SEASONALLY PRECIPITATED IRON-OXIDES IN A VERTISOL OF SOUTHEAST TEXAS

The composition and crystallinity of Fe oxides in soils determines the reactivity and toxicity of Fe through redox and solubility reactions. The mineralogy and crystallinity of Fe oxides seasonally precipitatin g on ped surfaces and within soil pores and those forming around rice (Oryza sativa L.) roots were investigated by x-ray diffraction, electr on microscopy, and electron microprobe analyses. Iron oxides precipita ted on exposed surfaces of the League soil (fine, montmorillonitic, hy perthermic Oxyaquic Dystrudert), which is hooded during rice productio n, differed from Fe phases precipitated around rice roots. Iron oxides precipitated on ped surfaces and within soil pores were relatively po orly crystallized while those precipitated on rice-root surfaces were well crystallized. The presence of soluble Si and P during flooding ma y be responsible for precipitation of the less crystalline Fe oxides. Infrared and electron-diffraction data on the precipitate suggest the presence of PO4 groups either adsorbed or coprecipitated with Fe oxide . Depletion of Si and P from the rhizosphere is believed to contribute to the formation of well-crystallized lepidocrocite on root surfaces. The poorly crystalline Fe-oxide precipitate that forms on ped surface s upon draining and oxidation of the League soil has adsorbed or occlu ded Si and P. Thus, Fe oxides may influence the mobility of Si and P i n alternately flooded and drained soils. Electron-diffraction data sug gests that some of the Fe may be precipitated as strengite. Upon reduc tion and dissolution, these oxides release Fe, Si, and P into the soil solution and influence the nutrient dynamics in the rhizosphere of th e rice plant.