310 CHAPTER 12 Mineral and Soil Resources
Courtesy U.S. Dept. of AgricultureO-horizon:
Mostly organic matter and
humus; plant litter accumulates
and decays.A-horizon (topsoil):
Dark; high concentration of
organic matter.B-horizon (subsoil):
Light-colored; litter and nutrient
minerals leached from
A-horizon accumulate here.Bedrock
(parent material).C-horizon
(weathered parent material):
Below roots, often saturated
with groundwater.Photo by Ray Weil, courtesy Martin Rabenhorst.
b. A “typical” soil profile, as it
appears to the trained eye of a soil
scientist. Each horizon has its own
chemical and physical properties.a. This soil, located on a farm in Virginia,
has no O-horizon because it is used for
agriculture; the surface litter that would
normally compose the O-horizon was plowed
into the A-horizon. The shovel gives an idea
of the relative depths of each horizon.-ÊÀV
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Humus is partially decomposed organic material, primarily
from plant and animal remains.
Soil rich in humus has a
loose, somewhat spongy
structure with several
properties, such as
increased water-
holding capacity, that
are beneficial for plants
and other organisms
living in it.main compo nent of soil. It provides anchorage and essen-
tial nutri ent minerals for plants, as well as pore space for
water and air. Litter (dead leaves and branches on the soil’s
surface), animal dung, and the remains of plants, ani mals,
and microorganisms constitute the organic portion of soil.
Microorganisms, particularly bacteria and fungi, gradually
decompose this material. Organic matter increases the
soil’s water-holding capacity by acting much like a sponge.
The black or dark brown organic material that remains
after extended decomposition is called humus (Figure
12.12). Humus, which is a mix of many organic com-
pounds, binds to nutrient mineral ions and holds water.
WHAT A SCIENTIST SEES
Soil Profile