Lecture 11 Clay Minerals Clay and organic matter in the soil provide the negative absorptive sites or Cation Exchange Capacity (CEC) Clay can hold positive cations from leaching Soil CEC is the most important indicator of the soil’s ability to hold onto and retain + cations like Ca, Mg, K, NH4+,
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Lecture 11 Clay Minerals Clay and organic matter in the soil provide the negative absorptive sites or Cation Exchange Capacity (CEC) Clay can hold positive.
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Lecture 11Clay Minerals
Clay and organic matter in the soil provide the negative absorptive sites or Cation Exchange Capacity (CEC)
Clay can hold positive cations from leaching
Soil CEC is the most important indicator of the soil’s ability to hold onto and retain + cations like Ca, Mg, K, NH4+,
Organic matter as a source of negative charge
1. Soil Humus - SOM
R-CO + OH <---> R-CO-O- + H+
This charge is pH dependent or as the pH increases more OH- is available and therefore more CEC and conversely acid soils have a lower CEC from OM.
OH- orO- Ca++ or K+
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Clay minerals as a source of negative charge- internal and external
Clay as a source of negative charge
2. Source 1:1) broken bonds at
edge of clay mineral
Edge Effects are pH Dependent
Source 2 for negative charges:
2) internal charge not satisfied because of ionic substitution -called Isomorphous Substitution
Al 3+ for Si 4+ or Mg2+ for Al 3+ or Zn2+ for Al3+
No substitution Zn++ for Al+++
Neutral -1 charge
Silicate Clay Minerals
Aluminosilicates have a definite crystalline structure. Formed by the alteration of existing minerals or by synthesis from elements.
< .002 mm in size (only seen with aid of electron microscope)
Two basic kinds of clay minerals 1 : 1 and 2 : 1
Basic Building Blocks for Clay Minerals
Silicon - Oxygen Tetrahedron
Repeat Unit: (Si2O5)-2
SiO4-4
Four sides
Basic Building Blocks - 2
Aluminum Octahedral (Gibbsite Sheet) Al(OH)6
-3 Repeat Unit: Al2(OH)4+2
Al(OH)3
Eight Sided
Types of Clay Minerals
1) Kaolinite a 1: 1 clay mineral - 1 silica sheet and 1 gibbsite sheet