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Soil chemical extractions are widely used to predict the nutritional status of soils. However, the correlation between extracted elements and plant uptake is often poor, especially if compared over a range of soil types. The aim of this study was to examine a new method called Diffusive Gradients in Thin films (DGT), which measures the diffusive supply of elements, thereby mimicking a plant root. The ability of DGT to assess plant-available P, Zn and Cu was tested in a wide range of typical Scandinavian agricultural soils along with conventional methods (EDTA and DTPA for Cu and Zn; NaHCO for P and soil solution concentrations). Extracted soil concentrations were compared to that of the element in the youngest fully developed leaf of barley ( Hordeum vulgare L.) grown in pots. For Zn and P, only DGT could predict plant uptake while conventional extraction methods and soil solution analyses performed poorly. All soil tests could predict Cu concentration in leaves, but the DGT technique proved to be most accurate followed by the soil solution concentration of Cu. We conclude that DGT is much more accurate at predicting plant-available P, Zn and Cu than commonly used methods for analysing plant-available nutrients in soil. [ABSTRACT FROM AUTHOR]

Liquid polyphosphate fertilisers have shown advantages in field experiments as a phosphorus (P) source for crops grown on calcareous soils. Polyphosphate fertilisers contain orthophosphate (oP), pyrophosphate (pP) and other condensed P species. A double labelling technique was developed using ion chromatography for separation of oP and pP, the major P species in polyphosphate fertilisers, in order to measure the isotopically exchangeable oP, pP and hydrolysed pP. Isotopically exchangeable P was measured in soils incubating for zero, three and 7 days after applying oP or pP to simulate a fertiliser band concentration at one g P kg−1 soil. The data from this incubation study suggest that pP addition initially resulted in less isotopically exchangeable P than oP addition but there was no significant difference in the total isotopically exchangeable P for the two different P sources after 7 days of incubation. The addition of pP to soil resulted in an increase in iron, aluminium and dissolved organic carbon in soil solution, and a decrease in calcium concentration in soil solution. This study has shown that the availability of P added as pP is governed by a complex series of processes that differ from the chemistry of oP reactions in soil. [ABSTRACT FROM AUTHOR]

Despite a long history of application of phosphorus fertilisers, P deficiency is still a major limitation to crop production on calcareous soils. Recent field research conducted in highly calcareous soils in southern Australia has demonstrated that both grain yield and P uptake of wheat (Triticum aestivumL.) is greater when fluid forms of P are used compared to granular forms. To improve our understanding of the mechanisms underlying this response to P in the field, we compared the lability, solubility and mobility of P applied as either a fluid (3 products) or granular (3 products) form to two calcareous and one alkaline non-calcareous soils in the laboratory. Over a five-week period, between 9.5 and 18 % of the P initially present in the fertiliser granules did not diffuse into the surrounding soil. The degree of granule dissolution was independent of the soil type. In contrast, P solubility, lability and diffusion were significantly greater when fluid products were applied to the calcareous soils, but not to the alkaline non-calcareous soil. These findings are discussed in relation to field trials results where fluid products outperformed granular fertilisers. [ABSTRACT FROM AUTHOR]