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SOIL CHEMISTRY

Evidence for Different Reaction Pathways for Liquid and Granular Micronutrients in a Calcareous Soil

^a Soil and Land Systems, School of Earth and Environmental Sciences, Univ. of Adelaide, Waite Campus, Glen Osmond, SA 5064, Australia
^b CSIRO Land and Water, PMB 2, Glen Osmond, SA 5064, Australia
^c National Risk Management Research Lab., U.S. Environmental Protection Agency, 5995 Center Hill Ave., Cincinnati, OH 45224
^d GSECARS, Univ. of Chicago, Chicago, IL 60637

^* Corresponding author (ganga.hettiarachchi{at}adelaide.edu.au).

The benefits of Mn and Zn fluid fertilizers over conventional granular products in calcareous sandy loam soils have been agronomically demonstrated. We hypothesized that the differences in the effectiveness between granular and fluid Mn and Zn fertilizers is due to different Mn and Zn reaction processes in and around fertilizer granules and fluid fertilizer bands. We used a combination of several synchrotron-based x-ray techniques, namely, spatially resolved micro-x-ray fluorescence (µ-XRF), micro-x-ray absorption near edge structure spectroscopy (µ-XANES), and bulk-XANES and -extended x-ray absorption fine structure (EXAFS) spectroscopy, along with several laboratory-based x-ray techniques to speciate different fertilizer-derived Mn and Zn species in highly calcareous soils to understand the chemistry underlying the observed differential behavior of fluid and granular micronutrient forms. Micro-XRF mapping of soil–fertilizer reactions zones indicated that the mobility of Mn and Zn from liquid fertilizer was greater than that observed for equivalent granular sources of these micronutrients in soil. After application of these micronutrient fertilizers to soil, Mn and Zn from liquid fertilizers were found to remain in comparatively more soluble solid forms, such as hydrated Mn phosphate-like, Mn calcite-like, adsorbed Zn-like, and Zn silicate-like phases, whereas Mn and Zn from equivalent granular sources tended to transform into comparatively less soluble solid forms such as Mn oxide-like, Mn carbonate-like, and Zn phosphate-like phases.

Abbreviations: EDAX, energy dispersive x-ray analysis • EXAFS, extended x-ray absorption fine structure • LCF, linear combination fitting • MAP, monoammonium phosphate • SEM, scanning electron microscopy • TGMAP, technical-grade monoammonium phosphate • XANES, x-ray absorption near-edge structure • XAS, x-ray absorption spectroscopy • XRD, x-ray diffraction • XRF, x-ray fluorescence