# Fire-Induced Changes in Soil and Implications on Soil Sorption Capacity and Remediation Methods
## 3 Fire-Induced Changes in Soil Properties and the Implications on Soil Sorption Capacity
### Fire-Induced Changes in Soil pH and Implications on Soil Sorption  
Reports of the effects of fire on soil pH are contradictory. Whereas fire-induced increase in soil pH has been reported by [PERSON] and [PERSON] [16], [PERSON] [26], [PERSON] and [PERSON] [49], and other researchers [70; 71; 72; 73] have shown that heating between 250 and 500 \({}^{\circ}\)C lowers soil pH. Most of the studies which reported a decrease in pH were, however, carried out in the lab, which does not take into consideration the effect of the ashes produced from burning plant residues in the soil, hence the observed decrease in soil pH in some studies. However, laboratory studies carried out on contaminated soil in South Africa show that soil pH increases with increase in temperature up to 150 \({}^{\circ}\)C, after which there is no obvious effect of fire on soil pH (Figure 3a) [44]. These changes were observed after up to about 4 h of heating at various temperatures (Figure 3b-e). The values obtained in this study are similar to those reported by [PERSON] and [PERSON] [16] in burnt soils. These results highlight the role of fire intensity and duration on the changes in soil pH observed during fire events. Combustion of organic matter and ash production which releases large quantities of base forming ions calcium (Ca), magnesium (Mg) and potassium (K) during burning is the main explanation used to justify the increase in pH values observed with increase in fire intensity and duration. [PERSON] et al. [72], [PERSON] and [PERSON] [16], and [PERSON] et al. [74] have all shown that changes in soil pH caused by fires only last for about 3 years though [PERSON] and [PERSON] [75] reported up to 50 years.  
Soil pH is one of the major properties influencing the soil sorption complex. This role is associated with the fact that some charges in soils are pH-dependent and their contribution to the overall soil sorption capacity would vary with pH. The charges carried by organic matter and some clay minerals like kaolinite are pH-dependent, with increase in pH resulting in increased density of negative charges on these components, and vice versa. Increase in soil pH could therefore result in an increase in the density of negative charges on the soil and consequently their sorption capacity. However, fires reduce soil organic matter content, and so changes in pH may have no effect on the fraction of soil CEC contributed by organic matter because the functional groups contained in soil organic matter would have been destroyed or transformed. No correlation was observed between pH and CEC of soils in a laboratory study by [PERSON] [44] (Figure 4), which indicate that the reduction in CEC with heating of soils may have been due to changes in the lattice structure of the mineral components of the soil particles rather than by pH-dependent charges on the organic matter fraction of the soil. This further highlights the complexity of soil sorption. In minerals like kaolinite with hydroxyl groups on their outer edges, increase in pH would increase the negative charges on the mineral, which in turn will affect the CEC of the soil, and consequently its sorption capacity. This would be possible only  
Figure 2: Decrease in soil CEC with increase in temperature.  
if the soil temperature was not high enough to alter the structural composition of the kaolinite and consequently its surface charge density. Conversion of kaolinite to primary minerals during fires of high intensity would also mean that fire-induced changes in pH may not have any effect on the fraction of soil sorption contributed by the pH-dependent charges on kaolinite. Changes in soil pH caused by fire events could therefore either reduce or increase soil sorption capacity, depending on the content of organic matter in the soil, the presence of minerals with pH-dependent charges, and soil temperature.  
Figure 4: Relationship between pH and CEC of soils heated at different temperatures.  
Figure 3: Variation of soil pH with intensity and duration of heating.