Clay mineral dissolution following intensive cultivation in a tropical sandy soil

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anglais. Sandy soils of Northeast Thailand are predisposed to high acidification rates due to leaching and the export of alkalinity is associated with crop removal in these intensive cropping systems. Despite significant acid release, soil pH generally remains stable at a threshold value of around 4.0. Low organic matter content and the absence of weatherable primary minerals would suggest that clay mineral dissolution is responsible for the high degree of buffering commonly observed in these soils. The objective of this study was to investigate changes in nature and organisation of clay minerals following intensive cultivation in a typical sandy soil from Northeast Thailand. Surface soils were sampled under a forest (FS) and an adjacent area cultivated for 50 years (CS); they were compared with parent material sampled at 3.5 meter depth (PM) with the aim of characterising the evolution of clay minerals through pedogenesis and cultivation. The proportion of small particles (mode 0.1 µm) decreased according to pedogenesis – from parent material to soil, and land use – from forest to crop. Under the cultivated and forest soils, particles of kaolinite appeared to be very small (0.02-0.10 µm), poorly crystallised and eroded, often organised as aggregates of 1-2 µm. Expandable 2:1 clay minerals were associated with kaolinite. Chemical data of individual particles revealed that kaolinite contained iron and that expandable 2:1 clay minerals were smectite, vermiculite and mixed-layer illite/smectite. X-ray diffraction patterns of <2 µm-fractions indicated that kaolinite was the main phase, 78%, 88% and 88% in CS, FS and PM respectively, smectite being a minor phase with 20%, 6% and 12% respectively. Our results suggest that the dissolution of kaolinite was accelerated in cultivated system (CS), with a correlative neoformation of smectite, which buffers potential declines in soil pH.