An Introduction to Environmental Chemistry

(Rick Simeone) #1

106 Chapter Four


Laterite

Red oxisols (feralsols)
(kaolinitic, Fe- and Al-rich)

Dark vertisols
(Smectitic, base cation-rich)

Base cations
Smectite clays


  • Low cation exchange capacity

  • Low fertility

  • Residual Fe and Al can be toxic
    to plants

    • High cation exchange capacity

    • High fertility

    • Deep soil cracking can affect
      agriculture in dry seasons




Fig. 4.16Diagram illustrating the soil end-members of a tropical catena.

Box 4.11 Base cations

The term base cation, or non-acid cation, is
often used in soil chemistry and refers to
cations of the alkali metals and alkali earth
metals (see Section 2.2), most importantly
Ca^2 +, Na+, K+and Mg^2 +. The weathering of
these metals from crustal minerals is a slow,
but very important process that helps
neutralize acidity. For example, weathering

of the (Na)-rich feldspar, albite, proceeds
as in equation 1.
The acidity (H+) contained in H 2 CO 3 is
neutralized and dissolved cations (Na+),
bicarbonate (HCO 3 - ) and silicic acid (H 4 SiO 4 )
are released. These ‘released’ base cations
thus become available in soilwater to take
part in exchange reactions (Section 4.8).

eqn. 1

292 NaAlSi O 38 ()sl++H O 2 () H CO2 3( )aqÆAl Si O OH 225 ( ) 4 ()s++ 224 Na()+aq HCO- 344 ()aq+H SiO()aq
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