10 0Natural systems and greenspaces
earth or soil. For example, sandy soils have more porosity, better drainage, more
oxygen, and less compaction susceptibility. In contrast, clay-dominated material
tends to drain poorly, form surface puddles, erode, compact readily, and hold
mineral nutrients better.
Soil,ontheother hand, as the upper portion of earth altered by organisms,
is a rich dynamic mixture of mineral particles, water, air, roots, organic matter
(blackish dead material), fungi, bacteria, and tiny soil animals. (This is not dirt
which refers to unclean or filthy material). Soil types differ in organic matter,
texture, chemistry, and other attributes, and are patchily distributed across the
ground surface, as represented in soil maps. Vertically a typical soil profile is
usually composed of somewhat distinctsoil horizons. The topsoil or A-horizon has
an organic layer of leaf litter and humus over a layer of mineral particles and
organic matter, that in turn is on a leached layer with most mineral nutrients
washed out by percolating rainwater. Beneath the A-horizon layers is a subsoil
or B-horizon and beneath it a C-horizon mainly of decomposed rock materials.
Organic matter and available nutrients are scarce in these two horizons.
Soil profiles and soil types partially reflect underlying rock types, but mainly
reflect differences in climate and microclimate. Planners and designers have
used soil maps showing the water-holding capacity (primarily determined by
soil texture) of each soil type, together with the juxtaposition of types, to absorb
stormwater and thus minimize flooding in communities (Woodlands New Com-
munity1973--74, Morgan and King1987,Galatas2004).
Urban soils are significantly modified by human activities, especially related
toconstruction history (Craul1999). Indeed, where impermeable surface cover
due to roads, parking lots, and buildings is extensive, very little soil of any sort
exists. Some urban parks have little or no natural soil remaining; New York’s
340 ha Central Park was entirely covered by 10 000 wagon-loads of sand from
nearby Long Island (Phillip Craul, personal communication). Near metropolitan
areas remnant woodland/forest is normally on poor agricultural soil and poor
building sites, while farmland is on good agricultural soil.
The preceding structural attributes of soils have major effects on tiny soil
animals, water, and chemistry, which largely determine how soil works (Cothrel
et al.1997, Steinberget al.1997). Soil animals in prolific numbers, such as earth-
worms, snails, slugs, and beetles, move up and down in the soil (except where
it is highly acid). This increases porosity, drainage, and oxygen conditions, and
mixes organic matter and mineral nutrients, all of which generally benefit plant
growth.Onthe other hand, high soil temperature from the urban heat-island
effect accelerates decomposition and disappearance of soil organic matter and
physiologically stresses plants.