Chapter 5 etc.

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Chapter 5 etc. Soils formation and fertility
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Outline

• 1. Nutrient Cycles
• What are nutrient cycles? major cycles (Ch 5-1)
• Water Cycle (Ch 5-2)
• Carbon Cycle (Ch 5-3)
• Nitrogen Cycle (Ch 5-4)
• Phosphorus Cycle (Ch 5-5)
• Sulfur Cycle (Ch 5-6)
• 2. Nutrient Cycling Sustainability
  (Ch 5-9)
• 3. Soil (Ch 5-8)
• layers/profiles, texture porosity, acidity
• Soil Erosion and Degradation (Ch 14-6)
• Soil Conservation (Ch 14-7)

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Soil Profile
A soil profile is a description of the soil
horizons (layers) from the surface where the
plants are growing to the geologic material (rock
or sediment) at the base of the soil. This is
usually considered the parent material from
which the soil formed.
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Soil Types

• Soils can be categorized into two major types
• Organic soil - a soil in which more than half of
  the soil profile thickness is composed of organic
  matter (gt30 cm thick, gt20 organic matter).
• Mineral soil - a soil whose properties are
  determined by mineral (rock) matter, rather than
  organic matter.

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Organic Soil example Surface organic layer
(O-horizon) is gt 30 cm thick.
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• Humus-partially decomposed organic matter.
• Organic matter (OM) importance
• Organic acids and colloids in OM bind
  contaminants and help make nutrients available to
  plants.
• OM holds onto water and releases it slowly.
• OM in soil is a carbon reservoir.
• Dark brown or black color in soil usually
  indicates high nitrogen
  content (and lots of humus).

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Components of Soil Organic Matter (OM)
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Soil Organisms

• By-products from growing roots and plant residue
  feed soil organisms. In turn, soil organisms
  support plant health as they
• decompose organic matter,
• cycle nutrients,
• enhance soil structure,
• and control the populations of soil organisms
  including crop pests.
• - USDA/NRCS

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3. Soil Characteristics
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3. Soil Characteristics
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Mineral Soil Horizons

• A horizon - Surface horizon of a mineral soil.
  It has most of the organic matter. Often dark
  brown or dark gray.
• E horizon - Present in some acid (low pH), forest
  soils. A zone of Eluviation (washing out) of
  clays, and iron and aluminum oxides. Often light
  gray or white.
• B horizon - A zone of accumulation of clays, and
  iron and aluminum oxides. May be reddish in
  color. May have blocky or prismatic texture.
• C horizon - A zone that is relatively unaffected
  by biological activity. It may be the parent
  material of the soil.

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How do horizons form?

• soil horizon animation

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• Particle Size
• Clay- Very fine (microscopic grains)
• Silt- Fine
• Sand- Medium
• Gravel- Large

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What type of soil is 20 Clay, 70 silt, And 10
Sand?
100 Clay
Silty Loam
100 Sand
100 Silt
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Loams Roughly equal mixture of clay, silt, sand
and humus.
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Soil Texture Nutrient-Holding Capacity Water- Infiltrating Capacity Water-Holding Capacity Permeability
Clay Good Poor Good Poor
Silt Medium Medium Medium Medium
Sand Poor Good Poor Good
Loam Medium Medium Medium Medium
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• Soil Porosity is the amount of pore space a
  soil has.
• Porosity is affected by the sizes, shapes and
  arrangements of particles.
• It is also influenced by plants and soil
  organisms that create channels and pores in
  soil.
• Compaction reduces pore space. Soil holds less
  water or air.

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Soil Permeability

• A measure of how connected the pore spaces are to
  each other.
• Determines the water and air capacity of the
  soil.
• Clay will hold onto water because the clay
  mineral grains are negatively charged at their
  edges. Some clays also swell when water is added.

START
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Soil Fertility

• Soil fertility is influenced by
• Porosity/compaction
• Drainage
• Nutrient content
• Nutrient availability
• Salinity and other toxins

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What are the major soil nutrients?

• macronutrients Sulfur, Phosphorous, Magnesium,
  Calcium, Potassium, Nitrogen, Oxygen, Carbon and
  Hydrogen.
• micronutrients molybdenum, copper, zinc,
  manganese, iron, boron, chlorine. (less
  important)
• Most soil nutrients are positively charged ions
  (cations).

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Nutrient availability

• The three biggest factors for nutrient
  availability for a moist soil are
• clay content, organic matter (OM) content, and
  pH.
• The Cation Exchange Capacity (CEC) is a measure
  of how available nutrients are to plants in the
  soil. It is a lab measurement.
• CEC can be estimated by determining the amount of
• Clay
• Organic matter
• IF the pH is about 7.0 (neutral)

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Why does clay matter?

• Nutrients are mostly cations (positively charged
  ions).
• Clays are negatively charged at their edges.
• Clays hang on to cations loosely enough, so they
  are available to plants.

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Why does OM matter?

• Its surface is negatively charged, so it has lots
  of places for positive ions to loosely attach and
  become available for plants to use.
• It tends to contain a lot of carbon and other
  nutrients in the big organic molecules that make
  up the OM itself.
• Its food for the decomposers, etc.
• The chemistry gets messy, but thats the gist of
  it

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Why does pH matter?
H competes with other nutrients for sites on OM
and clays. During acid leachingnutrients are
washed away.
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Tropical soils why do clay, OM and pH matter?

• Organic matter is broken down rapidly in the
  tropics (its hot!) and is removed.
• High rainfall leads to lots of leaching
• Leaching of soils in the tropics leads loss of
  clays in upper layers and laterization.
• Tropical soils are generally low in fertility as
  a result.

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Sodium poison in soil

• Sodium modifies the structure of some clays to
  seal the soil. Water does not penetrate.
• Too much sodium (salt) is toxic to most plants.
  Excess sodium can be tolerated by some plants
  that have evolved to deal with it.
• If salt builds up, it must be leached away by
  flooding, or other chemicals (calcium sulfate)
  must be applied to the soil to restore fertility.

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What is NOT generally considered a benefit of
organic matter

1. It contributes to overall soil fertility by
   helping make nutrients available to plants.
2. It binds contaminants.
3. It stains your clothes when you dig in the dirt.
4. It retains and slowly releases water.
5. It is a carbon sink, meaning that it sequesters
   carbon and keeps it out of the atmosphere.

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The B horizon is

1. A soil layer dominated by organic matter.
2. A mineral soil layer that has some deposits of
   iron oxides, aluminum oxides, and clays that turn
   it red. Or it may have a prismatic or blocky
   structure.
3. Parent material that is virtually unaltered.
4. A layer or zone of eluviation where iron oxides
   and aluminum oxides have been leached out by
   water.

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What is the difference between porosity and
permeability?

1. Only porosity can be improved by soil organisms,
   not permeability.
2. Porosity can be reduced by compaction (squishing
   the soil), but permeability cannot.
3. Porosity is a measure of total pore space, while
   permeability measures how connected the pores
   (holes) are.
4. You measure porosity by pouring water through the
   soil. You cant measure permeability this way.

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How does clay improve soil fertility?

1. It doesnt. Clay decreases soil fertility.
2. Clay increases the availability of nutrients to
   plants.
3. Clay increases the permeability of the soil.
4. Clay is a nutrient.

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What is the optimal pH for soil fertility for
most food crops?

• a. Less than 7.0 (acid)
• b. 7.0 (neutral)
• c. Greater than 7.0 (alkaline)