Soil physics

• Physical properties


  Characteristics, processes, or reactions of soil that are caused by physical forces and which can be described by, or expressed in, physical terms or equations
• Physical properties


  • They can be seen or felt
  • They help answer questions like: Is the soil loose so roots can grow easily through it or does water seep in easily? Is the soil tight hampering root growth and water absorption? How well does the soil supply air, water, and nutrients?
• Outline of Chapter 2

  • Soil Texture
  • Soil Structure
  • Soil Color
  • Bulk Density
  • Soil Porosity
  • Void Ratio
  • Soil Permeability
  • Soil Consistence
  • Soil Tilth
  • Soil Temperature
  • Soil Water

  
• Soil texture


  The relative proportion of the soil particles or separates - sand (coarsest), silt (medium), and clay (finest)

  
• Soil texture refers to the sensation when you rub the materials with your fingers
• Soil texture is a stable property, meaning it does not easily change with soil management
• Soil texture affects soil behavior, in particular its retention capacity for nutrients and water

  
• Sand and silt are the products of physical and chemical weathering; clay, on the other hand, results from chemical reactions between weathered minerals
• Internal surface area (ISA)

  The total surface area of all the soil particles

  
• Reactions occur on the soil surface
• Soils with the smallest particles, like silt and clay, have the largest surface area, they hold the most moisture

  
• Only the finest particles are charged, hence, are important for nutrient retention
• Soil pores


  Larger between large particles and smaller between small particles

  
• Macropores


  Large pores that allow water to drain rapidly
• Micropores


  Small pores that tend to retain water
• Soil separates


  • Sand
  • Silt
  • Clay
• Sand


  • Coarse, gritty, mostly primary minerals (quartz), cubic to spherical in shape, acts as individual grains
• Silt


  • Smooth, powdery, mostly primary minerals (quartz), cubic to spherical in shape, able to hold large amounts of water in a form plants can use
• Clay


  • Sticky and plastic when moist, mostly secondary "clay" minerals, very high specific surface area hence the most reactive component of the soil, plate-like or flake-like or sheet-like
• Influence of surface area on other soil properties

  • Greater surface area means greater capacity for holding water films
  • Greater surface area means greater rate of release of plant nutrients from weatherable minerals
  • Greater surface area means greater rate of release of plant nutrients and other chemicals
  • Greater surface area means greater propensity of soil particles to stick together in a coherent mass, or as discrete aggregates
  • Microbial reactions in soils are greatly affected by the specific surface area

  
• Generalized influence of soil separates on some properties and behavior of soils

  • Water-holding capacity
  • Aeration
  • Drainage rate
  • Compactibility
  • Total surface area (TSA)
  • Ability to store nutrients
  • Suitability for tillage after rain
  • Soil organic matter level
  • Decomposition of OM
  • Warm-up in spring
  • Resistance to pH change (Buffering capacity)
  • Susceptibility to water erosion
  • Shrink-swell potential
  • Sealing of ponds, dams, and landfills
  • Pollutant leaching potential
  • Susceptibility to wind erosion
• Soil scientists have simplified texture by dividing soils into 12 textural classes
• Soil textural classes


  • Coarse-textured soils: sand and loamy sand
  • Moderately coarse: sandy loam
  • Medium-textured soils: loam, silt loam, and silt
  • Moderately fine: sandy clay loam, silty clay loam, clay loam
  • Fine-textured soils: sandy clay, silty clay, and clay

  
• For most purposes, growers consider medium-textured soils (loam, silt loam, silt) to be ideal
• Determination of textural class

  • Indirect methods: Use of soil survey report, Use of soil map (paper and interactive), Use of soil databases (tabular and GIS/map-based)
  • Direct methods: Qualitative "feel" or "roll" method, Laboratory quantitative methods (dispersion, fractionation - sieving, sedimentation, laser diffraction)
• Stoke's law


  The settling velocity of particles in a liquid medium is directly proportional to the square of the particle diameter
• USDA size classification of rock fragments

  • Gravel or Pebbles (2- 75 mm)
  • Cobbles (75-250 mm if round)
  • Channers (75-250 mm if flat)
  • Stones (250-600 mm)
  • Boulders (>600 mm)

  
• Importance of soil texture

  • Governs the way water behaves in the soil (infiltration and percolation)
  • Guides irrigation and drainage management
  • Fine soils retain nutrients better than coarse soils
  • Influences how easily soil can be worked

  
• Soil structure


  The arrangement of soil particles (sand, silt, and clay) under natural conditions into bigger units or the way they clump together into large units called aggregates
• Conditions for a soil to have structure: there should be aggregation and a definite pattern or arrangement of particles

  
• Structureless soil conditions


  • Single-grained
  • Massive
• Types of soil structure

  • Spheroidal (granular and crumb)
  • Platelike
  • Blocklike
  • Prismlike (columnar and prismatic)

  
• Granular structure


  • Typically ranges from <1 mm to as large as 10 mm in diameter, characterizes many surface soils (usually topsoil or commonly found in the A horizon of arable soil), particularly those high in organic matter, the principal type of soil structure affected by management, water circulates very easily through such soils

  
• Crumb structure


  • Very porous
• Platy structure


  • Characterized by relatively thin horizontal sheetlike peds (plates), commonly found in E horizon and forest soils, may occur in part of A horizon, and in claypan soils, unlike other structure types, can be inherited from soil parent materials, especially those laid down by water or ice, compaction of clayey soils by heavy machinery can create platy structure

  
• Blocklike structure


  • Irregular, roughly cubelike, and range from about 5 to 50 mm, individual blocks are not shaped independently but are molded by the shapes of the surrounding blocks
• Spheroidal soil structure


  • Granular structure that may be separated from each other in a loosely packed arrangement
  • Very porous

  
• Platelike (platy) soil structure

  • Characterized by relatively thin horizontal sheetlike peds (plates)
  • Commonly found in E horizon and forest soils
  • May occur in part of A horizon, and in claypan soils
  • Can be inherited from soil parent materials, especially those laid down by water or ice
  • Can be created by compaction of clayey soils by heavy machinery

  
• Blocklike soil structure


  • Irregular, roughly cubelike, and range from about 5 to 50 mm
  • Individual blocks are not shaped independently but are molded by the shapes of the surrounding blocks
  • Usually found in B horizons (subsoil) in humid regions where they promote drainage, aeration, and root penetration
  • May occur also in A horizons
• Angular blocky soil structure


  • Distinct angles