Fig.1 Ferralsol, Malaysia

( Source: Brady and Weil 2002.)

• PZNC is critical to understanding the nutrient dynamics of variable-charge soils.
  
• Reflects the correlation between soil pH and negative + positive charges.
  
• The negative charge (CEC) increases with increasing pH.
  
• Positive charge (AEC) decreases with increasing pH.
  
• The point at which equal numbers of positive and negative charges coexist, i.e. the pH at which the soil is electrically neutral.
  
• Here (in Fig.1): PZNC is at ~ pH 4.4
  

1. pH < PZNC: at pH levels below this point, the soil takes on an increasing net AEC.
   
2. pH > PZNC: at pH levels above the PZNC, increasing net CEC prevails.
   
3. at PZNC: the variable-charge component of the soil provides no adsorptive capacity.
   

• PZNC of many tropical soils (contain soil organic matter and mixture of clay minerals of varying properties) range between pH 3.5 – 5.0. This lies within the range of natural soil pH variability.
  
• Consequently these soils may take on either ( CEC) or  AEC characteristics depending on local circumstances and type of management.