Chemical Properties of Secondary Phyllosilicates Isomorphous substitution •‘replacement’ of an ion by another of similar size, but differing charge •Creates net negative charge on mineral structure Cation Exchange Capacity •Measure of ability of soil to retain positively charged ions (meq/100 g) •Measured on basis of cations retained per 100 g soil Base Saturation •Fraction of total CEC that is counter balanced by ‘base cations’ (Ca, Mg, Na, K) •Remaining charge neutralization by H, Al is refered to as ‘exchangable acidity”
Chemical Properties of Secondary Phyllosilicates. Isomorphous substitution ‘replacement’ of an ion by another of similar size, but differing charge Creates net negative charge on mineral structure Cation Exchange Capacity - PowerPoint PPT Presentation
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Chemical Properties of Secondary Phyllosilicates
Isomorphous substitution•‘replacement’ of an ion by another of similar size, but differing charge
•Creates net negative charge on mineral structure
Cation Exchange Capacity•Measure of ability of soil to retain positively charged ions (meq/100 g)
•Measured on basis of cations retained per 100 g soil
Base Saturation•Fraction of total CEC that is counter balanced by ‘base cations’ (Ca, Mg, Na, K)
•Remaining charge neutralization by H, Al is refered to as ‘exchangable acidity”
Estimating soil clay mineralogy from CEC
CEC/100 g soil x 1/clay% x 100 = CEC/100g clay
(meq/100g soil)(100gsoil/g clay)(100)
Organic matter correction
CEC/100gsoil x C% x CEC/g C = corrected CEC (insert into equation above)
(meq/100gsoil)(gC/100gsoil)(meq/gC)
Example from Brazil
Correction of A horizon
CEC= 6.7 meq/100 g soil
C = 2.76% (x2 = OM)
Clay = 34.7%
CEC/100g clay =19.3
Mineralogy=kaolinite and geothite (~5 meq/100g clay)