soil Colloids

• Soil Colloids


  Extremely fine soil particles
• Soil Colloids


  • Are negatively charged
  • Have ability to adsorb cations
  • Occupy large surface area
  • Have the most active portion of soil
• Types of Soil Colloids

  • Organic Colloids (humus)
  • Inorganic Colloids (clay particle)
• Inorganic Colloids


  Represented by clay
• Types of Inorganic Soil Colloids

  • Phyllosilicate Clays
  • Non-Crystalline Silicate Clays
  • Oxides (Iron and Aluminium)
• Phyllosilicate Clays


  • Are clay minerals derived from Silica and generally contain aluminium, magnesium and hydroxide
  • Are responsible for the cracks that form when soil dry out because the phyllosilicate clays minerals shrink without water they create wide, loosely tetrahedral cracks
  • Help to hold nutrients in the soil
  • Protect the soil from weathering
• Groups of Phyllosilicate Clays

  • Kaolinite
  • Montmorillonite
  • Illite
  • Vermiculite
  • Chlorite
• Vermiculite


  Has high fertility because of high negative charges which adsorb high amount of cations
• Kaolinite


  Has less fertility because of less negative charges which adsorb less cations
• Montmorillonite 2:1 type


  • Form from weathering minerals such as feldspar or mica
  • Rich in minerals
  • Made up of clay minerals containing aluminium, magnesium and hydroxide
• Non-Crystalline Silicate Clays


  • Lack a well defined and structured like oxides and clay minerals
  • Formed from deposit of volcanic ash
  • Receptive and fertile minerals
• Oxides (Iron and Aluminium)


  • Also known as sesquioxides
  • Important for holding nutrients in soil during cation exchange
• structure of Inorganic Colloids

  • Have a small diameter which gives them a large surface area
  • Carry a negative electric charge which attract cations
  • Cations form external ionic layer
  • Colloid particle form inner ionic layer
  • Ionic double layer is formed
• Silicon combines with oxygen to form silica sheets
• Soil Acidity


  Created by the reaction of a solution which influences the degree of acidity caused by the relative concentration of H+ ions (acidity) or OH- ions present in it
• Acidity


  Due to the excess of H+ ions over OH- ions
• Alkalinity


  Due to the excess of OH- ions over H+ ions
• Neutral Reaction


  Produced by an equal activity of H+ and OH- ions
• Dissociation


  Breaking-up into different electrical charged parts
• Basic Cations


  • Calcium
  • Magnesium
  • Sodium
• Cation Exchange Capacity (CEC)

  The capacity of soil to hold and exchange cations
• External Ionic Layer


  The outer energy level of the atom
• Ion Exchange


  Exchange (moving) of ions from colloid to soil solution and vice versa
• Legume


  Plant which can bind atmospheric nitrogen to nitrate
• Murky


  Unclear liquid
• Phosphate Fixing


  Phosphorus becoming insoluble in soil
• Platelets


  Thin leaves of something
• Soil Buffering Capacity


  Capacity of soil to prevent a change in the reaction (pH)
• Soil pH


  The acidity of the soil which is measured by the number of hydrogen ions present in the soil solution, as well as the alkalinity which is measured by the number of hydroxide ions in the soil solution
• Subject to Decay


  Can become rotten
• Suspension


  Particles float in a liquid medium
• In soil there are two types of colloids: clay particles (inorganic colloids) and humus (organic colloids)
• Inorganic Colloids


  • Represented by clay
  • Silicate clays are the most common and can be divided into five groups: kaolinite, montmorillonite, illite, vermiculite and chlorite
• Clay Minerals


  • Have a layered structure and consist of flat platelets
  • Platelets are held together by forces of attraction
  • Some clays swell considerably when wet (e.g. montmorillonite)
  • Some clays have platelets held together strongly with little expansion when wet (e.g. kaolinite)
• Inorganic Soil Colloids


  • Have an extremely large specific surface area
  • Carry a negative electric charge which attracts positively charged ions (cations)
  • Cations form an external ionic layer around the colloid particle
• Organic Colloid (Humus)


  • Complex, stable mixture of dark brown colloidal substance
  • Originates from plant and animal matter through action of soil microbes
  • Has no specific composition, shape or form
  • Possesses negative charges which cause it to be surrounded by adsorbed cations
  • Has much greater cation adsorption capacity than inorganic colloids
  • Mainly consists of carbon, hydrogen and oxygen
  • Subject to decay by microbes
• Cation Adsorption


  Negative charge of clay and humus colloids attracts positively charged ions (cations) which serve as main source of nutrient elements for plants
• Common Cations in Agricultural Soil

  • Hydrogen ion (H+)
  • Aluminium ion (Al3+)
  • Calcium ion (Ca2+)
  • Magnesium ion (Mg2+)
  • Potassium ion (K+)
  • Sodium ion (Na+)
  • Ammonium ion (NH4+)
  • Zinc ion (Zn2+)
  • Iron ion (Fe2+/Fe3+)
  • Manganese ion (Mn2+)
  • Copper ion (Cu2+)
• Predominant adsorbed cation around colloid

  Determines chemical and physical properties of soil
• Soil Types Based on Predominant Adsorbed Cation

  • Acid soil (H+ predominant)
  • Sweet soil (Ca2+ and Mg2+ predominant)
  • Brackish soil (Na+ and K+ predominant)