Many agricultural soils worldwide in their natural state are deficient in phosphorus (P), and the production of healthy agricultural crops has required the regular addition of P fertilisers. In cropping systems, P accumulates almost predominantly in inorganic forms in soil, associated with aluminium, calcium and iron. In pasture soils, P accumulates in both inorganic and organic forms, but the chemical nature of much organic P is still unresolved. The P use efficiency (PUE) of fertilisers is generally low in the year of application, but residual effectiveness is important, highlighting the importance of soil P testing prior to fertiliser use. With increasing costs of P fertiliser, various technologies have been suggested to improve PUE, but few have provided solid field evidence for efficacy. Fluid fertilisers have been demonstrated under field conditions to increase PUE on highly calcareous soils. Slow release P products have been demonstrated to improve PUE in soils where leaching is important. Modification of soil chemistry around the fertiliser granule or fluid injection point also offers promise for increasing PUE, but is less well validated. Better placement of P, even into subsoils, also offers promise to increase PUE in both cropping and pasture systems. AbbreviationsDAP diammonium phosphate DCPD dicalcium phosphate dihydrate EDTA ethylenediamine tetraacetate MAP monoammonium phosphate MCP monocalcium phosphate NMR nuclear magnetic resonance OM organic matter P phosphorus P i inorganic P P o organic P PUE P use efficiency RPR reactive phosphate rock TSP triple superphosphate XANES x-ray absorption near-edge structure Plant Soil (

The solubility of Zn in Zn fertilizers plays an important role in the agronomic effectiveness of the fertilizer. On the basis of thermodynamics, zinc oxide (ZnO) nanoparticles (NPs) should dissolve faster and to a greater extent than bulk ZnO particles (equivalent spherical diameter >100 nm). These novel solubility features of ZnO NPs might be exploited to improve the efficiency of Zn fertilizers. In this study, we compared the Zn solubility and dissolution kinetics of ZnO nanoparticles and bulk ZnO particles coated onto two selected granular macronutrient fertilizers, urea and monoammonium phosphate (MAP). The main Zn species on coated MAP and urea granules were zinc ammonium phosphate and ZnO, respectively. Coated MAP granules showed greater Zn solubility and faster dissolution rates in sand columns compared to coated urea granules, which may be related to pH differences in the solution surrounding the fertilizer granules. The kinetics of Zn dissolution was not affected by the size of the ZnO particles applied for coating of either fertilizer type, possibly because solubility was controlled by formation of the same compounds irrespective of the size of the original ZnO particles used for coating.

Aim Biofortification of wheat with zinc (Zn) through foliar Zn application has been proposed as an agronomic strategy to increase grain Zn concentration, which could serve as a nutritional intervention in regions with dietary Zn deficiency. Methods Bread wheat (Triticum aestivum L.) was biofortified through foliar Zn applications at different growth stages. The concentration of Zn and associated micronutrient in harvested whole grains was determined by ICP-OES. Synchrotron-based X-ray fluorescence microscopy (XFM) was then used to investigate the localization of Zn and associated micronutrients in cross sections of these grains. Results The concentration of Zn and other micronutrients (Mn, Fe, and Cu) was higher in grains treated with foliar Zn during grain-filling (early milk/dough) than those treated at stem elongation. The increase in Zn concentration of wheat grain with foliar application during grain-filling can be attributed to the intense localization of Zn in the aleurone layer, modified aleurone, crease tissue, vascular bundle, and endosperm cavity, and to a modest localization in endosperm, which is the most dominant grain tissue. These tissues and the Zn they contain are presumed to remain after milling and can potentially increase the Zn concentration in wheat flour. Conclusions By using XFM, it was shown that foliar Zn spray represents an important agronomic tool for a substantial Zn enrichment of different fractions of wheat grain, especially the endosperm. Further investigation of the chemical speciation of Zn in the endosperm is recommended to assess Zn bioavailability in harvested whole grain of wheat that has been biofortified through different timing of foliar Zn application.

This study examined the effects of applied selenium (Se) species, time of application, method of application, and soil water management regimen on the accumulation of Se in rice plants. Plants were grown to maturity in a temperature-and humidity-controlled growth chamber using three water management methods: field capacity (FC), submerged until harvest, and submerged and drained 2 weeks before harvest. Two Se species, selenate (SeO 4 2−) and selenite (SeO 3 2− ), were applied at a rate equivalent to 30 g ha −1 . Four application methods were employed as follows: (i) Se applied at soil preparation, (ii) Se-enriched urea granules applied to floodwater at heading; (iii) foliar Se applied at heading; and (iv) fluid fertilizer Se applied to soil or floodwater at heading. Total Se concentrations in rice grains, husks, leaves, culms, and roots were measured, as well as Se speciation in grains from the Se-enriched urea granule treatment. Highest Se concentrations in the grain occurred with SeO 4 2− and with fertilizer applied at heading stage; SeO 4 2− -enriched urea granules applied at heading increased grain Se concentrations 5−6-fold (by 450−600 μg kg −1 ) compared to the control (no fertilizer Se applied) in all water treatments. Under paddy conditions other Se fertilization strategies were much less effective. Drainage before harvesting caused Se to accumulate in/on rice roots, possibly through adsorption onto iron plaque on roots. Rice grains contained Se mainly in the organic form as selenomethionine (SeM), which comprised >90% of the total grain Se in treatments fertilized with SeO 4 2− -enriched urea granules. The results of this study clearly show that of the fertilizer strategies tested biofortification of Se in rice grains can best be achieved in lowland rice by broadcast application of SeO 4 2− -enriched urea granules to floodwater at heading stage.

Zinc oxide (ZnO) nanoparticles may provide a more soluble and plant available source of Zn in Zn fertilizers due to their greater reactivity compared to equivalent micron- or millimetre-sized (bulk) particles. However, the effect of soil on solubility, spatial distribution and speciation of ZnO nanoparticles has not yet been investigated. In this study, we examined the diffusion and solid phase speciation of Zn in an alkaline calcareous soil following application of nanoparticulate and bulk ZnO coated fertilizer products (monoammonium phosphate (MAP) and urea) using laboratory-based x-ray techniques and synchrotron-based μ-x-ray fluorescence (μ–XRF) mapping and absorption fine structure spectroscopy (μ–XAFS). Mapping of the soil-fertilizer reaction zones revealed that most of the applied Zn for all treatments remained on the coated fertilizer granule or close to the point of application after five weeks of incubation in soil. Zinc precipitated mainly as scholzite (CaZn2(PO4)2.2H2O) and zinc ammonium phosphate (Zn(NH4)PO4) species at the surface of MAP granules. These reactions reduced dissolution and diffusion of Zn from the MAP granules. Although Zn remained as zincite (ZnO) at the surface of urea granules, limited diffusion of Zn from ZnO-coated urea granules was also observed for both bulk and nanoparticulate ZnO treatments. This might be due to either the high pH of urea granules, which reduced solubility of Zn, or aggregation (due to high ionic strength) of released ZnO nanoparticles around the granule/point of application. The relative proportion of Zn(OH)2 and ZnCO3 species increased for all Zn treatments with increasing distance from coated MAP and urea granules in the calcareous soil. When coated on macronutrient fertilizers, Zn from ZnO nanoparticles (without surface modifiers) was not more mobile or diffusible compared to bulk forms of ZnO. The results also suggest that risk associated with the presence of ZnO NPs in calcareous soils would be the same as bulk sources of ZnO.