Environmental Engineering Reference
In-Depth Information
2.8.1 Climate
Many people can appreciate that soils vary among climatic regions. Climate influences soil
formation by its control on factors such as precipitation amounts and intensity as well as
temperature regime. Both moisture and temperature are important drivers of abiotic activity
within soils; examples include (but are not limited to) leaching of materials down through
soil profiles, fracturing of rocks and minerals by heating and cooling processes, and the
dissolution of minerals by water as discussed in the surficial weathering processes section
earlier. Climate also drives biological activity in the soil profile and at the soil surface.
2.8.2 Parent Material
Parent material is the sediment or bedrock that weathers to form soils. It can range from
river alluvium to wind-deposited materials, to rock materials such as granites or basalts,
just to name a few. The mineral composition and hardness of a parent material influence
soil fertility and rate of soil formation. Soils developed from parent materials high in iron
and magnesium, such as basalts, tend to be rich in secondary clay minerals, fine-textured,
and highly fertile. In contrast, soils developed from silica-rich parent materials, such as
granites and sandstones in arid environments, typically retain more primary minerals
such as quartz and feldspar, have coarse textures, and have relatively low fertility.
2.8.3 Organisms
Organisms, one of the most important drivers of soil formation, range from macrofauna
such as burrowing mammals, earthworms, and ants to microfauna, which include fungi
and bacteria. Macrofauna are essential as this group disturbs soil materials by burrowing
and bringing materials at depth to the surface ( bioturbation ), by incorporation of organic
matter into the soil profile, and by their primary role in physically reducing the particle
size of organic matter. Microfauna play a pivotal role in cycling of nutrients, particularly
nitrogen, phosphorus, and potassium, in soil systems. Bacteria and fungi convert nutrients
from their organic form to inorganic forms. This is important, as the inorganic form of
nutrients is required for uptake by organisms. Plants are also important to the formation
of soils, in that they provide organic matter to the soil surface from litter and to the soil
profile by root inputs. In desert ecosystems, patchy shrub cover leads to the formation of
“islands of fertility” that concentrate nutrients below the individual shrubs. This is due
to a variety of interacting processes, including litter fall, uptake of certain nutrients by
the plant, interception of air- and water-borne dust, soil formation, soil erosion, animal
decomposition, and mineralization by soil biota.*
2.8.4 Relief
Relief includes the slope and aspect (facing direction) of a particular site. Relief influences
climatic affects, for example, soils on south-facing slopes, are often shallower than those
on north-facing slopes where temperatures are cooler and soils are moister for a greater
period of time. This promotes greater biological activity and formation of thicker soils.
Slope plays a role in soil formation by its influence on soil erosion. Typically, soils on
moderate to steep slopes are shallower due to constant erosion carrying particles down
slope, which relates to the previously described sediment-transport processes.
* For a more detailed discussion of spatial patterns of soil nutrient distribution, see Whitford. 1
Search WWH ::




Custom Search