The researchers studied how the point of zero charge (PZC) of soil changes as it ages in tropical Hainan Island. They collected soil samples from different ages and used a method called potentiometric titration to measure the PZC. They found that the PZC of the soil increased as the soil aged, likely due to changes in clay minerals and the presence of iron oxides. This change in PZC also affected the soil's ability to retain nutrients, with the soil becoming less fertile over time. The researchers suggest that the PZC can be a useful indicator of soil development and weathering in tropical areas.