Environmental Engineering Reference
In-Depth Information
darker and looser than deeper layers. A fertile soil that
produces high crop yields has a thick topsoil layer
with lots of humus. This composition helps topsoil
hold water and nutrients taken up by plant roots.
The roots of most plants and the majority of a soil's
organic matter are concentrated in a soil's two upper
layers. As long as vegetation anchors these layers, soil
stores water and releases it in a nourishing trickle.
The two top layers of most well-developed soils
teem with bacteria, fungi, earthworms, and small
insects that interact in complex food webs. Bacteria
and other decomposer microorganisms found by the
billions in every handful of topsoil break down some
of its complex organic compounds into simpler in-
organic compounds that are soluble in water. Soil
moisture carrying these dissolved nutrients is drawn
up by the roots of plants and transported through
stems and into leaves as part of the earth's chemical
cycling processes.
The color of its topsoil suggests how useful a soil
is for growing crops. Dark brown or black topsoil is
rich in both nitrogen and organic matter. Gray, bright
yellow, and red topsoils are low in organic matter and
need nitrogen enrichment to support most crops.
The spaces, or pores, between the solid organic
and inorganic particles in the upper and lower soil lay-
ers contain varying amounts of air (mostly nitrogen
and oxygen gas) and water. Plant roots need the oxy-
gen for cellular respiration.
Some precipitation that reaches the soil percolates
through the soil layers and occupies many of the soil's
open spaces or pores. This downward movement of
water through soil is called
infiltration.
As the water
seeps down, it dissolves various minerals and organic
matter in upper layers and carries them to lower layers
in a process called
leaching.
Most of the world's crops are grown on soils ex-
posed when grasslands and deciduous (leaf-shedding)
Wood
sorrel
Oak tree
Lords and
ladies
Dog violet
Grasses and
small shrubs
Organic debris
builds up
Earthworm
Mole
Rock
fragments
Millipede
Moss and
lichen
Fern
Honey
fungus
O horizon
Leaf litter
A horizon
Topsoil
Bedrock
B horizon
Subsoil
Immature soil
Regolith
Young soil
C horizon
Parent
material
Pseudoscorpion
Mite
Nematode
Root system
Red earth
mite
Actinomycetes
Fungus
Springtail
Bacteria
Mature soil
Active Figure 3-21
Natural capital:
soil formation and generalized soil profile. Horizons, or layers, vary in
number, composition, and thickness, depending on the type of soil.
See an animation based on this figure and
take a short quiz on the concept.
(Used by permission of Macmillan Publishing Company from Derek Elsom,
Earth,
New York: Macmillan, 1992. Copyright © 1992 by Marshall Editions Developments Limited)
