Visualizing Environmental Science

Soil Properties and Processes 311

Soil Organisms

Soil organisms, which are usually hidden underground,

are remarkably numerous. Organisms that colonize

the soil ecosystem include plant roots, insects such

as ter mites and ants, earthworms, moles, snakes, and

ground hogs (Figure 12.13). Most numerous in soil are

bacteria, which number in the hundreds of millions per

gram of soil. Other microorganisms that are abundant in

soil eco systems include fungi, algae, microscopic worms

such as nematodes, and protozoa.

In a balanced ecosystem, the relationship between

soil and the organisms that live in and on it ensures soil fer-

tility. Soil organisms provide several essential ecosystem

services, such as maintaining soil fertility, preventing soil

Many soils are organized into distinctive hori zontal
layers called soil horizons. A soil profile is a vertical sec-
tion from surface to parent material, show ing the soil
horizons (see What a Scientist Sees). (Layers shown here
are generalized; the specific hori-
zons that develop in any given soil
are a consequence of the interac-
tions during soil development or
human disturbance.) The topsoil
(or A-horizon) is some what nutri-
ent poor due to the leaching of
many nutrients into deeper soil layers. Leaching is the re-
moval of dissolved materi als from the soil by water perco-
lating downward.

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The diversity of life in fertile soil includes plants, algae, fungi, earthworms, flatworms, roundworms,
insects, spiders and mites, bacteria, and burrowing animals such as moles and groundhogs.
(Soil horizons are not drawn to scale.)

Root

nodules:

Nitrogen-

fixing

bacteria

Mite

Nematodes

Root

Protozoa

Fungus

Bacteria

Surface

litter (O-horizon)

Topsoil

(A-horizon)

Subsoil (B- and

C-horizons)

Parent material

(bedrock)

soil horizons

Horizontal layers into

which many soils are

organized, from the

surface to the under-

lying parent material.