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Zinc (Zn) is the micronutrient commonly found deficient in agricultural soils worldwide, limiting crop yield and reducing food quality. Zinc‐enriched fertilizers have been successfully used to tackle crop Zn deficiency. However, Zn solubility is reduced after addition to phosphate fertilizers due to the formation of sparingly soluble precipitates, decreasing Zn availability to crops. We hypothesized that the availability of Zn in the fertilizer to plants is related to its speciation in the fertilizer. We evaluated a range of ammonium phosphate fertilizers using X‐ray absorption near‐edge structure and assessed the water‐solubility of Zn in the formulations. Four Zn species were identified in these fertilizers, with zinc ammonium phosphate the most abundant one. The speciation of Zn in the fertilizer had little relationship with the water solubility of Zn in the final product. Zinc solubility was driven by fertilizer pH, with lower pH resulting in higher solubility. We concluded that added Zn reacts with the fertilizer matrix to form mainly zinc (ammonium) phosphates, and when the fertilizer is dissolved in water, hopeite controls the solubility. Based on these findings, we tested whether a barrier coating between the P granule and the Zn compound could prevent reaction between Zn and P and thus enhance Zn availability. Indeed, higher Zn uptake was observed in an isotopically labeled fertilizer growth trial when a barrier coating physically separated Zn and P in the fertilizer. In summary, Zn availability to crops can be maximized by decreasing fertilizer pH and separating P and Zn in the granule. Core Ideas: Water‐soluble Zn content affects the effectiveness of Zn fertilizers.Zn speciation is not a reliable predictor of fertilizer Zn solubility.Zn water solubility is maximized by decreasing granule pH.A coated barrier between the P matrix and Zn coating improved plant Zn uptake. [ABSTRACT FROM AUTHOR]

Knowledge of the dominant P species present in the soil following the application of organic amendments and fertilizer is important in understanding the fate of P in the environment. This study was performed to identify P species in two calcareous soils, an Osborne series (Typic Humicryert) and a Lakeland series (Typic Calciudoll), treated with organic amendments (biosolids and hog and dairy cattle manure) and fertilizer (monoammonium phosphate, MAP). Phosphorus 1s x-ray absorption near-edge structure (XANES) spectroscopy was used to speciate P compounds in these amended soils. The result showed that 'adsorbed P' was the dominant P species in both soils. For the Osborne soil, the unamended soil (control) and those amended with biosolids and MAP contained an appreciable amount of hydroxyapatite (HAP), the most thermodynamically favored Ca phosphate. In addition, soils amended with biosolids or hog or dairy manures contain [beta]-tricalcium phosphate (TRICAL), a more soluble form of Ca phosphate than HAP. The amended Lakeland soils contained a variety of species in addition to the dominant 'adsorbed P.' While TRICAL was found in all the amended soils except in that amended with hog manure, HAP was present in appreciable amount only in the unamended soil. Overall, the adsorbed P, most likely through innersphere complexation, in these amended soils may not be readily available as a source of P into the environment. In addition, the HAP and TRICAL will have limited solubility, and thus, are probably only a very minor source of P in the environment. Abbreviations: AQ-P, aqueous phosphate; DCP, dicalcium phosphate; DCPD, dicalcium phosphate dihydrate; HAP, hydroxyapatite; LC, linear combination; MAP, monoammonium phosphate; TRICAL, [beta]-tricalcium phosphate; XANES, x-ray absorption near-edge structure; XAS, x-ray adsorption spectroscopy.

The quantitative approach used in x-ray absorption spectroscopy (XAS) experiments is oftentimes based on statistical goodness-of-fit criteria, which do not explain the accuracy of the components obtained from the fittings. This study was performed to validate the linear combination (LC) approach used in quantitative XAS analysis by estimating the accuracy of this procedure. Near-edge K[[alpha].sub.1] fluorescence XAS spectra were acquired for known binary mixtures of Ca, Al, and Fe phosphates in varying proportions and for the individual compounds. All combinations of the spectra of model compounds were fitted to the spectra of the known mixtures to obtain their relative abundance. The binary combinations produced the best fit with [chi square] values ranging from 0.02 to 0.25. The relative error associated with the fitting ranged from as low as 0.8 to 17% for thoroughly mixed samples. The relative error was small when the proportion of Ca phosphate in the mixture was high but the error was large at low abundance of this component in the mixture. Because the interpretation of the XANES result largely depends on the relative proportion of species in the sample obtained by LC, we therefore recommend acquiring a spectrum for a mixture of certified reference compounds that mimics the composition of the sample being investigated at the beamline to estimate the accuracy of the proportions obtained from quantitative x-ray absorption near-edge structure (XANES) analysis. Abbreviations: HAP, hydroxyapatite; LC, linear combination; PSIDER, phosphosiderite; VAR, variscite; XANES, x-ray absorption near-edge structure; XAS, x-ray absorption spectroscopy.