An Introduction to Environmental Chemistry

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