Soil
This section is in addition to Chapter 3
Soil
Produced slowly (200-1000 years typically)
by weathering of rock, deposition of
sediments, and decomposition of organic
matter
Soil horizons – separate zones within soil
Soil profile – cross-section view of soil
Horizons
O horizon – surface litter
A horizon – top soil, made up of inorganic
particles (clay, silt, sand) and humus (organic
particles from decomposed organisms)
 Dark topsoil is richer in nutrients
Releases water and nutrients slowly
Provides aeration to roots
Healthy soil contains many nematodes and
bacteria, fungi, etc.
Oak tree
Fern
Word
sorrel
Lords and
ladies
Dog violet
Earthworm
Millipede
Mole
Honey
fungus
Grasses and
small shrubs
Organic debris
Builds up
Moss and
lichen
Rock
fragments
O horizon
Leaf litter
A horizon
Topsoil
Bedrock
B horizon
Subsoil
Immature soil
Regolith
Young soil
Pseudoscorpion
C horizon
Parent
material
Mite
Nematode
Actinomycetes
Root system
Red earth
Springtail
mite
Mature soil
Fungus
Bacteria
Fig. 10.12, p. 220
Poor topsoil
Grey, yellow and red are not the colors of
healthy topsoil
Generally means that soil is lacking nutrients
Best soil is called loam with equal parts sand,
silt, clay and humus
Leaching – dissolving and carrying nutrients
(or pollutants) through soil into lower layers
B – horizon and C - horizon
B – Subsoil mostly broken down rock
with little organic matter
C- parent material broken down rock on
top of the bedrock
Soils
Texture – relative amount of different
sized particles present (sand, silt, clay)
Porosity – volume of pore space in the
soil
Permeability – the ability of water to
flow through the soil
Water
Water
High permeability
Low permeability
Sandy soil
Clay soil
Soils
Clay – high porosity, low permeability
Sand – high permeability, low porosity
Acidity is another factor
Where rain is low, calcium and other
alkaline compounds may build up (sulfur
can be added – turns to sulfuric acid by
bacteria)
Forest litter
leaf mold
Acidic
lightcolored
humus
Humus-mineral
mixture
Light-colored
and acidic
Light, grayishbrown, silt loam
Iron and
aluminum
compounds
mixed with
clay
Tropical Rain Forest Soil
(humid, tropical climate)
Acid litter
and humus
Humus and
iron and
aluminum
compounds
Dark brown
Firm clay
Deciduous Forest Soil
(humid, mild climate)
Coniferous Forest Soil
(humid, cold climate)
Fig. 10.15b, p. 223
Mosaic
of closely
packed
pebbles,
boulders
Alkaline,
dark,
and rich
in humus
Weak humusmineral mixture
Dry, brown to
reddish-brown
with variable
accumulations
of clay, calcium
carbonate, and
soluble salts
Desert Soil
(hot, dry climate)
Clay,
calcium
compounds
Grassland Soil
(semiarid climate)
Fig. 10.15a, p. 223
Soil erosion
Causes – mainly water and wind
Human induced causes – farming,
logging, mining, construction,
overgrazing by livestock, off-road
vehicles, burning, and more (go us!)
Soil erosion
Types
Sheet
Uniform loss of soil, usually when water
crosses a flat field
Rill
Fast flowing water cuts small rivulets in soil
Gully
Rivulets join to become larger, channel
becomes wider and deeper, usually on
steeper slopes or where water moves fast
Global soil loss
 This is a major problem world wide
 Have lost about 15% of land for agriculture to soil
erosion
 Overgrazing
 Deforestation
 Unsustainable farming
 Also 40% of ag land is seriously degraded due to soil
erosion, salinization, water logging and compaction
Moderate
Severe
Desertification of arid and semiarid lands
Very Severe
Fig. 10.21, p. 228
Areas of serious concern
Areas of some concern
Stable or nonvegetative areas
Global soil erosion
Fig. 10.19, p. 226
Desertification
Turning productive (fertile) soil into less
productive soil (10% loss or more)
Overgrazing
Deforestation
Surface mining
Poor irrigation techniques
Poor farming techniques
Soil compaction
Salinization
 As water flows over the land, salts are leached out
 When water irrigates a field it is left to evaporate
typically
 This repeated process causes the dissolved salts to
accumulate and possibly severely reduce plant
productivity
 Fields must be repeatedly flushed with fresh water to
remove salt build up
Waterlogging
When fields are irrigated they allow water to
sink into the soil.
Winds can dry the surface
As more water is applied the root area of
plants is over saturated reducing yield
As clay is brought to subsoil levels it can act
as a boundary for water infiltration
Evaporation
Transpiration
Evaporation
Evaporation
Waterlogging
Less permeable
clay layer
Fig. 10.22, p. 229
Soil conservation
Conservation tillage – (no till farming)
disturb the soil as little as possible
Reducing erosion also helps – save
fuel, cut costs, hold water, avoid
compaction, allow more crops to be
grown, increase yields, reduce release
of carbon dioxide
Soil conservation
Terracing – making flat growing areas on
hillsides
Contour farming – planting crops
perpendicular to the hill slope, not parallel
Strip cropping – planting alternating rows of
crops to replace lost soil nutrients (legumes)
Alley cropping – planting crops between rows
of trees
Control planting and strip cropping
Fig. 10.24b, p. 230
Alley cropping
Fig. 10.24c, p. 230
Fig. 10.24a, p. 230
Terracing
Soil conservation
 Gully reclamation – seeding with fast growing native
grasses, slows erosion or “reverses” it
 Also building small dams traps sediments
 Building channels to divert water or slow water
 Windbreaks – trees planted around open land to
prevent erosion
 Retains soil moisture (shade, less wind)
 Habitats for birds, bees, etc.
 Land classification – identify marginal land that
should not be farmed
Windbreaks
Fig. 10.24d, p. 230
Soil fertility
Inorganic fertilizers – easily transported,
stored, and applied
Do not add humus – less water and air
holding ability, leads to compaction
Only supply about 3 of 20 needed nutrients
Requires large amount of energy for
production
Releases nitrous oxide (N2O) during
production, a green house gas
Soil fertility
Organic fertilizers – the odor is a problem
Animal manure – difficult to collect and
transfer easily, hard to store
Green manure – compost, aerates soil,
improves water retention, recycles nutrients
Crop rotation – allows nutrients to return to
soil, otherwise same crop continually strips
same nutrient, keeps yields high, reduces
erosion