SOILS
APES
WHAT IS SOIL?
• ROCK WEATHERED INTO
SEDIMENTS with some
organic matter, water and
air mixed in
• Weathering of rock may be
by wind, water, glacier, the
action of plants and
animals or heat. All of
these surface processes
break bedrock into smaller
fragments that form the
mineral (sediment)
component of soil
MECHANICAL (Physical)
Weathering
Using the force of pounding
rain, thermal expansion,
frost action of water
freezing in cracks large
parent rocks are broken
into smaller and smaller
fragments. This is the
beginning of soil. These
forces work by applying
pressure to widen existing
cracks, weakening the
rocks.
Chemical weathering
• Carbon dioxide in the
soil mixes with water
and forms carbonic acid
that breaks down rocks.
Clays are formed this
way
• Oxygen in the soil
combines with iron to
form iron oxides which
weakens and crumbles
the rocks
BARREN ROCK can turn to soil
• The roots of low growing
PIONEER PLANTS such
as mosses, lichens and
fast growing grasses can
break rock into smaller
particles
• When plant parts die and
decompose they add
humus and nutrients to the
soil, fertilizing it.
Humus is essential for forming arable soil.
• It increases the porosity and water holding capacity
of soil.
• It adds nutrients to the soil as it continues to decay.
Humus is BEST developed in grassland soils
SOIL PROFILE
•
Good Soil has layers, divided into horizons based on physical and chemical
characteristics:
– O horizon= humus= partially decomposed organic materials such as
leaf litter. Humus supplies nutrients when decomposed and retains
water in the soil
– A horizon= most fertile layer= most biological activity (biodiversity)
found here (topsoil). Large numbers of bacteria, earthworms and
organisms
– B horizon= accumulates iron, clay, Al and other leached minerals
(subsoil). Only deep plant roots penetrate this far
– C horizon= large unweathered rocks; as they break down they
become integrated into the B horizon
http://youtu.be/6Kr3Wj7SeSc
One centimeter of Soil takes
hundreds of years to develop.
As it is slow to renew, care
must be taken to use it
sustainably. Only a small
percentage of the earth’s land
is ARABLE, with soil suitable
for growing crops.
The temperature and rainfall
an area receives determines
how quickly soil forms and
how arable it will be
SOIL PROFILE AND BIOMES
•
•
•
•
Grassland biomes
– Most well developed soils rich in organic
content; ideal for growing crops
– Low rainfall means less leaching
– Decomposition during winter replenishes
organic content
Temperate deciduous forest
– Accumulates thick leaf litter in the fall when
leaves drop
– High rainfall means more leaching
Temperate coniferous forest
– Acidic soil from evergreen needles but thick
layer of humus
– Low rainfall keeps it fairly fertile
– Poor farmland due to short growing season
and acidic soils
Tropical rain forest
– High temps and rainfall accelerate
decomposition and leaching so poor, thin soil
– Most of the nutrients used immediately to
support plant growth
GRASSLANDS ARE IDEAL
FOR AGRICULTURE
• The rich organic soil in
grasslands provide the
basis for much of the
world’s agriculture. Most
grasslands are midlatitudinal, where wheat,
corn, potatoes, soybeans
are grown.
• Low rainfall makes these
biomes more susceptible to
desertification.
PHYSICAL PROPERTIES OF
SOIL
1. POROSITY
2. PERMEABILITY
3. TEXTURE
1. Determined by size of particles,
ranging from clay (smallest) to silt
(medium), and sand (largest)
particles.
2. Fine texture means more clay
particles.
3. Texture influences permeability.
Too much clay makes water
retention too great and soil
becomes waterlogged, depriving
roots of oxygen. Coarse sand grains
allow too much water to drain away
before it can be absorbed by roots
but allow the flow of oxygen
DETERMINING SOIL
TEXTURE
• Placing a sample of soil in
a jar of water and allowing
to remain undisturbed will
cause the particles to
settle out by gravity.
• The largest particles will
appear at the bottom. The
smallest clay particles will
settle on top. Relative
amounts can be estimated
as a percentage of the
total volume of soil
Classifying soils
• Once the amounts
of each particle is
determined the soil
may be classified
using the soil
triangle. A soil with
10% clay, 20% silt
and 70% sand is
called a sandy loam
PHYSICAL PROPERTIES OF
SOIL
1. POROSITY
2. PERMEABILITY
1. The ability of water to
move through a soil
2. Excess water not used by
plants, percolates down
and contributes to
groundwater recharging
and/or pollution
3. Highly permeable soils
lose water quickly and
leach nutrients quickly
4. A heavily compacted soil
restricts water movement
(impermeable)
3. TEXTURE
HIGH PERMEABILITY SOILS
DRAIN EASILY, recharging
aquifers
PHYSICAL PROPERTIES OF
SOIL
1. POROSITY
1. Plant roots live in pore
spaces
2. Some water is stored in
pores
3. Pores also contain oxygen
and other gases
4. Pores allow oxygen to
diffuse and water to drain
away
2. PERMEABILITY
3. TEXTURE
Aquifers store water because they have highly
porous and permeable sediments, allowing for
infiltration and percolation of water between
the particles
CHEMICAL PROPERTIES OF
SOIL
• NUTRIENTS IN SOIL
– A fertile soil is one that is able to supply
the dietary needs of a plant
– Essential macro-nutrients include:
– Nitrogen (produces rich green color of
leaves and influences the quality of fruit;
used in creating proteins)
– Phosphorus (abundant in processes
involving energy transfer such as the
formation of fats or moving food from
one part of the plant to another; needed
for creating nucleic acids; limiting factor
in plant growth)
• Soil pH
• SALINITY
• CATION EXCHANGE CAPACITY
Improving soil fertility
• ORGANIC FERTILIZERS
– Adds broad range microand macro nutrients to soil
but takes longer to be
effective as decomposition
may be slow
– Improves soil texture,
reduces runoff and
increases water retention of
the soil
– Helps to prevent soil erosion
– Examples include animal
manure, mulch or compost
(humus)
– It’s much more than just
fertilizer!
• INORGANIC
FERTILIZERS
– Add specific (target)
nutrient the soil may
need
– Easier to control
‘dosage’ and is
effective immediately
– Contributes to air
and water pollution
(cultural
eutrophication)
CHEMICAL PROPERTIES OF
SOIL
• NUTRIENTS IN SOIL
• SOIL pH
– Directly affects the availability of
nutrients
– Acidic soils leach heavy metals
– Soils in the pH range of 6.0 to 8.0
are generally best for biodiversity
– Soil pH can be changed by adding
lime to increase the alkalinity (or
decrease its acidity or by adding
sulfur to make it more acidic.
– Soils on the US east coast are
usually more acidic than those in
the west due to coal burning
• Soil tests for
chemical properties
can be done by
extracting a solution
from the soil,
filtering it and either
using a pH probe or
using test tabs and
a color comparator
to determine levels.
CHEMICAL PROPERTIES OF
SOIL
• SALINITY
– Results from shallow irrigation
waters that evaporate easily or
runoff from salted roads
– Plants use the water or it
evaporates but the salts are left to
accumulate in the soil
– Prevents roots from effectively
absorbing water by osmosis. With
enough salt, roots may actually
lose water, wilting the plants
– MITIGATION: Salts must be
washed out of soils by flooding
with pure water then allowing the
water to drain away. Reducing
evaporation with mulch also helps
CATION EXCHANGE
CAPACITY (how plants get nutrients from the soil)
• Clay particles have a negative charge
• Positively charged ions (cations) like K+, Ca2+
and Al3+ are attached to the clay particles.
• Water mixes with carbon dioxide in the soil to
form weak carbonic acid, H2CO3,solution in
the soil
• The positively charged K ions detach from the clay
and replace the H+ ions in the soil solution
(exchange) and may either be absorbed by roots of
plants, feeding them, or leach away into the
groundwater or runoff
Cation exchange releases
nutrients to the pore space so
roots can absorb them
ECOSYSTEM SERVICES OF SOIL and
why we should use it sustainably
1. Soil stores and purifies water
by filtering out large debris and
retain some long enough for
microbes to break down toxins
2. Soil microorganisms and
decomposers recycle important
nutrients like C, N, P,S
3. Charged soil particles attract
chemical waste and pesticides
(can serve as BUFFER zone)
4. Support plant growth
(productivity), as well as
biodiversity of life forms such
as ants, earthworms, bacteria
and termites
Maintaining healthy soil also has
economic value since money is
saved from not using expensive
technological solutions
The productivity of some soils
has been reduced by 50% due
to soil erosion and
desertification
• Poor farming practices have led to a
decline in soil texture and fertility
leading to erosion, compaction,
desertification, acidification, leaching,
salinization and nutrient depletion
Overused soil results in
DESERTIFICATION
• Definition: changes in climate
or human activities that
destroys the soils ability to
store water or maintain fertility
• Grasslands and semi-arid
areas are most vulnerable
since they get low rainfall
• China, Australia and Africa
are experiencing increasing
levels of desertification and
gigantic dust storms
DESERTIFICATION RESULTS IN
SOIL DEGRADATION
• Land degradation is caused
by poor farming practices,
deforestation, overgrazing,
converting rangeland to
cropland in areas that are too
dry and urban development.
• It is manifested in processes
such as soil erosion, nutrient
depletion, water scarcity,
salinization, changes in soil
texture and pH, and loss of
humus from soil
Desertification video
• http://youtu.be/w9RxnuBiFbg
OVERALL EFFECTS OF
DESERTIFICATION
1. Sandstorms or Dust
storms from increased
soil erosion, adding to
air pollution
2. Lower crop yield or
productivity of land
3. Lower biodiversity of the
land as increasing
numbers of habitats are
lost
SOIL EROSION
• Soil erosion is the removal of
topsoil by wind or water
• Agricultural practices such as
tilling (plowing), intensive
monoculture and over grazing
that lead to loss of plants to
cover the soil will increase
erosion. ROOTS BIND AND
ANCHOR SOIL.
• loss of topsoil leads to
decreased soil fertility and water
retention because the surface
horizons containing the decaying
humus to release nutrients are
Soil erosion is also caused by
1. Improper irrigation that
causes salts to build up
in the soil (salination)
due to evaporation of
water, leaving the salts
behind
2. Poor forest
management where too
many trees are
removed, exposing bare
soil to wind, making it
easier to move
SOIL CONSERVATION
• Soil Conservation Act
(1935), renamed the
Natural Resources
Conservation Service
(NRCS) in 1994 appraises
the status of soil and water
on non-Federal land and
advises need for
conservation programs
• Farming methods that
maximize vegetative cover
to reduce soil erosion are
increasing used worldwide
as a remedy to
desertification
HOW TO MITIGATE
DESERTIFICATION
1. Organic fertilizers (adds humus,increasing water retention and nutrients)
2. Crop rotation, intercropping or polyculture
1. Alternating plants with high nutrient demands one season with nitrogen
fixing plants such as legumes the next season
2. Also good for pest control since pests are adapted to a specific food
source and when it is not grown, they starve to death
3. Growing alternating rows or mixtures of different crops to ensure that
plants do not all withdraw the same nutrients from the soil at the same
time
3. No till or conservation tilling
•
Leaving the soil covered at all times to prevent wind erosion (often
requires the use of herbicides and fertilizers to fend off weeds)but some
old roots are always left to anchor the soil
4. Agroforestry (shelter belts)
1. Trees act as shelter or wind breaks to reduce wind erosion and
evaporation
2. Also good for pest management since trees often provide shelter and
nesting grounds for birds who might prey upon pest insects
Mitigating desertification
5. Contour farming (gentle
slopes) or terracing
(steep slopes)
• Plowing furrows
sideways, perpendicular
to the slope to capture
soil and prevent runoff
or cutting flat sections
into the sides of hill to
prevent water flowing
downwards
Review of soil
• https://youtu.be/mg7
XSjcnZQM
• Watch the video a
few times