Vertical and Spatial Variability
Properties in most natural profiles gradually grade from one horizon to
the next lower one, while some may exhibit abrupt changes. Urban soil profiles
show abrupt changes from one layer to another depending upon the
constructional history of the soil. If topsoil is scraped away and later backfilled,
two distinct layers result, particularly if the topsoil is from elsewhere. The abrupt
change is commonly referred to as a lithologic discontinuity (Figure 4) and an
interface is created. The importance of the interface will be discussed later.
This condition is common in home construction. Craul and Klein (1980)
observed this layer to range from 6 to 35 cm in streetside soils. The material
lying below the first discontinuity may be of the original soil profile. Each layer
may drastically differ in texture, structure, organic matter content, pH and bulk
density together with their related properties of aeration, drainage, waterholding
capacity and fertility. One layer may be hospitable to plant root growth
and survival, while the other may not. If the site has a constructional history of
fill, there may be several discontinuities present, each with an interface.
Therefore, great vertical variability exists which could present multiple problems
for plant root growth.