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The grain yield benefits of applying micronutrient fluid fertilisers over conventional granular products in calcareous sandy loam soils have been agronomically demonstrated. An understanding of the fundamental mechanisms and reactions occurring following application of these products to soils is critical to improve fertiliser management. We therefore examined the diffusion, solubility, and potential availability of manganese (Mn) and zinc (Zn) from both granular and fluid forms of Mn and Zn fertiliser in an alkaline calcareous and alkaline non-calcareous soil using laboratory incubation experiments in conjunction with an isotopic dilution technique with [sup.65]Mn and [sup.65]Zn. Enhanced mobility, solubility, and/or potential availability of Mn and Zn from fluid fertilisers were observed in comparison to Mn or Zn from granular fertilisers in both soils after 5 weeks of incubation. Differential behaviour of fluid and granular fertilisers for Mn and Zn appeared to be independent of their effects on soil pH. Most (~90%) of the Mn in granular fertiliser dissolved and diffused out of the granule but was retained within 4 mm of the point of granular placement, whereas most (~85%) of the Zn in the granular Zn fertiliser source remained in the granule. Our data suggest that the superior agronomic effectiveness of fluid Mn and Zn fertilisers observed in calcareous soils under field conditions may have resulted from the enhanced diffusion (Mn) and/or solubility/availability (Mn, Zn) of these micronutrients in soil when applied in fluid form., Introduction The major reason for the widespread occurrence of manganese (Mn) and zinc (Zn) deficiency in soils is the low availability of Mn and Zn to plant roots rather than [...]

Liquid polyphosphate fertilizers have shown significant yield increases compared with granular orthophosphate fertilizer on highly P-fixing soils in Australian cropping trials. Polyphosphate fertilizers contain mixed P species (predominantly pyrophosphate and orthophosphate) and are therefore chemically different from most other inorganic P fertilizers, which contain only orthophosphate. To determine the mechanisms causing the yield responses to polyphosphate fertilizer, a method for investigating the isotopic exchangeability (lability) of P added as pyrophosphate was developed and tested. This method enabled the quantification of the hydrolysis reaction of pyrophosphate to orthophosphate in soil solution. As the rate of pyrophosphate applied increased, the proportion of pyrophosphate added that was isotopically exchangeable increased, and the percentage of pyrophosphate added that hydrolyzed to orthophosphate decreased. The method was further developed to test the hypothesis that pyrophosphate addition to soil may cause the mobilization of native orthophosphate reserves. The results suggested that there was no mobilization of native P after 7 d of incubation in two calcareous soils. Abbreviations: OP, orthophosphate; PP, pyrophosphate.

Liquid polyphosphate fertilisers contain both orthophosphate and pyrophosphate, and have shown significant yield increases compared to granular orthophosphate fertiliser on highly phosphorus (P) fixing soils. The P sorption chemistry of the dominant P species in a polyphosphate fertiliser (pyrophosphate and orthophosphate) was compared on a range of Australian soils, and the effect of these P species on equilibrium solution chemistry was also examined. Phosphorus supplied as pyrophosphate had a stronger sorption affinity than orthophosphate in all soils. The greater efficiency of pyrophosphate-based fertiliser on some soil types is therefore not due to reduced sorption of this P supplied as pyrophosphate compared to P supplied as orthophosphate fertiliser. In general, the addition of pyrophosphate to soil resulted in a larger decrease in calcium concentration in solution compared to orthophosphate. In contrast, there was an increase in iron concentration in solution with pyrophosphate addition, indicating sequestration reactions associated with the dissolution of dissolved organic carbon into soil solution. The pyrophosphate ion generally mobilised more Fe into solution than orthophosphate in several soils, likely through complexation reactions leading to Fe desorption/dissolution from the soil solid phase. These findings highlight the differences in soil chemical reactions that occur with the addition of polyphosphate fertilisers, which may contribute to their yield advantage in some soil types. Additional keywords: phosphorus, partitioning, polyphosphate, ion chromatography, calcarosol., Introduction Polyphosphate fertilisers have recently gained attention in Australian agricultural research due to significant yield increases with the application of liquid polyphosphate compared to granular orthophosphate (OP) fertilisers on highly [...]

Phosphorus availability is often a limiting factor for crop production around the world. The efficiency of P fertilizers in calcareous soils is limited by reactions that decrease P availability; however, fluid fertilizers have recently been shown, in highly calcareous soils of southern Australia, to be more efficient for crop (wheat [Triticum aestivum L.]) P nutrition than granular products. To elucidate the mechanisms responsible for this differential response, an isotopic dilution technique (E value) coupled with a synchrotron-based spectroscopic investigation were used to assess the reaction products of a granular (monoammonium phosphate, MAP) and a fluid P (technical-grade monoammonium phosphate, TG-MAP) fertilizer in a highly calcareous soil. The isotopic exchangeability of P from the fluid fertilizer, measured with the E-value technique, was higher than that of the granular product. The spatially resolved spectroscopic investigation, performed using nano x-ray fluorescence and nano x-ray absorption near-edge structure (n-XANES), showed that P is heterogeneously distributed in soil and that, at least in this highly calcareous soil, it is invariably associated with Ca rather than Fe at the nanoscale. 'Bulk' XANES spectroscopy revealed that, in the soil surrounding fertilizer granules, P precipitation in the form of octacalcium phosphate and apatite-like compounds is the dominant mechanism responsible for decreases in P exchangeability. This process was less prominent when the fluid P fertilizer was applied to the soil.

Pyrophosphate is the main form of condensed P in the fluid fertilizer ammonium polyphosphate (APP). When APP is applied to soil, pyrophosphate is hydrolyzed to orthophosphate. Hydrolysis of pyrophosphate was investigated in a highly calcareous soil over 3 wk. Changes in P speciafion were measured using solid-state [sup.31]P nuclear magnetic resonance (NMR) spectroscopy for dried soil samples and ion chromatography for NaOH extracts. Both techniques showed a decrease in pyrophosphate and concomitant increase in orthophosphale concentration with time, but the rate of pyrophosphate hydrolysis calculated from the NaOH extract measured by ion chromatography was two times faster than the rate calculated from the NMR data indicating differences in P speciation between methods. The NMR technique employed spin counting to determine the proportion of P nuclei observable by NMR. Since this proportion (71-88%) was greater than the proportion of P extracted by NaOH and detected by ion chromatography (38-71%), we concluded that the NMR technique was able to quantify more of the pyrophosphate added than the extraction method. The results of the study suggest that solid-state [sup.31]P NMR is a powerful noninvasive technique for the investigation of pyrophosphate reactions in soil and that pyrophosphate has the ability to persist for several weeks in highly calcareous Australian soils in the presence of microbial activity. This has important implications for P availability processes where pyrophosphale is supplied as a fertilizer and is currently under investigation in Australian soil types.

Recent field trials on alkaline soils in southern Australia showed significant grain yield responses to liquid compared with traditional granular forms of P fertiliser. However the advantages of liquid over granular P forms of fertiliser has not been consistent on all soil types. In order to better predict the soil types on which liquid P fertilisers are likely to have potential, a glasshouse trial was conducted to compare the responsiveness of wheat to both liquid and granular forms of P on a wide range of Australian soils. A granular P fertiliser (triple superphosphate) and 2 liquid fertilisers (phosphoric acid and ammonium polyphosphate) were compared at a rate equivalent to 12 kg P/ha in 29 soils representing many of the soil types used for grain production in Victoria and South Australia. Wheat biomass was enhanced by P application in 86% of the soils tested. In 62% of the P-responsive soils, wheat dry matter was significantly greater when liquid P fertilisers were used compared with the granular form. Chemical analysis of the soils tested showed that the better performance of liquid P forms was not correlated to total P concentration in soil, P buffer capacity, or P availability as measured by Colwell-P. However, there was a significant positive relationship between calcium carbonate (CaC[O.sub.3]) content of soil and wheat responsiveness to liquid P fertiliser. Additional keywords: liquid fertilisers, fluid fertilisers, phosphorus, alkaline soils, wheat., Introduction Phosphorus is the most expensive major nutrient used in broadacre cropping in Australia, with the cost of P fertiliser to Australian farmers being approximately $1.645 billion per year (FIFA [...]

Phosphorus availability is a major factor limiting crop production in highly calcareous soils. Recent field trials on calcareous soils in southern Australia have shown that fluid fertilizers may provide a useful 'alternative to granular fertilizer products. Fluid sources of P enhance P uptake and yield when compared with granular fertilizers applied at the same rate. This work aimed to compare the behavior of one fluid (technical grade monoammonium phosphate, TG-MAP) and one granular (monoammonium phosphate, MAP) form of P fertilizer in a highly calcareous soil. Changes in soil pH, P diffusion, solubility, and lability (using isotopic dilution techniques) were determined at different distances from the point of application over 5 wk. Furthermore, rear, on products in MAP granules were investigated using spectroscopic techniques. The results indicated that P from fluid TG-MAP diffused more and was more available than P supplied as granular MAP. Also, X-ray diffraction (XRD) and energy' dispersive X-ray microanalyses (EDXMA) of the MAP granules indicated that a significant percentage (12%) of the initial P remained in the granules even alter 5 wk of incubation in the soil. The enhanced P availability of fluid fertilizers observed in field trials compared with granular forms is discussed in relation to differences in the dissolution, diffusion, and reaction processes in soils.