Assessing Iron Fractions and Their Availability in Lateritic Soils of Kerala Using Structural Equation Modelling

Abstract

Aims: The study aimed to investigate the different fractions of iron in the north central laterites of Kerala and their relative contribution to iron availability. Study Design:  Soil sampling, characterisation, fractionation, and structural equation modelling. Location and Duration of Study: Department of Soil Science and Agricultural Chemistry, College of Agriculture, Vellanikkara, Thrissur, from January 2024 to May 2024. Methodology: Thirty georeferenced soil samples were collected from Agro Ecological Unit (AEU) 10, representing the north central laterites of Kerala. The samples were characterised, and different iron forms were extracted. Fractionation procedures were carried out to separate specific iron pools. Structural equation modelling (SEM) was used to determine the soil properties and iron fractions contributing to iron availability. Results: The results showed that the dominant fractions in the soil were residual iron (32.06%) and crystalline iron oxide occluded iron (28.97%). Acid-soluble iron (Aci-Fe), crystalline iron oxide-occluded iron (Cry FeO-Fe), and manganese oxide-occluded iron (MnO-Fe) positively influenced available iron (path coefficients = 0.28*, 0.26*, and 0.55**, respectively). In contrast, fractions such as amorphous iron oxide-occluded iron (Am FeO-Fe), organic matter-bound iron (OM-Fe), and residual iron showed more indirect effects on iron availability. Conclusion: In the laterites of north central Kerala, most iron is present in stable forms and only a small fraction is present as plant-available iron. Its availability is controlled by dynamic interactions between amorphous, crystalline, and organically bound forms of iron. Although pH did not show a strong direct effect on iron availability, it may still play an indirect role by influencing the stability of organic matter and the transformation between different iron forms.