Chapter 4

• All soils contain mineral particles, organic matter, water and air. The combinations of these determine the soil's properties – its texture, structure, porosity, chemistry and colour.

  
• Major components of soils

  • Mineral particles
  • Organic matter
  • Water
  • Air

  
• Mineral components of soil

  • Coarse fragments
  • Sand
  • Silt
  • Clay

  
• Coarse fragments


  Greater than 2 mm and include coarse quartz, rock fragments and cemented material

  
• Sand


  Comprise quartz and resistant primary minerals such as mica, between 2 mm and 20 microns in size
• Silt


  Typically composed of quartz and small mineral particles such as feldspars and mica, between 2 and 20 microns in diameter

  
• Clay


  Made up of secondary clay minerals and oxides/ oxyhydroxides of iron and aluminum, less than 2 microns in diameter
• Soil properties that influence plant growth

  • Physical
  • Chemical
  • Biological
• It is the combination of these properties that determine soil health and the ability of the soil to provide ecosystem services.
• Soil properties influence plant growth and guide fertilizer decision making.
• Information relating to soil properties can be used to help guide investment decisions on-farm to maximize the benefit, for minimal investment.
• Important physical properties of soils

  • Soil texture
  • Soil structure
  • Surface area
  • Soil density
  • Soil porosity
  • Soil colour
  • Soil consistence

  
• Soil texture


  Indicates the relative content of particles of various sizes, such as sand, silt and clay in the soil

  
• Texture influences the ease with which soil can be worked, the amount of water and air it holds, and the rate at which water can enter and move through soil.

  
• Classification of particle size


  • Fine earth (< 2 mm)
  • Coarse fragments (2 to 7.5 cm gravel, 7.5 to 25 cm cobbles)
  • Rock fragments (> 25 cm stones and boulders)

  
• Stones and gravels may influence the use and management of land because of tillage difficulties but these larger particles make little or no contribution to soil properties such as water holding capacity and capacity to store plant nutrients and their supply.
• Components of fine earth

  • Sand
  • Silt
  • Clay
• Systems of naming soil separates

  • American system (USDA)
  • English system (BSI)
  • International system (ISSS)
  • European system
• Soil textural class characteristics are described for different soil types like sand, loamy sand, sandy loam, loam, silt or silt loam, sandy clay loam, clay loam, silty clay loam, sandy clay, silty clay, and clay.

  
• Methods to determine soil textural class

  • Feel method
  • Sieve method
  • Pipette method
  • Hydrometer method

  
• Using the texture triangle, the soil type can be determined based on the percentages of sand, silt and clay.

  
• Soil structure refers to the arrangement of soil separates into units called soil aggregates. Aggregates are groups of soil particles held together by organic matter or chemical bonds.
• Soil textural class


  • sand
  • loamy sand
  • sandy loam
  • loam
  • silt loam
  • silt
  • sandy clay
  • sandy clay loam
  • clay loam
  • silty sandy clay loam
  • silty clay
  • clay
• Soil textural class


  Feel by hand texturing
• Coarse to very coarse soil textural class

  • sand
  • loamy sand
• Coarse to very coarse soil textural class


  • Gritty - does not ribbon or leave a stained smear on hand
• Moderately coarse soil textural class

  • sandy loam
• Moderately coarse soil textural class

  • Gritty - leaves smear on hand, does not ribbon - breaks into small pieces
• Medium soil textural class

  • loam
  • silt loam
  • silt

  
• Medium soil textural class

  • Smooth and flour-like, does not ribbon, breaks into pieces about 1/2 inch long or less
• Moderately fine soil textural class

  • sandy clay
  • sandy clay loam
  • clay loam
  • silty sandy clay loam
  • silty clay
  • clay
• Moderately fine soil textural class

  • Forms ribbon; clays from longer ribbons than clay loams
• Clay loam soil textural class

  • Feels gritty
• Figure 4.3 shows the Soil Textural Classification of Kalinga based from DENR-CAR available Shapefile

  
• Soil Structure


  Arrangement of soil separates into units called soil aggregates
• Soil aggregates


  • Groups of soil particles held together by organic matter or chemical forces
  • Possess solids and pore spaces
  • Separated by planes of weakness and dominated by clay particles
  • Silt and fine sand particles may also be part of an aggregate
  • Act like a larger silt or sand particle depending upon its size

  
• Natural processes that aid in forming soil aggregates

  1. Wetting and drying

  2. Freezing and thawing

  3. Microbial activity that aids in the decay of organic matter

  4. Activity of roots and soil animals

  5. Adsorbed cations

  
• Wetting/drying and freezing/thawing action as well as root or animal activity


  Pushes particles back and forth to form aggregates
• Decaying plant residues and microbial byproducts

  Coat soil particles and bind particles into aggregates
• Adsorbed cations


  Help form aggregates whenever a cation is bonded to two or more particles