Transcript Chapter 2 The Soil

Chapter 2
The Soil
Perhaps the greatest evidence of knowing
little about the soil is to label it dirt.
 Soil is the underground environment of
plants.
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Soil is the thin outer layer of the earth’s
crust, made up of weathered minerals,
living and nonliving organisms, water, and
air.
A cross-sectional slice made
down into the earth’s surface
to show the different layers of
soil formations is called a soil
profile.
 Over time, distinctive layers
develop in undisturbed soils.
 The subsoil is between the
parent layer and the top soil.
 The subsoil is finely weathered
like the topsoil, but it lacks
organic matter in the quantity
found in the topsoil layer.
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Organic matter comes from the decomposition
of plant and animal tissue.
Humus- organic compounds that do not
decompose quickly, eventually succumb to
enzymatic action, forming a complex mixture.
Humus
Plowed under green plants are known as
green manure.
 Organic material increases both water and
mineral holding capacity of the soil.
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Water and air exist around and between the
soil particles.
 As much as 50% of the topsoil may be air
and water in liquid or vapor form.
 The ratio of air to water depends on the
texture of the soil and how wet it is.
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Even though the original parent stone may
be the same or similar, differences in the
subsoil and topsoil may result from
variations in five things.
– Weathering elements
– Soil movement
– Topography
– Climate
– Amount of organic matter
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Soils that weather from bedrock and remain
in place are termed sedentary.
Transported soils have been moved by forces
of nature.
– Colluvial soils have moved in response to gravity.
– Alluvial soils are carried in water such as rivers.
– Aeolian soils are transported and deposited by
winds
– Glacial till is soil deposited by glaciers
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The best agricultural soils are usually alluvial and
glacial till.
Weathering parent material forms particles of
different sizes, called soil separates.
In decreasing order of size, the separates are:
– Gravel, sand, silt, and clay
The relative
proportion of soil
separates of different
sizes in any one soil
create the soil
texture.
 Most soils in nature
contain sand, silt, and
clay in some
proportions.
 Loam soil proportion40% sand 40% silt
20% fine clay.
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Sand
Particles have assorted shapes and sizes,
depending on how they were weathered
 Spaces between particles are large
 Water passes through quickly because of
the large pore space
 Air is present in the greatest quantity
 Low in mineral nutrients
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Silt
Particles are irregularly shaped and much
smaller than most sand particles
 Has greater surface area than sand
 Holds water in the soil better than sand,
but it does not provide as much space for
air
 Low nutrient level
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Clay
Very small, plate-like particles
 Possesses the greatest surface area of all
the separates
 Water is held tightly to the particles and
passes very slowly through the soil
 Has an adhesive quality when moistened
and squeezed
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– This is what sometimes creates a sticky, hardto-plow field
At least 17 separate chemical elements has
been proven repeatedly through tests that
demonstrate growth abnormalities when any
one of these essential elements is lacking.
 An element is essential if the plant cannot
grow and develop normally without it.
 Essential elements are broken down into 2
categories, depending on how much of the
element is needed
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– Macronutrients
– Micronutrients
Essential Elements
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Macronutrients
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Calcium (Ca)
Carbon (C)
Hydrogen (H)
Magnesium (Mg)
Nitrogen (N)
Oxygen (O)
Phosphorus (P)
Potassium (K)
Sulfur (S)
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Micronutrients
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Boron (B)
Chlorine (Cl)
Copper (Cu)
Iron (Fe)
Manganese (Mg)
Molybdenum (Mo)
Nickel (Ni)
Zinc (Zn)
Plants only obtain carbon, hydrogen, and
oxygen from sources other than the soil
 All other nutrients must be obtained as
minerals from the soil around the plant’s
roots
 Beneficial elements have been found to
promote plant growth in many species,
but have not been proven to be absolutely
necessary for completion of the plants’ life
cycle.
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Plants may exhibit symptoms of nutrient
deficiency for several reasons
– The element may be lacking totally or not be
present in sufficient quantity
– The element may be bound in a chemical
form unavailable or too slowly available to the
plant
– There may be an overall imbalance of
nutrients in the soil
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Nitrogen is the most common element
lacking sufficient quantities in the soil
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Leaching is when nitrogen, in the nitrate
form, is not absorbed by the colloidal
particles of the soil, it passes quickly through
the root region of the soil.
Soil particles adhere together to form
larger particles called aggregates.
 The soils’ water, held between the
particles and granules of the soil contains
dissolved mineral salts. This liquidation is
known as the soil solution.
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When soil contains more hydrogen ions
than hydroxyl ions, this makes them
acidic.
 When soil contains more hydroxyl ions
than hydrogen ions, this makes them
alkaline.
 When a soil contains equal concentrations
of hydrogen and hydroxyl ions, its termed
neutral.
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pH=7 neutral
pH<7 acidic
pH>7 alkaline
Cation exchange refers to the capacity of
colloidal particles to attract positively charged
ions (cations) and to exchange one ion for
another.
 Without cation exchange, nutrients would be
readily leached from the soil.
 To make a soil more acidic (lower pH), sulfur
is usually added to the soil
 To make a soil more alkaline (raise pH)
calcium or calcium-magnesium compounds
are used
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The uptake of water and the uptake of
minerals are independent processes.
 Minerals enter root cells through a
permeable membrane when the
concentration f of the mineral salts in the
soil solution is greater than in the root cell
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Fertilizers are nutrient additives applied to
the soil periodically to maintain optimum
crop productivity.
 The need for fertilization may result form a
deficiency of one or more mineral elements
in the soil, their presence in a form
unavailable to the plant, or the leaching of
elements into the soil to a depth below the
root zone.
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Soil elements used in greatest quantity by
the green plant: nitrogen, phosphorus,
potassium.
 A fertilizer which provides all three elements
is termed a complete fertilizer.
 Examples of organic fertilizers: dried blood,
cocoa meal, animal manures, dried sewage
sludge, bone meal.
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Ammonification is the conversion of nitrogen in
organic compounds to ammonia.
 Nitrification is the conversion of ammonia to
nitrate.
 Phosphorus and nitrogen are present in the soil in
very small amounts.
 Potassium is present in the soil in much larger
quantities than either nitrogen or phosphorus.
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