Soils are composed of rocks and minerals, salts or ions, organic matter, water, and air. Factors which influence their formation are the type of parent material (what kind of rock), climate, biological activities, the length of time exposed, and topography (including the internal soil environment, drainage, moisture, aeration, erosion, and degree of exposure to sun and wind). Soils are the product of the chemical breakdown of the underlying rock units. A one-foot depth of soil usually takes between several hundred and a thousand years to develop. Generally, the older the soil, the deeper it is and the more capable it is of storing moisture and nutrients and making them available to plants. Soil particles vary from coarse and sandy to fine and clayey. Clay particles are microscopic. It is the physical chemistry of clay particles that holds plant nutrients in the soil. Water also adheres to the surface of soil particles - the smaller the particle, the more surface area is available per unit of soil, thereby increasing the water capacity of the soil.

In Poly Canyon there are seven main soil types, as classified by the Soils Conservation Service: Los Osos, Maymen, Diablo, Cibo, Lodo, Gazos, Henneke, and Obispo-rock outcrops. There are also minor pockets of Gaviota sandy loam. This discussion first locates each of the soil types in the Canyon (Figure 11). Then each soil type is described in terms of its appearance (color and texture) and parent material; its permeability, drainage, and runoff; the effective rooting depth; pH; and vegetative cover (Table 4).

Los Osos loams occur above the sandstone and shale in the mélange (Franciscan Formation). They have developed in the hills around the beginning of Poly Canyon Road (along the fire break above the electrical substation to the east of the Road and in the Ecological Study Area on the west side of the Road). They are also found in the hills east of the main Poly Canyon ridge, northeast of the Botanic Garden. Los Osos clay loams appear dark grayish brown and fine textured at the surface. Underneath they are primarily brown to yellowish brown heavy clay loam. They have relatively slow permeability, are well drained, and have medium runoff. The effective rooting depth is 20 to 40 inches. The pH is slightly to medium acid to neutral. Vegetation is mostly annual grasses and forbs with some perennial grasses, coastal sagebrush (Artemisia californica), and coast live oak (Quercus agrifolia).

Table 4
SOILS OF POLY CANYON

Maymen loams are often included in the Los Osos-Diablo complex. They are weathered from sandstone, shale, and conglomerate and are found on the slopes northeast of the Botanic Garden and along Brizzolara Creek from just below the Peterson Ranch houses upstream approximately half a mile. The surface layers of Maymen include brown gravelly loam underneath undecomposed leaves. Beneath is a light yellowish brown gravelly loam over bedrock. Maymen sandy loams have relatively slow permeability, are well drained, and have medium runoff. The effective rooting depth is approximately 15 inches, with a few large woody roots that grow through the rocky substrate to 60 inches in depth. Maymen soils are medium to strongly acidic. Vegetation is usually open stands of chaparral consisting of chamise (Adenostoma fasciculatum), scrub oak (Quercus berberidifolia), and, in protected sites, scattered coast live oak (Quercus agrifolia).

Diablo and Cibo clays develop primarily from shale and occur above Toro and Mélange. They are seen in a narrow fringe along the lower eastern flank of the main ridge in lower Poly Canyon, in the area developed as Design Village, on the hill above Design Village (to the southwest), and in large patches among the hills of the upper part of the Canyon. Diablo soils are a very dark gray silty clay at the surface. Under this thin layer is a dark grayish brown to olive gray calcareous layer over sandstone, shale, or unconsolidated sediments. Diablo soils have characteristically slow permeability and are very well drained. Their effective rooting depth ranges from 40 to 60 inches. The pH of Diablo soils ranges from neutral to slightly or moderately alkaline. Vegetation consists primarily of annual grasses and forbs. Cibo clays are dark brown cobbly clays over brown to dark brown heavy clay loam or clay. They develop through the weathering of basic igneous rocks. They have slow permeability, are well drained, and have medium to very rapid runoff. The effective rooting depth ranges from 20 to 40 inches. Cibo clays can be slightly acid to mildly alkaline. Cover is usually annual grasses and forbs.

Lodo clay loams have developed over mélange. They are found along the western flank of the main ridge to the east of Poly Canyon Road (underlying the "P," the area of the fire break uphill from the electrical substation, and the landfill). They are also on the eastern side of the hill just west of Poly Canyon Road and north of the Ecological Study Area. Lodo is a grayish brown to very dark grayish brown shaly clay loam over dark grayish brown hard shale. It has moderate permeability, is somewhat excessively drained, and has medium to rapid runoff. The effective rooting depth is from four to 20 inches. It is slightly acid. Native vegetation is primarily chaparral, with some buckwheat (Eriogonum fasciculatum.) and scattered oaks (Quercus spp.). Naturalized cover includes annual grasses and forbs.

Gazos clay loams are found in conjunction with Lodo loams in the uppermost portion of the Canyon, from above the railroad tracks down to the base of the foothills. Gazos is dark to very dark grayish brown at the surface. It develops from brown to very pale brown sandstone. It has moderately slow permeability, is well drained, and has medium to rapid runoff. The effective rooting depth is 20 to 40 inches. It is slightly acid. Vegetation is mostly annual grasses and forbs with some brush and occasional coast live oaks (Quercus agrifolia).

Henneke loams and clays develop from serpentinite and are closely associated with Obispo-rock outcrops. They occur in two adjacent areas on the main ridge just east of lower Poly Canyon: in a narrow strip along the top of the ridge and in a broad strip on the eastern flank of that same ridge, and in a large patches north and south of Design Village. Henneke soils have a half inch covering of fresh or partly decomposed leather oak leaf litter mixed with dark reddish brown very gravelly loam. Beneath this surface layer is brown, very cobbly or gravelly clay loam or clay over serpentinite. The permeability of Henneke soils is moderately slow. They are somewhat excessively drained. They have an effective rooting depth of approximately ten to 20 inches. Their pH ranges from mildly to moderately alkaline. Obispo-rock outcrops are very dark gray at the surface. Beneath the top layer is black, yellowish brown, or olive colored serpentinite. Permeability is very slow. Obispo rock-outcrops are very well drained and have rapid to very rapid runoff. Their effective rooting depth is between eight and 20 inches. Their pH ranges from moderately alkaline to neutral. The sparse vegetative cover on Henneke and Obispo soils consists of scattered shrubs such as leather oak (Quercus durata), toyon (Heteromeles arbutifolia), and sagebrush (Artemisia fasciculatum), as well as grasses and forbs. Yucca scrub may develop here, as also do communities of rock outcrops.

Gaviota sandy loam is on the east face of the hill just south of the railroad tracks and adjacent to Serrano Canyon. Gaviota soils are a brown to dark grayish brown fine sandy loam. They develop from light yellowish brown sandstone. They have moderately rapid permeability, are well drained, and have medium to rapid runoff. Their effective rooting depth is from six to 20 inches. They are medium acid to neutral. Uncultivated areas have a cover of annual grasses and forbs. Steeper areas usually have a cover of brush.

Of all the geologic and edaphic (soil-related) influences in the Canyon, serpentinite has most captured the interest of biologists because of its unique chemical and physical characteristics and the distinctive vegetation growing on it. As serpentinite weathers, the soils that develop (here the Henneke soils and the Obispo-rock outcrops) have high concentrations of iron, chromium, nickel, and cobalt, levels which prove toxic to many species. However, serpentinitic soils have relatively low concentrations of nitrogen, phosphorus, and potassium (the primary plant nutrients), as well as sulfur and often calcium. Serpentinitic soils have a low clay content, and the few clay minerals available have a low ion exchange capacity. This means that the few minerals present are unavailable as nutrients to plants because of pH and/or other environmental conditions localized within the soil. Serpentinitic soils are extremely well drained, which contributes to the problems of reduced availability of nutrients and moisture. These soils tend to support much less plant cover than adjacent non-serpentinitic soils. Consequently, there is relatively little organic matter (live plant parts or plant and animal detritus) present in serpentinitic soils. Thus, these soils typically have low water-holding capacity. Serpentinitic soils are granular and quite porous; they are well aerated and well drained. These soils are dark, almost always blackish. Their dark color, in conjunction with low plant cover (which results in increased exposure), allows more radiant heat from the sun to be absorbed by day and dissipated by night. All of this would suggest that there are relatively high daily temperature fluctuations within the soil, one more aspect to consider when observing plant and animal survival. How such factors may affect plant growth is discussed in the section dedicated to serpentinite communities.