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