Soil
is a mixture of weathered rock and
organic matter. It also contains
mineral fragments, water, and air.
Soil Formation
Soil starts as weathered rock. Plants grow
in weathered rock. Worms, insects,
bacteria and fungus live among the plant
roots and help soil evolve by adding
organic matter.
Humus

Humus is dark-colored
organic matter made of
pieces of decaying plants
and animals

Worms and insects mix the
humus with the fragments of
rock.
Soil Profiles AB&C Horizon

The O horizon is filled
with organic matter
(leaf litter). The A
horizon, known as
topsoil, it is the most
fully evolved layer in a
soil profile. It is dark &
contains more humus &
smaller sediments.
B - Horizon
The B layer is lighter in
color & contains little or
no humus & contains
minerals that were
washed down (leaching)
from the topsoil.
C-Horizon & Below
The C horizon is
the bottom layer
in a soil profile,
it contains partly
weathered rock,
but no humus.
Below the Chorizon is parent
rock.
Types of Soil

The thickness of the horizons & the soil
composition depend on many conditions.
1.
Rate of weathering


2.
3.
4.
Hot & Moist = increased chemical weathering & more
vegetation
Cool & Moist = increased mechanical weathering
Climate (humidity and temperature)
The type of rock the soil has evolved from.
Slope of the land & the amount of humus in the soil.

Grassland vs. Mountain slope
Soil Loss

Desertification or desert formation


occur where soils are damaged by overgrazing &
poor farming practices in areas that receive little or
no rain.
Overgrazing strips plants from the land. This
results in erosion of topsoil & slows soil evolution.
The Dust Bowl

In the 1930s, the “Dust Bowl” was created by poor farming
practices, overgrazing, & a drought in the Great Plains. It
caused soil to be carried away by water & wind, creating
dust storms and widespread famine.
Overgrazing in the Tropics creates
Desertification

Deforestation, clear-cutting, and slash &
burn farming removes all trees & many
nutrients, it also gives a very short growing
period before the existing nutrients in the soil
are used up.
Preventing Soil Loss


Crop rotation – Rotating crops so nutrients
are not used up.
Cover crops Keeps soil in
place.

Barriers –
reduce wind and
water erosion