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
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.
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.
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
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
6
Q
Soil organic matter (humus) CEC content
A
200 meq/100g
7
Q
Vermiculite Clay CEC
A
150 meq/100g
8
Q
Montmorillonite Clay CEC
A
100 meq/100g CEC
9
Q
Illite Clay CEC
A
30 meq/100g CEC
10
Q
Kaolinite Clay CEC
A
10 meq/100g
11
Q
Soil Contains
- 3% clay
- 20% montmorillonite
A
CEC of 26 meq/100g
(0.03x200 + 0.20x100)
12
Q
As pH increases. . .
A
. . .CEC increases and AEC decreases
*most important in weathered soils of tropical climates
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
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
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.
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
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.
18
Q
Calcareous Soils
A
soil that contains free calcium carbonate (CaCO3)
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
Clay particle size
\<0.002mm
26
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
27
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
28
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
29
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
30
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
31
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
32
Field Capacity of different soil textures
* sand .03
* silt loam .2
* clay .3 g/g
33
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)
34
Define Soil Structure
* arrangement of soil particles (sand, silt, clay) into larger units (aggregates)
* structural units named peds
35
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
* Prismatic / Columnar
* like a column of soil with well defined edges along the column
* prismatic - no rounded top
* columnar - rounded top
36
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
37
Example of poor soil structure and good soil structure
* Good
* 25% Micropores
* 25% Macropores
* Poor
* 40% Micropores
* 10% Macropores
38
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
39
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
40
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
41
Particle Density of mineral soils
* average 2.65 g/cm3
42
Calculate Percent Porosity
* % Porosity = 100 - 100\*BD/PD
* % Porosity ranges from
* 36% sandy soils
* 58% in clay soils
43
Size of Pores _________ with Texture
Vary
* sandy soils have mostly large pores
* clay soils have small pores
44
Bulk density can be increased by \_\_\_\_\_\_\_\_
Bulk density can be decreased by \_\_\_\_\_\_\_\_\_\_
* compaction
* improving soil structure
45
Changes in bulk density usually due to changes in \_\_\_\_\_\_\_\_\_
* soil structure
46
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
