Where grows? Where grows it not? If vain our toil we ought to blame the culture not the soil.
Alexander Pope, quoted by E.J. Salisbury in The Living Garden (1935)
The parks and open spaces of cities provide, as William Hammond Hall said with the piety we expect from nineteenth-century reformers, “great oases in the arid desert of business and dissipation, for the refreshment of the city’s soul and body.” During the intervening century parks have become important not merely to our refreshment, but to our very survival, so intensely do we pursue our business and flagrantly dissipate the earth’s resources. The increasing importance of urban greenery is causing us to look at it more closely, but not yet as plants and animal are studied in the wild. Today wilderness is seen as a series of interdependent ecosystems in which soil, air, water, and every species of plant and animal interact and sustain the whole. Seldom, if ever, do we view these important urban spaces as ecosystems, with intricate relationships among their many components. Urban green spaces need to be examined in the same way as wilderness so that we will be able to slow their loss from inevitable decay and prevent their destruction from overuse.
The most obvious and commonly studied members of any ecosystem are plants and animals. Additional, but often overlooked, parts of ecosystems are soils — living components of our environment that lie beneath our feet or, in most cities, beneath concrete, steel, and asphalt. Golden Gate Park in San Francisco offers a fascinating case study of how people have created an urban ecosystem out of shifting sand. Some of the park’s plants have been discussed in Pacific Horticulture by Elizabeth McClintock. Here we focus on the park’s soils and examine their evolution in the century since the park was first established.
General Characteristics of Soils
Most soils, including those of Golden Gate Park, consist mainly of mineral matter. Humus, dark brown to black colloidal matter that is the partially decomposed remains of plants and animals, makes up less than five percent of most soils. What humus lacks in volume it makes up for in importance to fertility and in its dynamic nature. It is broken down by soil microorganisms slowly over the years, and is replenished from new layers of debris. Most of the humus is converted to carbon dioxide gas, but its nutrients are released to be absorbed by the roots of living plants.
The breakdown of plant litter and soil humus by microorganisms also helps change the nature of the mineral matter of the soil. However, the time needed for minerals to change to a measurable degree is long, usually involving hundreds to thousands of years. The minerals of a soil are derived from the rock or sediment in which the soil forms. Most minerals in rocks are unstable at the earth’s surface and they slowly react with water, and with the acids that the water may contain, to produce new minerals that differ in color, texture, and chemical composition. These mineral changes are greatly accelerated in soil by the breakdown of humus, which contributes organic acids and other compounds to the water.
Over time, the decay of litter and humus, the passage of rainwater downward through the soil, and the reaction of the water with the minerals, creates a soil profile — a sequence of layers or horizons. The accumulation of leaf litter at the soil surface is called the O horizon. This horizon contains little mineral matter. The A horizon, which lies below the O, is mainly mineral matter but has a noticeable darkness and richness that comes from the presence of small amounts of humus. B horizons, which are features of geologically older, and more developed soils, are commonly redder than soil above them, and contain more clay. The C horizon is the parent material, rock or sediment little altered by activities above. Unfortunately, soil profiles in nature can be viewed only from excavations and roadcuts, so few people have the opportunity to appreciate their great diversity and beauty. However, gardeners working with meadowland or other undisturbed sites can see a portion of a profile when they first dig.
Environment and Soils of Golden Gate Park
Golden Gate Park, like much of San Francisco, lies on a large sand dune field, one of the largest along the California coast. The sands are derived from the bedrock along the coast, both north and south of Golden Gate Park. The eroded rock has been broken down, transported, and deposited on the beaches of western San Francisco by oceanic currents. Prevailing westerly winds have blown the sand from the beaches inland across the peninsula, forming the dune field.
The shifting dune sand, along with the cool, moist climate, made western San Francisco an unsuitable location for an urban park in the eyes of some. Frederick Law Olmsted, despite his success with New York’s Central Park, could see little potential for an urban park there. In a report to the city in 1866, he said, “… neither in beauty of greensward, nor in great umbrageous trees, do the special condition of the topography, soil, and climate of San Francisco allow us to hope that any pleasure ground it can acquire will ever compare, in the most distant degree, with those of New York or London.” However, by the 1880s, the ambitious land reclamation program initiated by William Hammond Hall had successfully stabilized the dunes and established vegetation. During reclamation many areas of the park were covered with day-rich fill from San Francisco Bay and from soils in surrounding areas. Most of the present lawns in the park have one or two feet of this artificial fill overlying fresh dune sand. Other areas in the park received little fill and, instead, were stabilized with grasses and other plants into which trees were later planted. While changes have been made in the park since then, many of the trees that exist today are from those original plantings. These plantings offer an opportunity to measure the ecological change that has occurred during the past century.
To appreciate the changes that have occurred within the park, one need only travel down the Pacific Coast a few miles to the Golden Gate National Seashore, opposite Lake Merced. There, the shifting sand, scattered vegetation, and virtual absence of recognizable soil development, which characterized pre-park conditions in western San Francisco, can be observed first hand. The soil profile, if one chooses to consider this a soil, consists only of alternating layers of light and dark sand grains. The dark layers contain large amounts of magnetite, a black, iron-rich mineral that can be easily removed from the sand with a common magnet.
Within the park boundaries, there are areas of contrasting vegetation that either were planted or naturally established on otherwise unaltered dune sand. The soils formed on dune sand, as distinct from those covered by artificial fill, provide an interesting comparison with unaltered dunes to the south. Even prior to park development, isolated stands of coast live oak (Quercus agrifolia) existed in what is now the northeastern portion of the park, near McLaren Lodge. Soils under these trees probably had insufficient opportunity to form until the surrounding dunes were stabilized, so that much of the soil development has occurred during the past century. Leaf litter from oak trees decomposes rapidly because it contains few chemical compounds that inhibit microbial degradation. Thus, only a thin litter layer, or O horizon, exists here. However, the partially decomposed litter has been intimately mixed with mineral matter to form a thick A horizon. Tree roots penetrate the entire soil and tend to concentrate at certain depths.
At the center of the park, near the buffalo paddock, blue gums (Eucalyptus globulus), with a dense groundcover of German ivy (Senecio mikanoides), grow on an otherwise undisturbed dune. Eucalyptus litter is less susceptible to microbial breakdown than oak litter, and the soil has a thicker litter layer than is found under the oaks. In addition, smaller quantities of decomposed litter have mixed with the underlying mineral matter and, as a result, the A horizon is thinner and less prominent. An interesting feature of this soil are the nearly parallel horizontal bands of humus in the A horizon. The bands appear to form as humus is moved downward by rainwater.
Not far from the eucalyptus grove is a site dominated by Monterey pines (Pinus radiata) and Monterey cypresses (Cupressus macrocarpa), with an understory of ivy, manroot (Marah sp.), and several species of shrubs. The soil that has evolved under this vegetation has the thickest O horizon of any examined, and the least prominent A horizon. Pine litter, because of its chemical nature, resists decomposition, and its litter tends to accumulate quickly under mature trees.
The soils are all too young to have B horizons. As a result, they are classified as entisols or “recent soils.” New iron-bearing minerals are being formed as minerals originally present in the dune sand slowly break down. In time — possibly several hundred years — a weak O horizon, reddish brown, and possibly with higher clay content, should appear.
Trees Trap Carbon Dioxide
What significance does Golden Gate Park have as an urban ecosystem? Certainly, one of the most apparent characteristics is the richness and density of vegetation, along with the accompanying accumulation of litter and humus in the soils. The soils we examined contain approximately 45,000 pounds of carbon per acre, nearly all of which has accumulated in the past century. Since the carbon is derived from plants, which take up carbon dioxide from the atmosphere, the accumulation of humus and litter in the soil is an important means of removing carbon dioxide from the atmosphere. If all the carbon stored in the trees themselves is also considered, one can easily appreciate the significance that urban parks, along with other terrestrial ecosystems, have in ameliorating the continuous additions of atmospheric carbon dioxide from our automobiles, homes, and factories.
Besides interacting with the atmosphere, the urban ecosystem of Golden Gate Park has greatly modified the local climate. Soil temperatures within the park are commonly four to six degrees Fahrenheit lower than those in the dunes because the dense vegetation of the park shades the ground. Fog dripping from trees also affects temperatures and may add significant amounts of water to park soils.
Importance of Soil Formation to Horticulture
The natural changes that slowly occur in soils as they form ultimately change properties important to horticulturists. In Golden Gate Park, the accumulation of litter and humus greatly increases the water-holding capacity of the soil since the original dune sand alone retains little moisture. The accumulation of humus also decreases the compactness of the surface horizons.
A somewhat detrimental aspect of soil development in Golden Gate Park is that soils tend to become water repellent, or hydrophobic. The water repellency occurs because sand grains become coated with organic waxes and fats that come from the decomposing litter. The water repellency is especially noticeable in initially dry soils. However, during rain or irrigation, water eventually overcomes the hydrophobic forces and the soils are then able to absorb water.
The type of vegetation has an interesting effect on the acidity of the soils. The pH of unvegetated dune sand is about 7, or neutral. The pH of oak and pine soils lies between 5 and 6, which is slightly acid. The more acidic nature of the vegetated soils is normal because the decomposing litter adds organic acids to the soil. Surprisingly, the eucalyptus soil is alkaline and has a pH (7.9) higher than that of the dune. It has been reported that eucalyptus leaves sometimes contain calcium carbonate, or lime, which may account for the alkalinity of this soil.
Urban Soils in Perspective
Urban parks are areas of beauty and solitude for city dwellers, yet they also provide an exciting means of understanding ecological processes. The soils of Golden Gate Park, like those in many urban areas, are young from a geologic perspective and are only in the early stages of evolution. Yet, as generations pass and the processes continue, the soil profiles will become more prominent, their fertility will change, and their importance to urban environments will continue to grow. It is unfortunate that soils, which are such a significant part of any ecosystem, are so difficult to observe and, as a result, tend to be underappreciated or ignored.
As we walk through the dense vegetation of an urban park and inhale the rich aroma of its gardens and forests, we should keep in mind that many of these odors can be traced to the soil, to its microorganisms, and to the products of their activity. These odors should remind us of the constant transfer of nutrients from dead to living organisms that is needed to maintain the richness and diversity of life. Finally, as we view the vegetation and inhale its scents, we should remind ourselves that we are part of the ecosystem, and that our activities affect not only the plants, animals, and soils, but ultimately our own well being.