SSC001

• Colloids


  Very tiny particles (< 0.001 mm, or 1-micron [µ]), small enough to stay suspended in water; bear a slight negative charge, attract nutrient cations and water molecules; have a large surface area per unit amount; the seat of various chemical reactions in soils; chemically reactive because of the electrical charges (positive and negative) on their surface; classified into two general groups: inorganic colloids/mineral or clay colloids (silicate and non-silicate) and organic colloids
• Inorganic colloids


  • Formed or synthesized from primary minerals through physical and chemical alteration, and decomposition with subsequent recrystallization
• Types of inorganic colloids

  • Silicate clays (aluminosilicates)
  • Non-silicate clays or Sesquioxide clays
• Types of silicate clays

  • Crystalline Silicate clays
  • Amorphous (non-crystalline) Silicate clays
• Crystalline Silicate clays


  Composed of sheet-structured aluminosilicates of various types depending on the ratio of silica sheet to alumina sheet in the crystal structure
• Types of Crystalline Silicate clays

  • 1:1 nonexpanding type of clays
  • 2:1 expanding type of clays
  • 2:1 limited expansion type
  • 2:1 nonexpanding type
  • 2:2 type
• 1. 1 nonexpanding type (Kandites)


  Residues from extensive weathering in high rainfall, acidic soils; has only one sheet each of Si tetrahedra and Al octahedra per layer; almost no substitution of Al3+ for Si4+ or Mg2+ for Al3+ thus net negative charge (CEC) is low; represented by kaolinite, the most prominent member; kaolinites have strong H-bond between layers preventing water to enter that is why they do not shrink and swell, hence, good material for pottery; reactive area limited to external surface (10-20 m2/g); kandites have pH-dependent charges on their surface and thus can generate both a cation and an anion exchange capacity, depending on the pH of the soil solution; occur as hexagonal crystals; good bases for roadbeds and building foundations; commonly used in making tricks, and easy to cultivate for agriculture
• 1. 1 expanding type (Smectites)

  The swelling, sticky 2:1 (two Si tetra sheets sandwiching one Al octa sheet) expanding lattice clays; water easily penetrates between planes of adjacent O's; common in arid regions, poorly drained soils and those developed from alkaline rocks like limestone; the most common member is montmorillonite; the unit layers are loosely held together by weak oxygen-to-oxygen and cation-to-oxygen linkages; when saturated with water, basal spacing between layers can approach 20° A (2.0 nm) however, under dry conditions it may be reduced to less than 10° A (1.0 nm); the specific surface (total surface area) is large due to the contribution of internal surface which is 700-800 m2/g; it is a flake-like crystal shape and much smaller than kaolinite; most of the negative charge comes from isomorphous substitution of Mg2+ for Al3+ in the octahedral sheet; has high shrink-well capacity which means it has big tendency to form crack and unstable soil surface; solids dominated by smectites are very difficult to cultivate because they form aggregates or clods that are very hard and are poor bases for roadbeds and building foundation
• 1. 1 limited expansion type (Vermiculite)


  Like hydrous mica but have lost the interlayer K ions; has layers held weakly by hydrated Mg, thus, this clay swells but not as much as montmorillonite, e.g., limited expansion; water molecules along with other ions act as bridge holding the unit layers together rather; has specific surface of 500-700 m2/g and exhibits a high CEC
• 1. 1 nonexpanding type (Hydrous mica and illite)

  2. 1 lattice clays like montmorillonite but have large K ions holding the layers tightly preventing entry of water
• 1. 2 or 2:1:1 nonexpanding type (Chlorites)

  2. 1:1 lattice clays (2 Si tetra, 1 Al octa and 1 Mg octa); do not swell when wetted; have low CEC's
• Amorphous (non-crystalline) Silicate clays

  Allophane and imogolite; mixed or poorly defined combinations of silica and aluminum sesquioxide; lacks crystallinity; common in volcanic ash soils; charges are from accessible hydroxyl ions (OH-) which can attract a positive ion or lose the H+ attached; pH-dependent charge or depends on the H+ in the solution (the soil acidity); high CEC and AEC; Imogolite was first found in weathered volcanic ash or pumice beds in Japan called imogo; its chemical structure is better defined than allophone; electron microscopy shows the presence of hair-like or spaghetti-like crystal forms
• Non-silicate clays or Sesquioxide Clays


  Amorphous metal oxide and hydrous oxides of Fe and Al; remnant materials under conditions of extensive leaching and weathering of primary minerals in well-drained soils of warm, humid climates where most of the Si and Al are dissolved and leached; are mixtures of Al(OH)3, Fe2O3 or Fe(OH)3; also include TiO2 and MnO2,with lower solubilities; do not swell, non-sticky; coat large particles and form stable aggregates, absorb water as if they were fine sands; high P adsorption capacity makes P unavailable, a common problem in highly weathered tropical soils; include iron oxides like hematite, goethite, limonite and aluminum oxides like boehmite, gibbsite
• Organic Colloids - Humus


  Decay-resistant residue of organic matter decomposition; amorphous, dark brown to black, nearly insoluble in water but mostly soluble in dilute alkali solutions; contains about 30 % each of the N-rich proteins, the slow-to-decompose lignin, and the complex sugars (polyuronides – CHOs that "cement" soil aggregates); has about 50 % Carbon, 5 % Nitrogen, some Oxygen, and lesser amounts of Phosphorus and Sulfur; on a dry-weight basis, has CEC many times greater than clays; high negative charges associated with partially neutralized carboxylic (-COOH) and phenolic (-OH) groups, which are pH-dependent
• Classes of humus based on solubility

  • Fulvic acid
  • Humic acid
  • Humin
• Origin of negative charges on colloids


  • Isomorphous Substitution (permanent charge not affected by change of pH)
  • Deprotonation (pH dependent)
  • Dissociation of H ions from the functional –OH and –COOH groups of soil OM (pH dependent)
• Cation Exchange


  The replacement of one cation for another on exchange sites; the exchange between a cation in solution and one adsorbed on a soil colloid
• OH-ion concentrations


  Reduced, and the negative charge is reduced
• Dissociation of H ions from the functional –OH and –COOH groups of soil OM

  1. -OH → -O- + H+

  2. -COOH → -COO- + H+
• Organic acids containing acidic functional groups, such as carboxyl or phenolic/hydroxyl groups, tend to donate protons (H⁺) more readily at high pH further increasing the negative charge
• Ca-clay micelle + 2H+ < -- -- > 2H-clay micelle + Ca++; replacement of Ca by H occurs not only due to mass action but also because under comparable conditions (chemically equivalent amounts) H+ is adsorbed more strongly than is the Ca++
• Exchangeable Cations


  Cations that can be replaced on exchange sites, e.g. exchangeable K, Ca, etc.
• Factors affecting the strength of adsorption

  • Charge (valence)
  • Hydrated size
  • Ionic size
  • Concentration of solution
• Order of adsorb-ability or replacing power of cations commonly found in soils


  • (Al3+, H+) > Ca2+> Mg2+ > K+ = NH4+ > Na+
• Order of adsorb-ability or replacing power of cations in humid regions

  • (H+, Al3+) > Ca2+ > Mg2+ > K+ > Na+
• Order of adsorb-ability or replacing power of cations in arid regions

  • Ca2+, Mg2+ > Na+, K+ > H+
• Cation Exchange Capacity (CEC)

  Refers to the capacity of the soil to hold (in a readily available form) several cations that can be exchanged for others; the total number of exchangeable cations a soil can adsorb; a measure of the soil's ability to hold nutrients; arises from the electrical charges on the colloid surfaces of the clay and organic matter, attracting ions of opposite charge; expressed in terms of equivalents or as milliequivalents per 100g of soil (me or meq/100 g soil) or centimoles of positive charge per kg of soil (cmolc / kg soil)
• Equivalent


  1. gram atomic weight of H or the amount of any other ion that will combine with or displace this amount of H
• Factors Affecting CEC


  • Amount of clay (soil texture)
  • Type of clay
  • OM content
  • pH
• CEC of different colloids

  • Humus: 100-300
  • Vermiculite: 80-150
  • Montmorillonite: 60-100
  • Illite: 25-40
  • Kaolinite: 3-15
  • Sesquioxides: 0-3
  • Chlorite (2:1:1 or 2:2): very high, but access blocked; 2-5
  • Allophane: 50-150
• pH: amorphous clay minerals and organic matter have a CEC that varies with pH. As pH increases, so does the CEC. Under acid conditions, these have an anion exchange capacity. For organic matter, the rule of thumb is that for every pH unit above 4.5, there is a 1 meq/100 g increase for each percent organic matter
• Characteristics of Cation Exchange Reaction

  • Reversible
  • Speed/rate (instantaneous)
  • Stoichiometry
  • Extent
• Anion Exchange / Storage


  At the anion exchange site, an extra H joins the OH group to produce a net positive charge. This attracts anions. For most soils, anion exchange capacity is quite low. Anion exchange is greatest in acid soils high in oxide clays.
• Because there is little adsorption of the anions, many (particularly nitrate) are easily leached, which can lead to groundwater contamination. This can even happen in organic farming situations if not well managed. Nutrients that are usually supplied by anions are nitrogen (NO3-), phosphorus (H2PO4-, HPO42-), sulfur (SO42-), chlorine (Cl-), boron (H4BO4-), and molybdenum (MoO42-)
• Anion exchange capacity (AEC)


  The sum of exchangeable anions that a soil can adsorb usually expressed as centimoles or millimoles of charge per kilogram of soil (or of other adsorbing material such as clay). An exchangeable anion is a negatively charged ion held on or near the surface of a solid particle by a positive surface charge and which may be replaced by other negatively charged ions (e.g. with a Cl- salt)
• Mechanism of Anion Exchange


  Electrostatic attraction between positively charged colloids and the anions - due to protonation

  Replacement of exposed hydroxyl groups - phosphate readily replaces other anions but is not easily replaced once it occupies the exchange site because it is held tightly by the soil. This is the reason why phosphate is relatively immobile in the soil. As the reaction progresses, the available P becomes insoluble. Conversion from available to unavailable P
• y charged ion


  Negatively charged ion held on or near the surface of a solid particle by a positive surface charge and which may be replaced by other negatively charged ions (e.g. with a Cl- salt)
• Anion properties


  • Atomic mass (g)
  • Valence
  • g/cmolc
• Anion properties (milliequivalent)


  • Atomic mass (g)
  • Valence
  • Equivalent mass or weight (g)
  • Mass or weight of 1meq (g/meq)
• Mechanism of Anion Exchange

  1. Electrostatic attraction between positively charged colloids and the anions - due to protonation

  2. Replacement of exposed hydroxyl groups - phosphate readily replaces other anions but is not easily replaced once it occupies the exchange site because it is held tightly by the soil. This is the reason why phosphate is relatively immobile in the soil. As the reaction progresses, the available P becomes insoluble. Conversion from available to unavailable is termed phosphate fixation. The replacement also occurs in equivalent amounts.