Basic Soil Properties Flashcards Preview

IPNI Soil and Water Management > Basic Soil Properties > Flashcards

Flashcards in Basic Soil Properties Deck (46):


  • negatively charged atom or molecule
  • Examples found in soils
    • phosphate (H2PO4-, HPO42-)
    • sulfate (SO42-)
    • nitrate (NO3-)
    • chloride (Cl-)
  • ions carry 1, 2, or 3 charges called monovalent, divalent, trivalent



  • positively charged atom or molecule
  • Examples in soils:
    • Calcium (Ca2+)
    • magnesium (Mg2+
    • sodium (Na+)
    • potassium (K+)
    • ammonium (NH4+)
  • ions that carry one, two, or three charges are called monovalent, divalent, or trivalent.


Cation exchange capacity

  • Cation exchange capacity is that amount of positively charged cations which can be held by a given weight of soil.
  • Cations are held by negative charges in clay and organic matter.
  • units
    • centimole charge per kg soil (cmolc/kg soil)
    • which is equivalent to meq/100g soil.


Soil has a CEC of 10 cmolc/kg


What is the CEC per meq/100g soil?


10 meq/100g soil



10 cmolc/kg  =  10 meq/100g soil


Anion Exchange Capacity (AEC)

  • Anion exchange capacity is that amount of negatively charged cations which can be held by a given weight of soil.
  • Anions are held by charges positive charges in clay and organic matter.
  • units are the same as CEC


Soil organic matter (humus) CEC content

200 meq/100g


Vermiculite Clay CEC 

150 meq/100g


Montmorillonite Clay CEC

100 meq/100g CEC


Illite Clay CEC

30 meq/100g CEC


Kaolinite Clay CEC

10 meq/100g


Soil Contains

  • 3% clay 
  • 20% montmorillonite 

CEC of 26 meq/100g

(0.03x200 + 0.20x100)


As pH increases. . . 

. . .CEC increases and AEC decreases


*most important in weathered soils of tropical climates


Some clay minerals have holes that fit. . .

. . . K+ and NH4+ ions. 


  • when those ions enter the holes, the clay collapses around them making them more plant available
  • weathering reactions can slowly release these cations to more available forms


Parent material and minerology influence on background fertility

  • determines many soil properties which influence background fertility
    • pH
    • CEC, AEC
    • soluble salts
    • Clay minerology
    • organic matter
  • insoluble minerals also serve as a nutrient resevoir that can become plant available over time


Saline Soil

  • contains sufficient soluble salt to impair plant growth
    • electrical conductivity greater than or equal to 0.4 siemens per meter in saturation extract.


Sodic / Natric Soil

  • has from 13 to 15 percent (or more) of the CEC occupied by sodium 
  • have poor structure and accompanying poor plant growth


Saline-Sodic Soils

  • soils have ECs > 0.4 siemens per meter and from 13 to 15 percent of the CEC (or more) occupied by sodium.  
  • these soils have good physical properties until the salt is removed and they revert to sodic soils.


Calcareous Soils

soil that contains free calcium carbonate (CaCO3)


Acidic Soils

soils with a pH less than 7


Alkaline soils 

soils with a pH greater than 7


Define Soil Texture

  • percentages of sand, silt and clay in a soil determines soil texture
  • sand, silt and clay are called soil separates


Gravel particle size



Sand particle size



Silt particle size



Clay particle size 



Soil particle size affects surface area and reactivity of soils

Relative Surface areas of Soil Separate

  • Sand 1
  • Silt 250
  • Clay 17,000

Clay holds more water and retains more nutrients than Sand or Silt


Soil Properties change as amounts of sand and silt decrease and amounts of clay increases

  • bulk density, particle size and pore size decrease
  • pore volume and surface area increases


***add graphs on page 47 to flashcards


Soils with higher surface areas tend to be __________ reactive because of ________ charge 

Soils with higher surface areas tend to be more reactive because of higher charge.


  • higher surface area = more clay/organic matter= higher CEC and AEC


  • as well as more surfaces upon which reactions take place


How does Soil Texture affect

  • water holding capactiy
  • available water
  • wilting point of soils

  • water holding capacity
    • pore sizes in soil impacts soil drainage
      • larger soil pores are required for excess water and O2 to move into and through soil
      • smaller porse retain water for plant use


How does Soil Texture affect

  • water holding capactiy
  • available water

  • plant available water is water which can be extracted from plants
  • maximum value of available water is  (field capacity) minus (wilting point)
  • intermediate textures have the most available water

**add graphs from page 47


How does Soil Texture affect

  • water holding capactiy
  • available water
  • wilting point

  • amount of water in a soil where the plants will wilt and not recover.
  • sand .05 wilting point
  • silt loam .75 wilting point
  • clay .2 wilting point


Field Capacity of different soil textures

  • sand .03
  • silt loam .2
  • clay .3 g/g


Amount of water in a soil is measured as

  • weight (on a dry soil basis) percentage 
  • volume percentage
  • height of water (centimeters or inches)
  • energy of retention (units are bars, atmospheres, or pascals)


Define Soil Structure

  • arrangement of soil particles (sand, silt, clay) into larger units (aggregates)
    • structural units named peds


Major kinds of Soil Structure


  • Massive
    • category: structureless
    • soil particles cling together
      • do not break into smaller units
    • structure of puddled soils (lost structure)
  • Single Grain
    • sand


  • Granular
    • small round aggregates
    • porous. common to plow layer
  • platy
    • aggregates are thin form like a stack of plates
  • Blocky
    • irregular six sided aggregates
      • angular blocky - sharp edges, subangular
      • blocky - rounded edges
  • Prismatic / Columnar
    • like a column of soil with well defined edges along the column
    • prismatic - no rounded top
    • columnar - rounded top


Soil structure affect

Soil texture - properties within aggregates

Soil Structure - properties between aggregates

  • Good structure in plow layer or topsoil changes pores
    • alters soil areation
    • water relations (infiltration vs runoff)
    • soil tilth for proper germination / growth
    • Granular structure preferred for seedbed
  • Good Structure in Subsoil
    • soil aeration
    • water relations
    • root penetration


Example of poor soil structure and good soil structure

  • Good 
    • 25% Micropores
    • 25% Macropores
  • Poor
    • 40% Micropores
    • 10% Macropores


Soil organisms and Organic matter effect on soil structure

  • microorganisms affect soil structure thru decomp of soil organic matter, crop residues, and organic amendments
    • short term
      • decomp can increase aggregation (glue soil particles together)
    • long term
      • conditions that favor decomp 
        • frequent tillage
        • optimum temp
        • moisture
        • Oxygen
      • decrease soil organic levels and aggregation


Macro Organisms affect on Soil Structure 

  • Macroorganisms
    • ants, termites, earthworms, moles
    • mix soil, create large channels
    • termites 
      • contribute to decomp of organic materials at or near the surface
        • but material secreted is low in organic matter
    • Earthworms
      • consume soil, excrete granular structure
      • create macropores for aeration / drainage


Soil Bulk Density

  • BD of a soil is weight of dry soil in grams per cubic cm of soil
  • typical BD
    • 1.7g/cm3 (sandy soils)
    • 1.1g/cm3 (clayey soils)
  • Organic soils BD
    • .5g/cm3 


Particle Density of mineral soils

  • average 2.65 g/cm3 


Calculate Percent Porosity

  • % Porosity = 100 - 100*BD/PD
  • % Porosity ranges from
    • 36% sandy soils 
    • 58% in clay soils


Size of Pores _________ with Texture


  • sandy soils have mostly large pores
  • clay soils have small pores


Bulk density can be increased by ________

Bulk density can be decreased by __________

  • compaction
  • improving soil structure


Changes in bulk density usually due to changes in _________

  • soil structure


Increasing Bulk density

  • soil organic matter decreases (due to incorporation, burning, removal crop residues)
    • INCREASE bulk density
  • Tillage over years can increase bulk density 
    • cause formation of tillage pans
  • Compaction from wheels increase BD