What are the FACTORS
that make SOIL?
TIME
1000’S TO 10,000’S OF YEARS
LAY OF THE LAND
TOPOGRAPHY
LIVING ORGANISMS
ANIMALS AND PLANTS
CLIMATE
HOT/COLD, WIND/RAIN
PARENT MATERIAL
ROCKS AND MINERALS
Physical land features
Why is soil important?
Soil is an essential part of the ecosystem:
 supporting the growth of plants
 recycling nutrients from dead
organisms
 providing a habitat for organisms
such as bacteria and worms.
There are different types of soil,
depending on its composition. These
support different types of plants and
influence the ecosystem that
develops.
The Rock Cycle
http://www.youtube.com/watch?v=pm6cCg_Do6k
How is soil created?
Soil is created in two main stages:
1. Erosion of the underlying parent rock
Fragments of rock accumulate at the site of
erosion. These are the source of minerals in the
soil. The mineral composition of the soil therefore
depends on the type of parent rock.
2. Production of topsoil
This is where the other components are added.
The accumulation of organic matter is affected by the climate and
topology of the area.
Uses of soil
Most plants grow in soil. Soil provides
anchorage to hold the plant in one place.
Soil is a source of minerals. Plants produce
their own food, but have to obtain minerals
from the environment. Plants growing on soils
that are lacking in minerals often show
symptoms related to deficiencies.
Soil also provides plants with water. Plants obtain
water via their roots. Soil traps water,
providing a reliable supply for plants.
Soil separate particle diameter (mm)
____________________________
Sand . . . . . . . . . 2.0 - 0.05
Silt . . . . . . . . . . 0.05 - 0.002
Clay . . . . . . . . . . <0.002
SAND
SILT
CLAY
Note: clays are microscopic in size !
True or false?
Soil Properties
 Texture - Determined by the size of mineral particles within
the soil.
 Too many large particles leads to extreme leaching.
 Too many small particles leads to poor drainage.
Oxygen and water
Most of the organisms living in soil require oxygen and water
for respiration. The amount of water and oxygen that a soil
can hold depends on the soil’s structure and texture.
Sandy soil
Large soil particles
have large spaces
between them, and
water can drain
away rapidly.
Clay soil
Smaller
particles have
smaller spaces
and retain more
water.
Air can only remain in the spaces that are not filled with water.
Waterlogged soils have very low oxygen levels. This is
beneficial to anaerobic bacteria but not to aerobic organisms.
Clay minerals
photographed
with an electron
Microscope.
Note: they are plate
or flake like and
are stacked on top
of each other.
They are electrically
charged and act like
magnets that attract
and hold plant
nutrients.
Soil Structure
 When subjected to pressure, pores can
collapse and total pore space decrease. This
is called compaction. Compaction reduces
permeability of soil to water and air.
Infiltration Rate
 The rate of gravitational flow of water through soil
is referred to as the infiltration rate.
 Soil with a high infiltration rate is easily leached.
These soils can quickly become infertile without
the addition of fertilizers
 When soil become dry, water can be drawn upward
from the water table by capillary action.
Permeability vs. Porosity
 Permeability is a measure of how quickly water
infiltrates soil.
 Highest in soil with large pores – sandy soils
 Lowest in soil with small pores – clay
Porosity is a measure of the space between particles of
the soil.
Clay has the greatest porosity
Sand has the lowest porosity
Porosity has an inverse relationship to
permeability.
Soils and pH
 The pH of most soils is 4 – 8.
 The solubility of certain nutrient minerals
varies with differences in pH.
 Examples:
 At a low pH, the aluminum and manganese in
soil water are more soluble and are sometime
absorbed by the roots in toxic concentration.
 Calcium phosphate become less soluble and
therefore less available to plants at a higher
pH.
Soils and pH
 Soil pH affects the leaching of nutrient minerals ;
 Cation exchange capacity is a soil’s ability to hold
cations (preventing them from leaching out of the
soil)
 Acidic soil has a reduced ability to bind positively
charged ions to it (cation exchange ). Mineral ions
such as potassium (K+) are readily leached out.
 The optimum pH for soil is 6.0 – 7.0 because most
nutrient mineral needed by plants are available in that
pH range.
Neutralizing acidic soil
The pH of soil influences plant
growth. Acidic conditions can
affect enzyme function in soil
organisms and alter the
availability of mineral ions.
Acidic soils often occur in areas of higher rainfall because
the alkaline calcium is leached out. Farmers add lime to
neutralize acidic soil, a process which is called liming.
Lime is a mixture of calcium compounds.
Liming releases carbon dioxide into the atmosphere.
This may contribute to global warming.
Earthworms
Earthworms are present in most
soil throughout the world, except
in Antarctica. They improve its
quality by:
 burrowing through the soil,
which mixes and aerates it,
and improves drainage
 breaking up large pieces of organic matter by dragging it
into the soil, and consuming parts of it.
Soil and matter pass through their intestines and emerge as
castings. Castings are higher in mineral content than topsoil.
They increase the surface area of the material which is
available for decomposition by bacteria and fungi.
Erosion - Wearing away and
transportation of soil by wind,
water, or ice
The Dust Bowl
How did the Canyon Lands
of Utah form?
Sheet Erosion
The Problem of Soil Erosion
A. Topsoil is eroding faster than it forms on about
1/3 f the world’s cropland.
B. Losing topsoil makes soil less fertile and less able
to hold water.
C. Sediment is the largest source of water pollution
It clogs irrigation ditches, boat channels, reservoirs
and lakes.
2. It causes the water to heat up by absorbing solar
energy causing fish to die.
3. Increases flood risk by filling in streams and rivers.
1.
The Problem of Soil Erosion
D. 2/3 of the seriously degraded lands are in Asia and
Africa.
E. In the U.S., 1/3 of the nations original prime
topsoil has been washed or blown into streams,
lakes, and oceans by over cultivating, over grazing
and deforestation.
Desertification:
A.Causes:
1. Overgrazing
2. Deforestation
3. Surface mining
4. Irrigation techniques that lead to increased erosion
5. Salt build up and water logged soils
6. Farming on land with unsuitable terrain or soils
7. Soil compaction by farm machinery and cattle hoofs
Salinization
 1.Irrigation water contains small amount of
dissolved salts.
 2. Evaporation and transpiration leave salts behind
 3. Salt builds up in soil
 Waterlogging
 1. Precipitation and irrigation water percolate
downward
 2. Water table rises
Desertification:
B. Consequences:
1.
2.
3.
4.
Worsening drought
Famine
Declining living standards
Swelling numbers of environmental refugees whose
land is too eroded to grow crops or feed livestock.
Desertification:
C. Solutions:
1. Reduce overgrazing , deforestation, and the
destructive forms of planting, irrigation, and mining.
2. Planting trees and grasses to anchor soil and hold
water.
3. Removing salt from the soil.
D. Soil Erosion Act of 1935: U.S.
established the Soil Conservation
Service
Soil Conservation – Figure 14-21
 A. Tillage farming
 1. Conventional tillage farming: land is plowed
and then soil is broken and smoothed to make a
planting surface.
 2. Conservation tillage farming: disturb the soil
as little as possible.
Other methods:
 1. Terracing: series of broad level terraces
 2. Contour planting: planting crops in rows across
rather than up and down the slope
 3. Strip cropping: a row of crops alternates in strips
with another crop.
 4. Alley cropping: several crops are planted
together in strips or alleys between trees and
shrubs.
 5. Gully reclamation: planting of shrubs, trees to
prevent water runoff that forms gullies
Alley Cropping
Gully Reclamation
Soil Conservation Practices
8. Windbreaks - Planting of trees or other plants that
protect bare soil from full force of the wind.
Other methods:
7. PAM: a chemical added to water during the 1st
hour of irrigation. Negative PAM particles bond
to positive clay particles and increase
cohesiveness of surface soil particles.
8. 1985 Farm Act: established a strategy for
reducing soil erosion in the U.S.
How can soil fertility be maintained?
A. Organic fertilizers
B. Inorganic fertilizers