Basic Soil Chemistry - dcr.virginia.gov · Size affects Reactivity With water With chemicals With...
Transcript of Basic Soil Chemistry - dcr.virginia.gov · Size affects Reactivity With water With chemicals With...
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Basic Soil Chemistry
Soil Science
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Properties of colloids Properties of soil clays Cation Exchange Capacity Base Saturation The Soil Solution Soil pH and acidity in soils Effects of Aluminum on soil acidity
Objectives
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Size affects Reactivity With water With chemicals With biological components
Surface Area Colloids
clay sized (< 2 micrometers)
Charge
Properties of Colloids
0.3 x 0.3 x 6=0.54 0.1 x 0.1 x 6 x 27=1.62
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Sources of Charge
Permanent Charge Clay (layer silicates) Minerals
Variable Charge (pH dependent) Mineral Edges Oxides and Hydroxides of Fe and Al Organic Matter
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Permanent Charge Soils high in primary minerals, Si, Al, Mg, pH Layer silicates with 2 Si and 1 Al Layers If layers contain only Si and Al
Balanced Uncharged Stable
But ….. Si
Al
Si
2:1
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Permanent Charge: Ionic Substitution
If some Si+4 is replaced by Al+3 Or some Al+3 replaced by Mg+2
Creates charged sites (ground)
With Increasing substitution Increasing charge Decreases stability
Location of Charge (Si or Al layer) Affects Strength Expandability K Fixation
Si
Al
Si
2:1
-
-
- Al
Mg
Al
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Permanent Charge Soils low in soluble Si, Mg, and primary minerals, lower
pH, and high in Al Layer silicates with 1 Si and 1 Al
“No” substituion Low Charge Highly Stable
Si
Al
Si
Al
1:1
- Stacked Plates - Low surface area
Why is this important?
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Silicate Clays
1:1 Layer Silicates Kaolinite
2:1 Layer Silicates Micas
2:1 Layer Silicates Smectites & Vermiculites
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Variable Charge Minerals
Edges of clay minerals Edges of oxides, hydroxides of Fe and Al Exposed Edges of amorphous coatings Raise pH, increase charge
Al - O H
Al - O H - H + OH + OH
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Organic Matter
What is it? High carbon Leftovers Very high surface area Very complex chemistry
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Organic Matter - Humus
R-COOH + OH R-COO - + HOH
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Distribution of Charge
Type Colloid Charge Constant Variable cmol/kg % %
Organic 200 10 90 Vermiculite 150 95 5 Smectite 100 95 5 Kaolinite 8 5 95 Geothite 4 0 100
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Silicate Clays - Crystal
Ca2+
Ca2+ Ca2+
Ca2+
H+
H+
H+
H+
Mg2+
Mg2+ Mg2+
Mg2+ K+
K+
K+ K+
H+ Ca2+ Mg2+ K+
- - - - - - - - - -
- - - - - - - - -
- - - - - - - - -
Ca2+ H+ Mg2
K+ H+ Ca2+
H+ K+ Ca2+
Ca2+ H+ Mg2
K+ Ca2+ H+
NH4+ Ca2+
Ca2+
H+
Al3+
External Surfaces
Internal Surfaces
Enlarged edge of crystal
NH4+
Cl-
M+
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How Nutrients are Held
Permanent Charge pH dependent charge Minus Meets Plus
- - - - - - -
+
+
+
+
+
+
+
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0 P
/ | \ H0 H0 0
Strength of Retention Type of Charge ( + or -) Amount of Charge Reactivity (H2PO4
-)
K +
Mg++
Al +++
Ca++ K
+ K +
\ Al /
- - - - - - - - - - -
H +
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Soil Science
Cation Exchange Add Cations (fertilizers, root exudes,
rainfall, decomposition, etc.)
Mg++ K
+
Mg++
Al +++
Ca++
Ca++
Ca++
Ca++ K
+ K +
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Soil Solution
Plants depend primarily on what is in solution, not what is on the exchange complex (solids).
Typical concentrations 4 da to 7 da, readily available
Change readily Mobile (can be lost) Must be replenished
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The Soil Solution Weathering of mineral/
organic compounds Dissolution of soluble
forms into cations and anions
Hydrolysis reactions- Water with minerals
Oxidation/Reduction Reactions
Exchange reactions of "ions” held in an exchangeable form on solid soils
Soil Science
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How does Nutrient Uptake Occur?
Root Interception Mass Flow Diffusion
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Mg++ K
+
Mg++
Al +++
Ca++ K
+ K +
H +
Root Interception
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Mass Flow
Mg++ K
+
Mg++
Al +++
Ca++ K
+ K +
H +
Ca++ K
+ K +
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Diffusion
Mg++
Mg++
Al +++
Ca++ Na
+ H +
H +
Ca++
K +
K +
Note: If you take something from the soil, it must be replaced with something else!!
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How are soils buffered against change?
Soil Solution
Exchange Complex . . . . . . .. . . . . . . .
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Cation Exchange Capacity (CEC)
Cation Exchange Capacity cmolc kg-1
Sum of all cations (H, Na, K, Ca, Mg, Al, etc.) held by soil charges on an equivalent basis (per 1000 g)
1H + 3 Al +2 K + 4 Ca +2 Mg = 12 cmolc kg-1
1000 grams of soil (1 kilogram)
H +
Mg++
Al +++ Ca
++ K +
K +
Ca++
- - - - - - - - - - - -
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Buffering Capacity
Soil Science
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How does CEC Affect Nutrient Availability?
Solution depletion (uptake or leaching) causes: Exchange Dissolution
Solution additions cause: Exchange Precipitation
Al, Fe Oxides and P CaCO3, P
Dynamic Equilibrium
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Acidity, pH, and Source of
Acid in Soils
Soil Science
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Soil Science
Definitions
Acid A substance which gives up hydrogen (H+)
Base A substance which accepts hydrogen (H+)
pH pH = - (log 10 ([H+]) ) measures H+ in solution
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pH Concept Hydrogen Ion Concentrations
pH is a way to express very small numbers with a wide range easily The activity of Hydrogen (H+) and Hydroxyl (OH-) ions in solution
determine if the pH is acid, neutral or alkaline. Pure water provides these ions in equal concentration.
H20 H+ + OH-
The equilibrium for this reaction is far to the left (meaning most of the molecules occur as water). A tiny amount of H+ and OH- ions result from this reaction.
The ion product of the concentrations of H+ and OH- is a constant, (Ksp), which at 25 degrees C is known to be 1 x 10-14, thus
pH = - log H OR pH = log 1/(H+) OR pH = 7 at neutrality At pH = 7 the molar concentration of Hydrogen is 0.0000001 moles/L At pH = 5 the molar concentration of Hydrogen is 0.00001 moles/L
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pH vs. pOH
Soil Science
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Soil Science
Types of Soils, Based on Soil Reaction
Acid Soils – Al and H dominated (pH<7) Neutral Soils – equal mix of acid and alkaline
(pH=7.0) Calcareous soil – Ca dominated (pH 7.1 to 8.2) Saline soils – Ca + Sodium (Na) (pH> 8, free
salts) Sodic soils- Sodium dominated (free salts)
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Soil Science
How do soils become acid? Addition of Hydrogen (H+)
Acid Rain Root exudates Organic matter decay Ammonium-based fertilizers Breakdown of minerals and release of Al
Removal of Ca, Mg, and K Leaching Crop uptake of basic cations
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Soil Science
How does Al affect pH?
Al+3 is bound to the soil or in solution
Al3+ + HOH = Al(OH)2+ + H+
-Al+3
and, Al(OH) 2+ + HOH = Al(OH)2
+ + H+
Al(OH)2+ + HOH = Al(OH)3(s) + H+
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Soil Science
Acid Soil Toxicity
Hydrogen Toxicity Injury to roots at less than pH 4.0 Decrease in uptake of Ca and Mg
Aluminum Toxicity Decreased root growth Decreased Ca and P uptake Decreased Microbial Activity (Rhizobia)
Manganese Toxicity pH less than 5.0 with high total manganese
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%Base Saturation 100 x (Sum of Bases / CEC)
BASIC Cations are Na, K, Ca, and Mg CEC= 1 H + 3 Al +2 K + 4 Ca +2 Mg = 12 cmol/kg
Base saturation = 100 x (2 + 4 +2) /12=75%
1000 grams of soil (1 kilogram)
H +
Mg++
Al +++ Ca
++ K +
K +
Ca++
Soil Science
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Relationship of Soil pH and % Cation Holding Capacity
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Effect of O.M. on Aluminum and Soil Acidity
Soil Science
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Soil Science
How do liming materials affect pH
CaCO3 + 2 H2O = Ca + 2 OH + H2CO3, then
H+
H+
Al3+
Al3+
= 2 H2O + + 2 OH- + Ca2+
+ 6 OH- + 3 Ca2+ = Al(OH)3(s) + H2O + Ca++
Ca++
Ca++
Ca++
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Limed and Non-Limed Soils
Soil Science
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Phosphorus Generally very low in natural soils Exceptions:
Alluvial landscapes where O.M. is deposited by water with sediments
Where P bearing minerals are abundant in soil parent materials
Where Phosphorus tests of surface soils are High Due to P that has been added:
Manures and or chemical fertilizer Much is tied up in the Organic Fraction Some is tied up as oxides of Al and Fe and as such is
relatively unavailable
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Soil Science
Forms in Soils
Acid Soils Al and Fe phosphate Organic P Soil Solution - H2PO4
-1
Calcareous/Alkaline Soils Ca phosphates Organic P Soil Solution - HPO4
-2
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Fertilizer P Reactions
Acid Soils
Calcareous/Alkaline Soils Ca+2 + HPO4
-2 CaHPO4
OH
Al OH
OH + H2PO4
-2
H2PO4
Al OH
OH + OH-
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Movement in Soils
Low Mobility Moves primarily by diffusion Rates are very low - less than 1/4 of an inch
Factors Affecting Movement Water Content of Soil Clay Content: connection of water films Organic binding - phytate-P in manures more
mobile Amount of added P: Capacity to hold P limited
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Soil Science
Factors Affecting Availability
Amount of Available P Soil pH:
Acid soils: Al inhibits root growth, precipitates P in solution
Calcareous soils: Free carbonates precipitate P in solution
Soil Properties Amounts of Fe and Al oxides, hydroxides;
higher in soils with 1:1 clays than 2:1 clays
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