Clays Flashcards
What are the 3 most common elements in the Earth’s crust and in what %?
- Oxygen 49.2%
- Silicon 25.7%
- Aluminum 7.5%
What elements are required to produce silicates?
O, Si, Al, Fe, Ca, Na, K, Mg
What are the 6 silicate RFM?
- Olivine
- Hornblende
- Augite
- Micas
- Feldspares
- Quartz
Where does clay come from?
- Silicate RFM form igneous rocks
2. Weathering of RFM producing CLAY minerals, nutrients, quarts and silts
Why is clay important?
- significant component in soil
- contributes up to 40% in sedimentary rocks
- stores nutrients
- contributes to development of soil structure
- soil moisture holding capasity
What is the chemical structure of the silica tetrahedron?
SiO4 4-
What part of the clay is responsible for the negative charge?
3 of the 4 O atoms are bonding with other Si atoms so the negative charge only comes from the O on the edge of the sheet…. the internal atoms are neutralized
What is a clay crystal composed of?
2 connecting tetrahedrons (8 O and 2 Si) but 3 O are shared leaving 5 unbonded.
The formula is then: Si2O5 -2
6 silica tetrahedrons come together and for a ring. What is the chemical formula and what is found in the middle?
Si2O5(OH)-3
In the middle is a hydroxyl (OH-)
Concept:
O from silica tetrahedrons can bond with Al, Mg or Fe to form the 8 sided octahedron.
So now there is a bonded tetrahedron and octahedron
-
What is another word for clay microcell?
Micelles
What are the 2 most common clay patterns?
How are they arranged?
1: 1 and 2:1 and less common 2:1:1
1: 1 = 1 tetra and 1 oct
2: 1 = 1 oct sandwiched between 2 tetra
What are the 4 most common clay groups found?
Kaolinite
Smectite
Vermiculite
Chlorite
What are the clay microcell properties?
- clay has a huge internal surface area
- the microcell has a net negative charge with capasity to attract cations
What are the more communications found in our region?
H + Ca2 + Al3 + Mg2 + K + Na + and lesser NH4 +
What is the cationExchange Capacity? (CEC)
a measurement of the negative charge per unit mass of soil
or
soils ability to hold bases in the adsorption layer
Are compounds tightly held in clay’s crystal lattice available for plant uptake?
NO
Where are the available nutrients held?
Loosely held or adsorbed on the outer portion of the clay microcells —-> this is called the Defused double layer
How is the CEC measured?
What is measured?
In milligram equivalents (meq) of hydrogen
H + is measured
If all the exchange sites in 100g of soil could be occupied by 1 milligram of H+, then the soil would have _____ CEC or _____ meq, or _____ of hydrogen.
would have __1__ CEC or __1__ meq, or __1 milligram__ of hydrogen.
All the cations on the exchangeable cation exchange sites are replaced with the equivalent number of H+ ions
How does the CEC range of clay differ from the range of OM?
Clay = 0.4-0.5 meq/gram OM = 2.5-4.0 meq/gram ( higher because the SA of OM is so much greater then clay)
CEC associated with OM is highly dependent on what?
pH
high pH high CEC
What is the Percent Base Saturation?
The mount of bases held on the cation exchange site
What are the main anions of soil?
SO4 -2
borate B(OH)4 -
ammonia NO3 -
Phosphates H2PO4 -
How are bases held in the clay complex?
Cation bridging
What is soil reaction?
Another way of referring to soil pH and the effects of pH
Why is soil pH so important?
Storage of nutrients and elements
Why is clay referred to as “clay minerals”?
b/c it is a secondary mineral
Where does clay come from?
Chemical weathering of silicates
What is isomorphic substitution?
When Si is replaced with another element… this increases the net negative charge
What limits # of nutrient holding sites on clays?
Size and Isomorphic substitution
What are the factors that allow for Isomorphic substitution?
Atomic radius (function of the sub-atomic particles) and the valance
What is the difference between the diffuse layer and the secondary diffused layer?
Diffuse layer = there the cations are located
Secondary Diffuse Layer = where anions are found
The amount of cations soil can hold if a function of:
the amount of clay and the type of clay
What dictates the # of nutrients present in soil?
the humus fraction and the clay fraction
How does clay help with soil structure development?
(development of PEDs) acts as a binding agent!
What is expansive clay?
clay prone to large volume changes that are directly related to water content.
