Basic Soil Properties Flashcards
Anion
- 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
Cation
- 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
76-2.00mm
Sand particle size
2.0-0.05mm
Silt particle size
0.05-0.002mm
Clay particle size
<0.002mm
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
- pore sizes in soil impacts soil drainage
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
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
- irregular six sided aggregates
- 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
- conditions that favor decomp
- short term
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
- contribute to decomp of organic materials at or near the surface
- 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
- .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
Vary
- 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
