Location: Windy Hill Open-Space Preserve, San Mateo County, California Taxonomic name (to Great Group): coarse-loamy mixed thermic Argixeroll Landscape position: shoulder Slope: 25% Aspect: West Parent Material: Soft sandstone/shale Vegetation: annual grass and forbs, few shrubs
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Taxonomic name (to Great Group): coarse-loamy mixed thermic Argixeroll
Landscape position: shoulder
Parent Material: Soft sandstone/shale
Vegetation: annual grass and forbs, few shrubs
Described by: Karen Carney
Date: November 14, 1999
Horizon Depth (cm) Moist Color Texture Grade Size Structure Consistence Roots Boundary
A1 0-12 10 YR 3/2 loamy sand strong fine granular loose many f ,vf clear
A2 12-29 10 YR 3/2 loamy sand strong fine sub-angular blocky loose many f, vf clear
AB 29-47 10 YR 3/3 sandy loam strong medium angular blocky loose many f, vf gradual
Bt 47-94 10 YR 3/3 sandy loam strong medium angular blocky loose common f, vf N/A
Hans Jenny identified five primary factors that determine the rate and direction of soil development:
- parent material
In this presentation, I will examine in detail the influence of organisms on soil development and soil properties in a grassland in San Mateo County, California.
SOIL GENESIS AND CLASSIFICATION
The soil of the grasslands of Windy Hill Preserve was designated a Fagan loam (Soil Survey of San Mateo County, Eastern Part, and San Francisco County, CA 1991- (SSSMC)). However, because of the relatively coarse texture of the soil I excavated, I would argue against its being classified as a Fagan loam. Fagan loams are well-drained soils found on uplands, having weathered from soft sandstone and shale(SSSMC); this part of the description matches well to the site I excavated. The vegetation overlying the soil, the topography of the area (slopes of 15-50%), and the soil color of this site all also match well to the description of a Fagan loam (SSSMC). However, Fagan loams typically haveloam texture in the upper layers of the soil, and clay loam in the lower horizons; these textures are associated with low infiltration rates, as well as moderate to high shrink/swell potential. The site I excavated had loamy sand high in the soil profile, which gave way to sandy loam (see Table1). This soil would likely demonstrate high infiltration rates and low shrink/swell potential.
The soil had 4 different horizons, including two A layers, an AB layer, and an argillic B layer. A1 and A2 differed only in their structure (see Table 1). The AB layer was dominated by the characteristic s of A, but definitely had A and B influences mixed together. The Bt horizon had noticeably more clay than the A horizons; I assumed that there was 1.2 times the clay as the A2 horizon and therefore called it an argillic horizon.
I classified the soil as a coarse-loamy mixed thermic Argixeroll (Table 1). Though I did not have data for the base saturation of the soil, I assumed it was >50% and classified the epipedon a mollic epipedon. The epipedon’s depth (>25cm), high organic content, soil color, moisture regime, and vegetation support this designation. The suborder is Xeroll due to the xeric moisture regime. The Great Group is Argixeroll due to the argillic horizon as well as the lack of any other unusual soil characteristics.
A1 (0-12 cm)
A2 (12-29 cm)
Table 1. Soil description
MATERIALS AND METHODS
One soil profile was excavated out of a road cut off of Skyline Boulevard on the west end of Windy Hill preserve on November 24, 1999. To minimize the influence of disturbance, the pit was excavated one foot from the edge of the cut.
Soil textures of each horizon were delineated using the “feel” method. Moist soil colors were determined using a Munsell Color Chart.
Keys to Soil Taxonomy (USDA 8th edition) was used to classify the soil to the level of Great Group. Assumptions made during classification of this soil are outlined in the”SOIL CLASSIFICATION” section.
AB (29-47 cm)
Bt (44-94 cm)
Figure 1. Map (http://www.openspacetrust.org/) and photograph of soil survey location. Red arrow on the second inset map marks the location of the survey site.
SITE LOCATION AND DESCRIPTION
Windy Hill is approximately 15 miles north-west of San Jose, CA (see Figure 1). The vegetation on the site is primarily annual grasses and forbs, and includes some shrubs. The annual average rainfall is 25 to 35 inches, the average annual air temperature is 56-58 degrees, and the average frost-free period is 275-330 days (Soil Survey of San Mateo County, Eastern Part, and San Francisco County, CA 1991). Windy Hill is an open-space preserve that was set aside for recreational and wildlife use during the 1970’s (http://www.openspacetrust.org/). It was likely used for pasture grazing prior to conversion to a preserve.
Figure 2. Soil profile to 94 cm.
Chemical weathering: Carbon dioxide production related to root respiration and decomposition of plant materials results in the formation of carbonic acid - the most significant contributor to chemical weathering in the soil.
Physical weathering: Plant roots also can contribute to physical weathering by growing into cracks in bedrock and breaking apart parent materials.
Organic matter: Plants are primary producers, turning energy from the sun into the organic compounds on which nearly all other life forms depend. Organic matter from plants is an important source of nutrients, is an important contributor to cation exchange capacity, and is critical to maintaining soil structure.
Soil stability: Root exudates, plant litter, and roots themselves all serve to stabilize soil materials.
Compaction: Because they weigh so much and have small feet, cattle are very effective at compacting soil. Soil macropores are compressed during this process, resulting in low infiltration rates and high water retention. This leads to anaerobic conditions which retards plant growth and slows decomposition of organic matter. Since little can penetrate a compacted layer of soil, compaction also slows the processes of both physical and chemical weathering.
Removal of organic matter: Obviously, cattle need to eat grass to persist, and this represents a loss of organic matter to the soil. Lower soil organic matter means lower cation exchange capacity as well as the loss of a nutrient source for plants.
Concentration of nutrients: Cows are not 100% efficient in their metabolism of grass. Their metabolic wastes (which are often nutrient rich) are deposited in very discrete patches, which can greatly affect the nutrient distribution in the grasslands on which they graze.
Organic matter decompositon: Ants are important players in the breakdown of plant litter, woody debris, and other organic materials. They fragment and digest these materials, leaving behind manageable pieces that soil bacteria and other microorganisms can eat.
Soil mixing: Ants, like burrowing mammals, transport soil from bottom layers of soil to the surface and vice versa.
Aeration and drainage: Again, like other burrowers, ants increase the aeration and drainage of soils, which can affect nutrient availability, weathering rates, and leaching losses.
Figure 5. Ant burrows in the soil on Windy Hill
Figure 4. Grazing cattle near Windy Hill
Figure 3. Grasses on Windy Hill
BURROWING MAMMALS (ground squirrels, moles, etc.)
- Aeration: Burrows aerate the soil, enabling high rates of decomposition of organic matter in the soils. The decrease in organic matter reduces cation exchange capacity. Decomposition also contributes carbon dioxide to the soil which may enhance chemical weathering if the gas isn’t immediately released to the atmosphere.
- Soil mixing: Soil from deep horizons can be brought to the surface (and vice versa) when animals dig burrows.
- Leaching potential: Water flow through burrows can greatly increase the leaching potential of soils. Soluble nutrients or toxins may move rapidly through the water column and be lost to downstream ecosystems or to groundwater.
- Soil structure: Animal burrows affect the structure and stability of the soil. Extensive burrowing can lead to high runoff through the burrows themselves. When the soil surrounding the burrow becomes saturated, it may collapse.
Soil fertility: Worms actually ingest soil, breaking down organic matter in the soil as it passes through them. The casts they leave behind while tunneling through the soil are full of the nutrient-rich residues of their metabolism which serve as a nutrient reservoir for other creatures in and above the soil.
Soil stability: The worm casts mentioned above are also critical to providing structure to the soil. The organic residues in the casts adhere to soil particles and enhance the formation of soil aggregates.
Soil aeration: As with other burrowing animals, worms improve aeration and water flow.
Figure 6. Earthworm
Figure 10. Animal burrow on Windy Hill
Nitrogen cycling:Soil bacteria are nitrogen cycling machines - they fix nitrogen gas, turn organic nitrogen into a form plants and other microbes can use, and they are responsible for the production of nitric and nitrous oxides, which can represent gaseous losses of nitrogen from the soil.
Toxin degradation: Microbes can evolve to metabolize compounds that are lethal to human beings, such as arsenic and selenium. With the help of these strange organisms, we may be able to help the process of toxic waste clean-up.
Organic matter decomposition: Yes, yet another organism critical to this important process! Bacteria degrade small pieces of organic matter, often releasing nutrients to the soil solution.
Woody debris decomposition: Fungi are able to break down the most recalcitrant plant tissues in the world. Without the action of fungi, twigs and logs would remain in forests for many years, and the nutrients and energy locked inside them would be unavailable to other organisms in the ecosystem.
Humus formation: Fungi not only break down organic matter in the soil, they also contribute to it. When fungi die, their cell walls persist for long periods of time in the soil where they contribute to cation exchange capacity and soil flocculation.
Soil stability: Fungal hyphae, like plant roots, have vast networks throughout the soil. Such networks are important physical stabilizers in soil.
Soil fertility: Like many other soil organisms, mites are an important part of the soil food web. They are actually predators of nematodes and other organisms that prey on bacteria. Without this crucial link, bacteria would be in insufficient numbers to carry out the business of decomposition and nutrient mineralization.
Figure 7. Fungal hyphae
Figure 9. Mite
Figure 8. Bacteria
( http://www micrbiol.sci.kun.nl/micrbiol/gallery/barkeri.gif)
The influence of biota on soil development and soil properties in the grasslands of Windy Hill Open Space Preserve, California