Figure 6. Roots from redwood trees have entered cracks in bedrock in search for nutrients and water.
Formation and morphology of the soils
This section discusses the factors of soil formation, relates them to the formation of soils in the survey area, and explains the processes of soil formation.
Soil is a natural body on the surface of the earth in which plants grow; it consists of organic and mineral matter (24). The characteristics of a soil at any given place are deter mined by the interaction of five factors of soil formation: (1) the climate under which the soil material has accumulated or weathered; (2) the influence of plants and animals; (3) the relief, or topography; (4) physical and chemical properties of the parent material; and (5) the length of time these factors have been active. Each of these factors affects the formation of every soil, and each modifies the effects of the other four. The importance of the individual factors differs from place to place.
Climate and plants and animals are the "active" factors of soil formation. They act on the parent material that has accumulated through the weathering of rocks and slowly change it into soil. Relief modifies the effects of climate and vegetation, mainly by its influence on runoff and temperature. The nature of the parent material also affects the kind of soil that is formed. Time is needed for changing the parent material into soil, and generally a long time is needed for distinct soil horizons to form.
The interactions among these factors are more complex for some soils than for others. In places, for example, the environment has changed, and the characteristics of a new soil have been superimposed on those of an older soil.
In the pages that follow the five major factors of soil formation are discussed in relation to their effects on the formation of the soils in Santa Cruz County.
The climate has a marked influence on the kind of soil that forms. Heat and moisture strongly influence the amount and kind of vegetation, the rate at which organic matter decomposes, the rate at which minerals weather, and the removal or accumulation of material in the different soil horizons.
There are different climatic regions in the county. These regions are the coastal areas and valleys that open to the coast and the higher, more rugged mountainous areas of the Ben Lomond and Santa Cruz Mountains.
The mean annual air temperature in the county only varies from 55 to 58 degrees F. There is, however, a great difference between the maximum and the minimum temperatures from one region to another. The mean maximum temperature in the coastal areas and valleys ranges from about 68 degrees along the coast to about 76 degrees along the foothills. In the mountains the temperature ranges from 76 to 80 degrees F. The mean minimum temperature in the coastal areas and in the valleys is about 38 degrees F, and in the mountains it is about 36 degrees. Summers are cool foggy, and dry, and winters are cool and moist.
Most of the precipitation in the county occurs from November through April. Mean annual precipitation ranges from 20 to 35 inches in the coastal areas and valleys to as much as 35 to 60 inches in the Ben Lomond and Santa Cruz Mountains. In the mountains, snowfall occurs about every other year and averages less than 5 inches (8). Additional climatic data is given in the section "General nature of the county."
Along the immediate coast, fog occurs in summer, humidity is higher, and the transpiration rate and temperature are lower. This mild climate influences the decomposition of the surface litter and results in soils that have a gray, dark gray, or grayish brown surface layer.
In the hills and mountains, the effects of higher precipitation and lower temperature are reflected in the kinds of vegetation and soil; woody and herbaceous vegetation is more abundant, and the organic matter con tent of the soils increases. Laboratory analyses indicate that the surface layer of the soils in these areas contains 3 percent to as much as 5 percent organic carbon. The soils are commonly dark colored and have a granular surface layer overlain by a layer of leaves, twigs, and decomposed organic matter.
In the mountains, rainfall is sufficient to leach the soils of bases, which results in lower soil reaction. The soils are commonly medium acid to very strongly acid.
Warm temperatures in spring, when the soils are moist, accelerates the soil-forming processes. The warm weather permits rapid chemical reactions to take place, and the water from the spring rains moves through the soils to translocate dissolved or suspended clay particles. The surface litter decomposes rapidly, which increases the organic matter content of the soil. Climate alone does not account for all the local differences among the soils.
Plants and animals are important biological forces that affect soil formation. They affect organic matter and nitrogen content and soil reaction and help improve structure and porosity.
The most extensive plant community in Santa Cruz County is woodland-shrub. The coastal and stream terraces are dominantly in grasses and forbs, but there are a few areas of oak, eucalyptus, and pine. Much of the Pajaro Valley area that is now cultivated was originally pass (5).
Soils that formed under grasses have a dark-colored A horizon (7). The A horizon is thickest and darkest in the Pajaro Valley and along the terraces, and it is thinnest on the southwest-facing slopes of the mountains. Soils represented are those of the Conejo, Elder, Fagan, and Los Osos series.
About 70 percent of the county is dominated by coniferous and broadleaf trees that occur mainly on north- and northeast-facing slopes. This type of plant cover reduces runoff, erosion, and evaporation. Soils that formed under coniferous forest commonly have a mat of fresh and decomposed bark, twigs, leaves, and needles 1 inch to 6 inches thick. Such material is acid, and it con tributes to the acid reaction of soils. When decomposed, this mat of organic matter gives a dark color to the surface; this dark color extends deeply into the soil profile. Trees and other plants take up plant nutrients from the soil and store them in their roots, branches,. and leaves. When these plants die and decompose, elements are returned to the soil to be used again. Accumulation of organic matter on and in the soil is an important process in horizon differentiation in the soils of Santa Cruz County (4, 18, 28), Soil analyses indicate 1 percent to about 6 per cent organic carbon in the surface layer.
The roots of coniferous and broadleaf trees follow cracks and fracture planes in the parent rock and help in the physical and chemical weathering processes (fig. 6).
Figure 6. Roots from redwood trees have entered cracks in bedrock
in search for nutrients and water.
In places roots make up more than 20 percent of the upper 2 or 3 feet of the soil. In most places the carbon-to-nitrogen ratio of these soils is more than 20 to 1. Soils that have dark color, porosity, and structure associated with wooded areas are those of the Ben Lomond, Nisene, Felton, Catelli, Lompico, and Sur series. The dominant tree species on these soils are redwood, Douglas-fir, tan oak, madrone, California live oak, black oak, and laurel (20).
On south- and southwest-facing slopes and in areas higher than the persistent summer fog belt, the steep and very steep slopes are covered mostly with brush and chaparral. Brush in these areas does not adequately protect the soils from erosion. Because of continual erosion, the soils in most of these areas are less than 20 inches deep. Examples are Maymen and Maymen Variant soils. The main species growing in these areas are Chamis, Manzanita, ceanothus, and coyotebrush.
Man has disturbed the soils by mining, clearing, or burning vegetation, harvesting timber, grazing livestock, and cultivating the soils. Burning has influenced the soils most by depleting organic matter, accelerating erosion, and changing the characteristics of the surface layer.
The effect of animals on soils in the county is less apparent, and few major soil features are attributed solely to their activity (21). Ground squirrel and pocket gopher nest and burrow in the soils, bringing a large volume of soil material to the surface each year. This mixing alters the profile of some soils.
Relief has had an important effect on soil formation in Santa Cruz County. Steepness, shape, and length of slopes affect the runoff, erosion, and the amount of moisture available for soil development. For example, Maymen soils that have steep to very steep slopes have features that have been largely determined by the degree of slope. Because of rapid runoff and erosion, these soils are less than 20 inches deep. The surface layer is very thin, and little or no clay has accumulated in the subsoil. In nearly level to moderately sloping areas, however, where Danville soils formed, more water filters into the soil. A moderately thick to thick surface layer has formed. The increased moisture passing throughout the profile aids development of the subsoil by moving clay particles out of the surface layer and into the subsoil.
Aspect is especially important in the hills and mountains. It has an important effect upon the microclimate of the soils (15). For example, soils that have north-facing slopes have cooler temperatures and retain moisture longer than soils that have south-facing slopes. Consequently, the soils that have north-facing slopes have a denser plant cover, are deeper, have a thicker, darker colored surface layer, and have a greater clay increase in the subsoil than do soils that have south-facing slopes. Differences in soil characteristics as a result of aspect are readily evident in Ben Lomond or Felton soils that have north- and northeast-facing slopes and Bonnydoon and Maymen soils that have southwest-facing slopes.
Elevation also influences soil formation, mainly through its effect on soil climate. Elevation in the county ranges from sea level to about 3,000 feet.
On alluvial plains the water table is closest to the surface where the local relief is least. When the soil profile is saturated with water, many physical and chemical reactions are inhibited. Downward movement of water is restricted. Anaerobic reactions become dominant because there is inadequate oxygen. These soils tend to be colder than soils that have aerobic conditions. In Aquents, the ground water is very near the surface.
Soils on alluvial plains, because of their low-lying position, commonly receive additional sediment from flooding. This generally retards soil development, because each episode of flooding and deposition provides new soil parent material and initiates another cycle of soil development. An example of this is the stratified Mocho soil.
Parent material is the great variety of unconsolidated material from which the soil forms. Some features of the parent material that affect the kinds of soil that form are mineralogical composition, degree of consolidation, grain size, and presence or absence of salts.
Santa Cruz County consists of many geologic formations that are made up of a wide variety of igneous, sedimentary, and metamorphic rocks (3). These rocks differ greatly in age, hardness, and resistance to weathering. These differences in the rocks cause differences in the landscape, and they also affect the characteristics of the soils that form.
The Sur Formation, which consists of metamorphosed rock, is the oldest formation in the county. The soils in the northeastern part of the Ben Lomond Mountains and near the University of California are commonly moderately steep to very steep. Schist is the most common rock. The soils that formed in material derived from schist have a loamy texture. Examples of soils that formed in this material are Aptos, Nisene, Felton, and Lompico.
Most of the northeast-facing landscapes of Ben Lomond Mountain consist of intrusions of granitic rock. Most areas consist of material derived from quartz diorite, but some areas consist of material derived from granodiorite (12). The soils that formed in material derived from these rocks have a loamy texture. The dominant soils are Ben Lomond, Catelli, and Sur. Sur soils have a high content of rock fragments.
Rock of the Paleocene Epoch occupies relatively small areas in Santa Cruz County. It consists mostly of micaceous sandy siltstone and sandstone of the Locatelli Formation. Some of the soils that formed in material derived from siltstone are Felton and Lompico. The soils that formed in material derived from sandstone are Ben Lomond and Sur. The sedimentary rock of the Eocene, Oligocene, and Miocene Epochs is most extensive rock in Santa Cruz County. Several formations of sedimentary rock have been recognized, including the Vaqueros (Oligocene), Butano (Eocene), Monterey and Santa Margarita (Miocene), and Purisima (Pliocene) Formations.
The rock of the Vaqueros Formation consists largely of arkosic sandstone and distinct interbeds of mudstone and shale. This rock is exposed in large areas, mainly along the summit and on ridgetops in the Santa Cruz Mountains. Where the rock is coarse-grained sandstone, the soils that commonly formed are Sur, Catelli, and Ben Lomond; where the rock is weathered fine-grained sand stone, the soils that formed are Los Osos, Fagan and Diablo. Felton and Lompico soils formed in weathered mudstone and shale. The stony, shallow Maymen soils occur along the summit and are underlain by consolidated shale and mudstone. Large areas of exposed rock outcrop are common on the Maymen and Sur soils.
The Butano Formation consists largely of arkosic sand stone and interbeds of mudstone, shale, and siltstone. Where these rocks are on northeast- and north-facing slopes, they commonly are deeply weathered. Where they are on ridgetops and southwest- and south-facing slopes, the soils that formed in these areas are commonly sandy loam and fine sandy loam. Maymen and Sur soils generally are stony, and they formed in hard sandstone, shale, or mudstone. Catelli, Ben Lomond, and Pfeiffer soils formed in weathered coarse-grained sandstone. Pfeiffer soils have a high content of rock fragments.
The Monterey Formation extends from Boulder Creek east to an area south of Glenwood. The rocks are porcellaneous or elastic mudstone, shale, and various amounts of diatoms. The Monterey Formation also includes small areas of siltstone. The rock is generally light gray or olive gray to white and has low bulk density. On weathering, the rock becomes highly fractured and individual pieces remain hard and firm. The soils that formed in this material are generally gray, dark gray, or grayish brown. Bonnydoon and Maymen soils formed over shale and are generally less than 20 inches deep. Aptos loam, warm phase; Lompico soils; and Felton soils formed in material weathered from mudstone and siltstone and are generally 20 to 60 inches deep. The soils that formed in alluvial material derived mainly from porcellaneous and elastic mudstone, shale, and siltstone have a grayish surface layer similar to that of the soils on uplands that formed in material derived from these rocks.
The Santa Margarita Formation generally consists of highly weathered arkosic sandstone. Laboratory analyses of this sandstone indicate that it is 85 to 90 percent sand, 7 to 8 percent silt, and 4 percent clay. Zayante soils formed in material weathered from this sandstone.
The rock of the Purisima Formation consists mostly of fine-grained sandstone, mudstone, and siltstore. This rock is commonly deeply weathered, especially on the north and northeast-facing slopes, where greater moisture con tributes to deep weathering. The soils that formed in these areas commonly are loam, fine sandy loam, and sandy loam. In some places where the slopes face southwest, the mudstone or siltstone is hard and firm and the soils commonly are 10 to 20 inches deep. The soils that commonly formed in material, derived from these rocks are Bonnydoon, Pfeiffer, Ben Lomond, Felton, Aptos, Nisene, and Lompico. The Pfeiffer soils have a high content of rock fragments.
The Pleistocene and Holocene deposits consist of unconsolidated sediment The oldest of these deposits is the Aromas Red Sands in the Aptos La Selva Beach area (1). The formation consists of unconsolidated, well-sorted quartzose, brown to red sand that has strata of silt and clay. Baywood soils formed in this material. Terrace deposits consist mostly of clayey and loamy alluvium. The soils on the terraces are moderately to strongly developed. In places the soils have been dissected by drainageways. The soils that formed in these terrace deposits include Watsonville, Elkhorn, Tierra, and Pinto.
The youngest geologic material consists of late Holocene alluvium (14) that eroded from the uplands north and east of Watsonville. The alluvium has mixed lithology because there is a wide variety of rock sources, mostly of sedimentary origin. These deposits have been in place for so short a time that organisms and weathering have had little affect on the soils. The alluvium on fans and foot slopes generally has texture and other characteristics similar to those of the material in the hills directly above. Soquel soils are an example. The alluvium in the larger valleys has been deposited by flooding streams and rivers. As a stream or river overflows its channel and the water spreads over the flood plain, the coarse-textured sediment settles first. The floodwaters continue to spread, but they move more slowly and the finer sediment is next deposited. Most of the clay is deposited when the flood has passed and the water becomes still on the lowest part of the flood plain. In the Pajaro Valley, the alluvial material that extends from west Salsipuedes Creek to the ocean is finer textured than the alluvium in the upper part of the valley. The soils that formed in loamy sandy alluvium are Elder, San Emigdio Variant, and Baywood Variant. The soils that formed in the finer textured alluvium are Conejo, Clear Lake, Cropley, Mocho, Fluvaquentic Haploxerolls, and Aquic Xerofluvents.
The factors of soil formation are interrelated, and the effect of any one factor depends on the length of time the factor has been operating. The oldest geologic formations are of Paleozoic and upper Cretaceous age. These formations, however, provided the parent material for some of the youngest soils. These soils commonly are steep or very steep, and geologic erosion and soil creep takes place at a rate that equals or exceeds the rate of soil formation or weathering of the parent material. The soil material originally derived from the other formations has been deposited and removed many times. The soils most strongly developed formed on alluvial terraces and fans of Neistocene age.
The oldest soils generally have distinct horizons and are those in which the parent material has been most altered. Soils are considered to be old if their horizons markedly differ in color, texture, reaction, structure, and other features.
Soils that have only a few or indistinct horizon differences are considered to be of intermediate age. Soils that have few if any horizon differences are considered to be young. Differences between horizons or layers marked by lithologic discontinuities are not considered in evaluating the relative age of a soil.
Young soils can be separated into three groups. The most recent soils are those that formed in eolian deposits or in sand dunes. An example is the Baywood soils. Other recent soils formed on alluvial fans, alluvial plains, and flood plains. Examples of these soils are San Emidgio and Elder. Baywood and San Emigdio soils have had very little time for the accumulation of organic matter.
Soils on somewhat older alluvial fans and flood plains are represented by Mocho, Conejo, and Elder. These soils have had time for organic matter to accumulate and, consequently, exhibit darkening of the surface layer. Another group of young soils formed in material derived from consolidated rock, with new soil material being added by further weathering of the parent material. In this group are such soils as Maymen and the Xerorthents-Rock outcrop complex.
Somewhat older soils have undergone changes other than additions of organic matter and the gain or loss of bases. Clay has been removed from the surface layer and has accumulated in the subsoil. As clay accumulates in the subsoil over time, pores become fewer or smaller and permeability is reduced. The difference between the surface layer and subsoil becomes greater, and the horizon boundaries become more distinct. Water can accumulate just above the clay subsoil, causing temporary saturation in the lower part of the surface layer.
There is a relationship between the age of soils and the amount of clay that has accumulated in them. Ben Lomond, the youngest soils in the county, have about a 1 percent clay increase in the upper 35 inches. Watsonville, the oldest soils, have a clay increase of more than 33 per cent within a 1-inch vertical distance.
Because the influence of the soil-forming factors varies greatly within the survey area, many different kinds of soil have formed. Many soils in the area have several prominent horizons; some have only one horizon, and others have several weak horizons. In places, the soils that have prominent horizons are adjacent to those that have less distinct horizons. The processes that have the greatest influence on horizon formation are (1) the weathering of parent material, (2) the accumulation of organic matter, (3) the removal of organic matter, cations, clay; and (4) the formation and translocation of clay (7, 22).
The relative importance of these processes in horizon differentiation is not uniform in all soils. For example, montmorillonitic clay cracks extensively upon. drying. Soil material falls into the cracks and swells when wet. The soil is churned to the depth to which the cracks extend, thus inhibiting the formation of major horizons. Clear Lake, Cropley, and Diablo are examples of these soils.
Some of the distinguishing features of soils that formed in material derived from bedrock are related to the degree to which the parent material has weathered. For example, where weathering has been slight, the soils have few horizons and the distinguishing features are generally inherited from the parent material. As weathering in creases, horizon differences are less directly related to the parent material and are more the product of alteration, such as occurs with gains in organic matter or trans location of clay. The deep Felton soils have a brown, red dish brown, and yellowish red clay loam or sandy clay loam Bt horizon. This horizon contrasts sharply with the light brownish gray, weathered sandstone that underlies it.
In all the soils of the county, enough organic matter has accumulated in the surface layer to form an A1 horizon. The A1 horizon ranges from a thin pale horizon that is low in organic matter to a thick dark horizon. At lower elevations, the soils have an A1 horizon, about 3 to 8 inches thick, that is less than 2 percent organic matter. Organic matter does not accumulate in large quantities in warm temperatures. In the wooded north and eastern parts of the county where cooler temperatures prevail, the A1 horizon is thicker, darker, and higher in content of organic matter. The Ben Lomond, Aptos, and Nisene soils have a thick, dark A1 horizon that is about 4 percent organic matter in the upper 15 inches.
The translocation of silicate clay minerals has taken place in many soils in the county. The clay films on the ped faces and in root channels, as well as colloidal bridges between the sand grains, indicate the movement of clay minerals from the A horizon to the B2t horizon. Ben Lomond, Catelli, and Sur soils have little or no trans located clay. Watsonville soils have a large amount of translocated clay. As much as 33 percent more clay is in the Bt horizon than in the A horizon. Another evidence of clay translocation is the many thick clay films in the Bt horizon.
Ben Lomond, Catelli, and Sur soils have had accumulations of organic matter, but they have had little downward movement of day. Because of the high amount of precipitation, more than 50 inches, the leaching process removes the bases from the profile, which increases the acidity of the lower horizons. Aptos, Ben Lomond, Catelli, Nisene, Lompico, Lompico Variant, Santa Lucia, and Zayante are examples of soils that have been subject to this process.
Additions of organic matter, leaching of bases, and translocation of day are among the most important processes that contribute to horizon differentiation in the soils of Santa Cruz County.