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IPNI Soil and Water Management > Basic Soil Properties > Flashcards

Basic Soil Properties Flashcards

1
Q

Anion

A
  • 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
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2
Q

Cation

A
  • 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.
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3
Q

Cation exchange capacity

A
  • 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.
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4
Q

Soil has a CEC of 10 cmolc/kg

What is the CEC per meq/100g soil?

A

10 meq/100g soil

10 cmolc/kg = 10 meq/100g soil

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5
Q

Anion Exchange Capacity (AEC)

A
  • 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
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6
Q

Soil organic matter (humus) CEC content

A

200 meq/100g

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7
Q

Vermiculite Clay CEC

A

150 meq/100g

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8
Q

Montmorillonite Clay CEC

A

100 meq/100g CEC

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9
Q

Illite Clay CEC

A

30 meq/100g CEC

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10
Q

Kaolinite Clay CEC

A

10 meq/100g

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11
Q

Soil Contains

  • 3% clay
  • 20% montmorillonite
A

CEC of 26 meq/100g

(0.03x200 + 0.20x100)

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12
Q

As pH increases. . .

A

. . .CEC increases and AEC decreases

*most important in weathered soils of tropical climates

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13
Q

Some clay minerals have holes that fit. . .

A

. . . 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
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14
Q

Parent material and minerology influence on background fertility

A
  • 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
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15
Q

Saline Soil

A
  • contains sufficient soluble salt to impair plant growth
    • electrical conductivity greater than or equal to 0.4 siemens per meter in saturation extract.
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16
Q

Sodic / Natric Soil

A
  • has from 13 to 15 percent (or more) of the CEC occupied by sodium
  • have poor structure and accompanying poor plant growth
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17
Q

Saline-Sodic Soils

A
  • 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.
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18
Q

Calcareous Soils

A

soil that contains free calcium carbonate (CaCO3)

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19
Q

Acidic Soils

A

soils with a pH less than 7

20
Q

Alkaline soils

A

soils with a pH greater than 7

21
Q

Define Soil Texture

A
  • percentages of sand, silt and clay in a soil determines soil texture
  • sand, silt and clay are called soil separates
22
Q

Gravel particle size

A

76-2.00mm

23
Q

Sand particle size

A

2.0-0.05mm

24
Q

Silt particle size

A

0.05-0.002mm

25
Q

Clay particle size

A

<0.002mm

26
Q

Soil particle size affects surface area and reactivity of soils

A

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

27
Q

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

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

***add graphs on page 47 to flashcards

28
Q

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

A

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
29
Q

How does Soil Texture affect

  • water holding capactiy
  • available water
  • wilting point of soils
A
  • 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
30
Q

How does Soil Texture affect

  • water holding capactiy
  • available water
A
  • 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

31
Q

How does Soil Texture affect

  • water holding capactiy
  • available water
  • wilting point
A
  • 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
32
Q

Field Capacity of different soil textures

A
  • sand .03
  • silt loam .2
  • clay .3 g/g
33
Q

Amount of water in a soil is measured as

A
  • weight (on a dry soil basis) percentage
  • volume percentage
  • height of water (centimeters or inches)
  • energy of retention (units are bars, atmospheres, or pascals)
34
Q

Define Soil Structure

A
  • arrangement of soil particles (sand, silt, clay) into larger units (aggregates)
    • structural units named peds
35
Q

Major kinds of Soil Structure

A

Structureless

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

Structured

  • 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
36
Q

Soil structure affect

A

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
37
Q

Example of poor soil structure and good soil structure

A
  • Good
    • 25% Micropores
    • 25% Macropores
  • Poor
    • 40% Micropores
    • 10% Macropores
38
Q

Soil organisms and Organic matter effect on soil structure

A
  • 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
39
Q

Macro Organisms affect on Soil Structure

A
  • 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
40
Q

Soil Bulk Density

A
  • 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
      ​
41
Q

Particle Density of mineral soils

A
  • average 2.65 g/cm3
42
Q

Calculate Percent Porosity

A
  • % Porosity = 100 - 100*BD/PD
  • % Porosity ranges from
    • 36% sandy soils
    • 58% in clay soils
43
Q

Size of Pores _________ with Texture

A

Vary

  • sandy soils have mostly large pores
  • clay soils have small pores
44
Q

Bulk density can be increased by ________

Bulk density can be decreased by __________

A
  • compaction
  • improving soil structure
45
Q

Changes in bulk density usually due to changes in _________

A
  • soil structure
46
Q

Increasing Bulk density

A
  • 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

IPNI Soil and Water Management (10 decks)

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