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Formation and Classification of the Soil

This section describes the major factors of soil formation, tells how these factors have affected the soils of theVentura Area, and explains some of the principal processes in horizon development . It also defines the current system for classifying soils and shows the classification of the soils by series and higher categories.
Factors of Soil Formation
Soil forms through the interaction of the major soil-forming factors--parent material, climate, vegetation and animal life, relief, and time.
Climate and vegetation are the active forces in soil formation. Relief modifies the effects of climate and vegetation, mainly through its influence on runoff and temperature. Parent material also affects the kind of profile that forms. Time is needed for changing the parent material into soil. Usually a long time is required for the formation of distinct horizons
Parent Material
Parent material, which is the weathered rock or unconsolidated mass from which the soil forms, determines the chemical and mineralogical composition of the soil. Soils of the Ventura Area formed in material weathered from sandstone, shale, and basic igneous rock, and in alluvium derived from mixed rock sources.
Marine sandstone, shale, and semi consolidated material occupy the major part of the uplands. Considerable interbedding occurs, and the material varies in hardness and lime content. In sandstone and shale the percentage of the slowly weatherable mineral quartz is relatively high. these rocks differ mainly in the size of the individual grains and the strength of the cementing agents. Sandstone is the coarser grained. Sandy soils, such as Arnold and Gaviota soils, formed in material weathered from sandstone. Loamy, silty, and clayey soils, for example, Balcom, Castaic, Diablo, Nmcimiento, and San Benito soils, formed in material weathered from shale.
Basic igneous rocks occur in the southern part of the Ventura Area, mainly in an area that extends from Long Grade Canyon and Conejo Mountain through the south side of Santa Rosa Valley. They also occur south of Newbury Park and in an area that extends from Sandstone Peak to the Ventura County-Los Angeles County line. In basic igneous rocks the percentage of weatherable minerals is high. Clayey soils, such as Gilroy and Hambright soils, formed in material weathered from these rocks.
Except for Vina soils, which formed in alluvium derived from basic igneous rocks, the alluvial soils in the Area are derived from mixed rock sources Some are relatively uniform in texture; some are stratified. The texture ranges from sand to clay, and the reaction from slightly acid to moderately alkaline.
Climate
The climate of the Area is characterized by mild winters, warm summers , and moderate rainfal1. Presumably it is similar to the climate under which the soils formed. Only the stony and cobbly soils in the Ojai and Santa Clara Valleys appear to have formed under a climate in which storms were of higher intensity. About 14 to 22 inches of rain falls annually. This amount is insufficient to leach bases from the soil profiles. Consequently, some soils, for example, Anacapa, Cropley, Pacheco, Salinas, and Sorrento soils, have a zone of carbonate accumulation.
Living Organisms
Vegetation, burrowing animals, insects, earthworms, bacteria, and fungi are important in the formation of soils. Plants generally have a greater influence on soil formation than other living organisms have. They provide shade and cover, thus reducing runoff and the erosion hazard, and their roots loosen the soil material and add organic matter, thereby influencing soil structure and physical condition. Bases move upward from plant roots to the leaves and stems and are eventually returned to the soil, unless they are removed by grazing animals. This process counteracts the leaching of bases by rainfall and adds organic matter to the soil. Scanty vegetation contributes no appreciable amount of organic matter. Hence, soils that developed under brush, Arnold and Gaviota soils, for example, are affected by droughtiness, are low in organic-matter content, and have a light-colored surface layer. In contrast, soils that developed under grasses and forbs, Diablo, Linne, and Nacimiento soils, for example, are fine textured, are high in organic-matter content, and have a dark-colored surface layer. Well-drained alluvial soils, such as Anacapa, Garretson, Mocho, and Sorrento soils, developed under annual grasses and scattered brush. Poorly drained soils, for example, Camarillo, Hueneme, and Pacheco soils, developed under salt-tolerant and water-tolerant plants.
Micro-organisms play an important part in transforming plant nutrients. Burrowing animals and earthworms loosen and mix the soil and thus slow down the formation of distinct soil horizons.
Relief
Relief, or the shape of the landscape, influences soil formation, mainly through its effect on drainage and erosion, and partly through variations in exposure to the sun and wind and in air drainage.
Camarillo, Hueneme, and Pacheco soils formed in low-lying, poorly drained areas under salt-tolerant and water-tolerant plants. They have mottled underlying horizons that contain segregated lime and gypsum. Anacapa, Garretson, and Pico soils formed on well-drained alluvial fans and plains. They lack mottles and segregated gypsum. Upland soils on north-facing slopes receive less direct sunlight, have cooler soil temperatures and retain moisture longer than those on south-facing slopes, and they therefore tend to develop a denser vegetative cover, and in turn, a deeper, darker colored surface layer. For example, San Bonito soils, which generally occur on north-facing slopes, have a deeper surface layer than Nacimiento soils, which commonly occur on adjacent south-facing slopes. On steep slopes, relief is the dominant factor in soil formation. In these areas the soil material is removed by erosion nearly as fast as it forms; consequently, a thick soil profile seldom develops. Examples of shallow, steep soils are the Calleguas, Gaviota, and Millsholm soils.
Time
A long time is generally required for soil formation. The length of time depends largely on the other four soil-forming factors. Presumably, under a good vegetative cover and the most favorable climate, the formation of a single inch of topsoil from the raw material of the subsoil takes from 200 to 1,000 years. The formation of Huerhuero and Rincon soils, for example, which have a strongly developed subsoil, or Chesterton soils, which have a silica- cemented hardpan, indicates a million or more years of soil-building processes. Soils that have been in place for a relatively short time have not yet been influenced enough by the other soil-forming factors to have developed well-defined and genetically related horizons. Examples are Anacapa, Garretson, and Pico soils, which formed in recent alluvium. Time is directly related to relief for young soils in areas where soil material is removed by erosion nearly as fast as it forms. Young soils on steep slopes, such as Arnold, Balcom, Castaic, Gaviota, Nacimiento, and Saugus soils, lack well- developed horizons.
Processes of Soil Formation
The accumulation of organic matter, the solution, transfer, and reprecipitation of calcium carbonate and bases, the liberation, reduction, and transfer of iron, and the formation and translocation of silicate clay minerals have been active processes in the formation of the soils of the Ventura Area.
Accumulation of organic matter in the surface layer of the soils has been an important process in the formation of an Al horizon. In general, the soils that formed under dense vegetation and have the thickest, darkest colored Al horizon are highest in organic-matter content.
Leaching of carbonates from the upper horizons has occurred in a few soils in the Area, Generally this process precedes translocation of silicate clay minerals. The Huerhuero soil is an example of a soil that has been leached of carbonates to a depth below the accumulated silicate clay minerals.
Silicate clay accumulates in pores and forms bridges across sand grains and films on surfaces along which water moves. In the soils of this Area, the leaching of bases and the translocation of silicate clays are among the more important processes of horizon differentiation. The Hambright soil is an example of a soil that has a minimum of translocated clay. In contrast, the Huerhuero soil an example of a soil that has maximum clay translocation.
The reduction of iron, a process called gleying, results in mottled or olive and gray colors. Gleying is associated with poorly drained soils, such as Camarillo and Pacheco soils.
Classification of the Soils
Classification consists of an orderly grouping of soils according to a system designed to make it easier to remember soil characteristics and interrelationships. Classification is useful in organizing and applying the results of experience and research. Soils are placed in narrow classes for discussion in detailed soil surveys and for application of knowledge within farms and fields. The many thousands of narrow classes are then grouped into progressively fewer and broader classes in successively higher categories, so that information can be applied to geographic areas.
Two systems of classifying soils have been used in the United States in recent years. The older system was adopted in 1938 (2) and revised later (9). The system currently used by the National Cooperative Soil Survey was adopted in 1965 (11). It is under continual study. Readers interested in the development and application of the system should refer the latest literature available (6, 7).
The current system of classification has six categories. Beginning with the most inclusive, the categories are the order, the suborder, the great group, the subgroup, the family, and the series. The criteria for classification are soil properties that are observable or measurable, but the properties selected so that soils of similar genesis are grouped together. The placement of some soil series in the current system of classification, particularly in families, may change as more precise information becomes available.
Table 7 shows the classification of each soil series of the Ventura Area by family, subgroup, order, according to the current system.
A detailed description of each soil series represented in the Ventura Area is given in the section ‘Descriptions of the Soils.”
TABLE 7.--SOIL SERIES CLASSIFIED ACCORDING TO THE CURRENT SYSTEM OF CLASSIFICATION
Series
Family
Subgroup
Order
Anacapa
Coarse-loamy, mixed, thermic
Calcic Pachic Haploxerolls
Mollisols
Arnold
Mixed, thermic
Typic Xeropsamments
Entisols
Azule
Fine, montmorillonitic, thermic
Mollic Haploxeralfs
Alfisols
Balcom
Fine-silty, mixed, calcareous, thermic
Typic Xerorthents
Entisols
Calleguas
Loamy-skeletal, mixed, calcareous, thermic
Lithic Xerorthents
Entisols
Camarillo
Fine-loamy, mixed, calcareous, thermic
Aquic Xerofluvents
Entisols
Castaic
Fine-silty, mixed, nonacid, thermic
Typic Xerorthents
Entisols
Chesterton 1/
Fine, montmorillonitic, thermic
Abruptic Durixeralfs
Alfisols
Cibo
Fine, montmorillonitic, thermic
Typic Chromoxererts
Vertisols
Corraljtos
Mixed, thermic
Typic Xeropsamments
Entisols
Cortina
Loamy-skeletal, mixed, nonacid, thermic
Typic Xerofluvents
Entisols
Cropley
Fine, montmorillonitic, thermic
Chromic Pelloxererts
Vertisols
Cropley, calcareousvariant
Fine,montmorillonitic, thermic
Chromic Pelloxererts
Vertisols
Diablo
Fine, montmorillonitic, thermic
Chromic Pelloxererts
Vertisols
Garretson
Fine-loamy, mixed, nonacid, thermic
Typic Xerorthents
Entisols
Garretson, calcareous variant
Fine-silty, mixed, calcareous, thermic
Typic Xerorthents
Entisols
Gaviota
Loamy, mixed, nonacid, thermic
Lithic Xerorthents
Entisols
Gazos 1/
Fine-loamy, mixed, thermic
Typic Haploxerolls
Mollisols
Gilroy
Fine-loamy, mixed, thermic
Typic Argixerolls
Mollisols
Hambright
Loamy-skeletal, mixed, thermic
Lithic Haploxerolls
Mollisols
Hueneme
Coarse-loamy, mixed, calcareous, thermic
Aquic Xerofluvents
Entisols
Huerhuero
Fine, montmorillonitic, thermic
Haplic Natrixeralfs
Alfisols
Kimball
Fine, montmorillonitic, thermic
Mollic Palexeralfs
Alfisols
Linne
Fine-loamy, mixed, thermic
Calcic Pachic Haploxerolls
Mollisols
Lodo
Loamy, mixed, thermic
Lithic Haploxerolls
Mollisols
Los Osos
Fine, montmorillonitic, thermic
Typic Argixerolls
Mollisols
Malibu
Fine, montmorillonitic, thermic
Abruptic Palexeralfs
Mollisols
Metz
Sandy, mixed, thermic
Typic Xerorthents
Entisols