Soil
Soil
Mixture of weathered rock fragments (regolith) and
organic matter (humus)
Lies on top of bedrock (parent material)
Soil Composition
Clay (0.002 mm = smooth), Silt (0.002 – 0.05 mm =
gritty), Sand (0.05 – 2 mm = sandy)
Proportions and composition determined by parent
rock
o Feldspar bedrock = Clay-rich soils
o Granite bedrock (quartz) = sandy soils
o Transported soil = from elsewhere
(water/wind/ice carries in)
Soil Profile
A cross-section (side-view) of the different layers
within a soil and their makeup
Layers are called horizons
Three horizons on top of bedrock
o A horizon (topsoil)
 Contains humus and rock particles
 Dark colored; most weathered
 Many pore spaces to allow water in
o B horizon (subsoil)
 Mostly clay, little humus, minerals leached
from the A horizon
 Sometimes tan or reddish
o C horizon (weathered parent rock)
 Partly weathered bedrock fragments, minerals
leached from the B horizon
 Bottom layer; just beginning to weather
o Bedrock
 Unchanged rock below the C horizon that
serves as the parent rock for the soil
Soil and Climate
Climate is a very important factor in soil formation and
soil type
Creates varying thickness and mineral content in the
horizons
Climate and soil type
o Humid Tropics
 Chemical weathering rapid
 A horizon is thinner – washed away by heavy
rain
 Minerals are leached to the B horizon =
thicker B horizon
o Desert and Arctic Soil
 Mostly mechanical weathering
 Very little chemical weathering
 A and B horizons small
o Temperate Soil
 Both mechanical and chemical weathering
 All horizons thick
Soil and Topography
Soil on flat ground = thick, soil on
slopes = thin
Soil Erosion and Conservation
Unwise use of the land has upset the balance between
soil erosion and soil formation
Rapid soil erosion occurs when natural plant cover has
been removed
o Farming, grazing animals, construction, etc.
o Causes gullying or sheet erosion
This leaves behind only subsoil or bare rock
Ways to prevent soil erosion = soil conservation
o Contour plowing – plow perpendicular to slope
o Strip Cropping – crops are planted in alternate
rows
o Terracing – plant crops on step-like ridges
o Windbreaks – plant grass breaks
and build fences
o Crop Rotation – alternate the type
of crop planted each year
o Best way – leave natural ground
cover on the land!