated soil phase consists of at least two dissimilar constituents that are not constantly associated geographically. This indicates the soil components of undifferentiated soil phase do not occur jointly in the same map (Natural Resources Conservation Service 2006).
The type of vegetation that grows in a specific region is determined by chemical and physical properties of the soil. Various vegetations require specific soil conditions to germinate and produce. This is the main reason why different vegetations grow in different soil phases, since the soil condition of all the major soil phases are different. The major soil chemical properties that determine the type of vegetation that grow in a region are cation exchange capacity and pH. On the other hand, the physical soil properties that determine the type of vegetation that grow in a region are organic matter content and the moisture content of the soil. Thus, the difference in chemical and physical properties of the three major soil phases is the main reason why different vegetations grow in different soil phases (Natural Resources Conservation Service 2012).
The formation of the three major soil phases is influenced by weathering process. Both physical and chemical weatherings are critical in the breakdown of large rocks to tiny soil particles. Therefore, the clay content that is present in all the soil phases is as a result of weathering process.
Natural Resources Conservation Service, (August 27, 2012), Custom Soil Resource Report for Santa Monica Mountains National Recreation Area, United States Department of Agriculture
Natural Resources Conservation Service, (2006), Soil Survey of Santa Monica Mountains National Recreation Area, California