Earth and soil 101Silt-dominated soils and loamy soils (with a fairly even mixture of sand,
silt, and clay) are particularly good for growing crops and other plants. This
is because excess water tends to drain away, the soil holds adequate water for
plant growth (except in droughts when wilting occurs), oxygen reaches relatively
fardown, and soil particles (especially the fine clay particles present) hold ample
mineral nutrients to support plant growth. Loamy and silt soils are also best for
septic systems in residential areas, because water neither puddles nor flows too
rapidly to a water body. Also, oxygen helps in decomposition of the organic mat-
ter, and the wastewater mineral nutrients may be held in the soil and absorbed
byroots.
Soil acidity (pH) is strongly affected by rock type, yet rainwater running over
the widespread mortar and concrete surfaces in built areas typically makes
urban soils less acid (higher pH), which affects the plant species that can thrive.
Soil acidity affects mineral nutrient availability and root absorption. Not sur-
prisingly urban soil chemistry is extremely diverse and particularly important
from mineral nutrient and pollutant perspectives (Sieghardt 2005).Mineral nutri-
entssuch as nitrogen, phosphorus, sulfur, calcium, magnesium, potassium, iron,
and so forth are required in modest amounts for plant growth. If in short sup-
ply, nitrogen--phosphorus--potassium (N--P--K) fertilizer is typically added. Some
elements, such as heavy metals, zinc, cadmium, nickel, and copper, are required
by plants in tiny or trace amounts.
However, at high concentrations, resulting from pollution, these trace met-
als or elements are often toxic (Sieghardt 2005). Also, many polluting organic
substances, e.g., from the plastics, paint, and automotive industries, are directly
toxic to plants, soil animals, and microorganisms in soil. Hydrocarbons, particu-
larly from fuel combustion and vehicle use, tend to coat urban soils, creating a
‘‘ hydrophobic” film or crust (Gingrich and Diamond2001).In consequence, water
from light rains does not readily soak into the soil and root area, but remains on
thesurface and is evaporated. Nitrogen oxides (NOX) from vehicles and machin-
ery also coat the soil, which often results in one or two plant species becoming
dominant, and many others therefore becoming scarce or absent (effectively a
terrestrial eutrophication process).
Several other attributes of urban soils are rather distinctive (Craul1999). The
horizontal and vertical variability is typically greater, patchier, at a finer scale,
and different than in rural areas. Fallenleavesmayberemovedfromunderthe
scarce vegetation, so the organic matter that holds mineral nutrients and facili-
tates root growth tends to be limited in urban areas. Human materials and
contaminants -- metals, plastics, glass, asphalt, masonry, pesticides, road salt,
and much more -- are patchily distributed, break down slowly if at all, and leave