Soils 201

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Soils 201

• Soil Chemical Properties

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Fig. 4.3. General relationship between particle
size and kinds of minerals present. Quartz
dominates the sand and coarse silt fractions.
Primary silicates such as the feldspars,
hornblende, and micas are present in the sands
and, in decreasing amounts, in the silt fraction.
Secondary silicates dominate the fine clay. Other
secondary minerals, such as iron and aluminum
oxides, are prominent in the fine silt and coarse
clay fractions.
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Kinds of Minerals in Soils

• Silicate minerals (the main kind in soils)
• Nonsilicate minerals
• The most common minerals in the sand- and
  silt-size fraction are primary silicate minerals.
• Most of the clay-sized particles in soils are
  secondary (silicate and nonsilicate) minerals
  formed in the soil from the weathering of primary
  minerals.

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Minerals

• Primary mineral A mineral that has not been
  altered chemically since deposition and
  crystallization from molten lava.
• Secondary mineral A mineral resulting from the
  decomposition of a primary mineral or from the
  reprecipitation of the products of decomposition
  of a primary mineral.

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Silicate Minerals

• Primary silicate minerals Quartz, feldspar,
  micas, hornblende, and augite.
• Secondary silicate minerals Vermiculite,
  montmorillonite, illite, chlorite, and kaolinite.

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Soil Colloids Inorganic and Organic Matter lt
0.001 mm in sizeFour Groups of Soil Colloids

• 1. Secondary silicate minerals (silicate clays)
• 2. Iron and aluminum oxides (other secondary
  minerals)
• 3. Amorphous clays (other secondary minerals)
• 4. Humus (decomposed organic matter)

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Figure 8.1  Simplified representation of a
silicate clay crystal (micelle), its complement
of adsorbed cations, and ions in the surrounding
soil solution.
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Figure 8.3  The basic molecular and structural
components of silicate clays.
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Silicate Clays

• Each silicate clay particle is a flat, platelike
  crystal made of a series of layers, much like a
  deck of cards.
• Each layer, in turn, is composed of two to three
  sheets.
• There are two types of sheets a silica sheet and
  an alumina sheet.
• Silica and alumina sheets form a layer when they
  bind to one another by sharing oxygen atoms.
• The different stacking arrangements and specific
  nature of these two sheets help determine the
  different types of silicate clays in the soil.

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Figure 8.3  The basic molecular and structural
components of silicate clays.
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Silica and Alumina Sheets

• The basic building block of the silica sheet is a
  unit called the silica tetrahedron a silicon
  atom surrounded by four oxygen atoms.
• An interlocking plane of numerous tetrahedra
  joined together by shared oxygens forms a silica
  or tetrahedral sheet.
• The basic unit of the alumina sheet is the
  alumina octahedron an eight-sided building block
  consisting of a core aluminum atom surrounded by
  six hydroxyls or oxygens.
• Large numbers of alumina octahedra, bound to each
  other by shared oxygens and arranged in a plane,
  comprise an alumina or octahedral sheet.

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Si-Tetrahedron a four-sided structure made up
of a silicon ion surrounded by four oxygen atoms.
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Silicate Clays and Negative Charge

• Silicate clay particles carry a negative charge.
• Silicate clays can hold hundreds or thousands of
  pounds of nutrient cations per acre furrow slice.
• These held cations are called adsorbed cations.
• In addition to these adsorbed cations, silicate
  clay particles hold numerous water molecules.

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Sources of Negative Charge of Silicate Clays

• Exposed crystal edges.
• Ionic or isomorphic substitution.

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Exposed Crystal Edges

• Exposed oxygen or hydroxyl groups at exposed
  crystal edges are partly responsible for what has
  been termed the pH-dependent charge of clay
  minerals.
• pH-dependent charge that portion of the total
  charge of the soil particle which is affected by,
  and varies, with pH.
• At pH values of about 6 and above, hydrogen ions
  can be replaced by other cations such as calcium
  and magnesium.

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Exposed Crystal Edge
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Figure 8.11  How pH-dependent charges develop at
the broken edge of a kaolinite crystal.
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Ionic or Isomorphic Substitution

• Ionic substitution is the replacement (during
  clay formation) of one ion for another of similar
  size in the clay layer and usually lower positive
  valence.
• The remaining excess negative charge attracts
  cations from the soil solution.
• These negative charges are not dependent on pH
  and are commonly referred to as permanent
  charges.

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Isomorphous Substitution
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Figure 8.4  Simplified diagrams of octahedral
sheets in a silicate mineral illustrating the
effect of isomorphous substitution on the net
charge.