STRUCTURE AND COMPOSITION OF CALCIFIED ROOTS, AND THEIR IDENTIFICATION IN CALCAREOUS SOILS

Fine white branching structures isolated from calcareous soils in southern France were shown by microscopy and by chemical and mineralogical analysis to be calcified roots. These structures occur mainly in Rendzinas, Calcic Xerosols and Cambisols that are dense or rich in fines and where drainage is slow. Visible to the naked eye, they can locally represent up to a quarter of the soil mass. Calcified roots consist of calcite elements whose mean size varies from 60 to 80-mu-m, and which retain the structure of the original cells of the root cortex. Each calcified cell has a central nucleus composed of two crystals on average and is coated with a thin mineral layer that seems to be poorly crystallized. Some retain organic remnants and the structure of cell walls. Calcified cells contain tubes, less than 1-mu-m in diameter, which appear to have formed by weathering. Although they have generally not been recognized as calcified roots, similar microstructures have been indicated in some calcareous soils and calcretes, both present day and ancient ones. That suggests that calcified roots are widespread and that their occurrence contributes to the genesis of some calcareous soils.